300PRC +P Load development

[FLIGHT762]

**For those interested**

Old forester 1920 115º is just a hint sweeter than the Knob Creek single barrel 120º. Buy them both, too close to call. If you like sweeter (like me) the O/F is great. Fuck it, I'm keeping them all. Now I need to go smoke a good Cigar.

Carry on with the primer pocket smokers.

 

[Frank Green]

Dude.......I built 3 Shermans. Never got the speeds advertised. Went on there discussing velocity and psi. How does said 6.5mm Sherman case with 71gr capacity somehow out perform 6.5mm case with 74gr capacity? No geometrical magic but HIGH PSI. Richard booted me from the Facebook group. Fuck that guy......he says he'll never persue saami, hell no he wont, saami performs psi testing. All his "efficiency" will be proven as just excessive psi.

I'll agree and again I reiterate....unless the ammo is run thru a pressure and velocity (P&V) test barrel you don't know what you have! It might look good on paper but until it's tested.....

Look at the 300WM...might not be an apple to apple comparison but it does show issues even with the Mod 0 ammo with the 220gr bullet. They pushed the seating of the bullet out to as much as 3.5" (saami is 3.360") on the overall length. Stuffed a 220gr bullet in it and called out the same velocity spec. as the proven 190gr bullet load. Guess what? The stuff runs over pressure. I'll say average is 72k psi. If you gave me a 1k rounds of free ammo of that stuff I wouldn't run it thru my gun. I'd pull the bullets and dump the powder and reuse the cases etc...(no I wouldn't reuse the powder).

Also if these stepped throats are anything similar to the bore rider throats....the throat of the chamber area doesn't wear uniformly. Guys running these types of throats in they're guns notice the gun loses accuracy sooner and or it's harder to keep the gun in tune. Your always chasing it.

Also by seating the bullet out further...yes it gives you in theory more case capacity but in the end your pushing the bullet out further to the throat anyways so your not offsetting anything at all in my opinion. The closer the bullet it is to the throat pressures typically go up. So what are you gaining?

Pressure test the ammo in a pressure test barrel and get real data. That's the only way to prove the concept etc...

 

[Feniks Technologies]

Software is showing 84gr H1000 @ 3117fps. So, fairly close to your 3150.

It also shows about 76,300 psi. Saami is 65,000 max.

So, I would definitely be asking the manufacturer for some sort of actual pressure testing data.

 

[Carole Baskins]

Nope....all you gotta do is dick out yer chamber and if you really wanna scorch the earth go
SHERMAN MEGA......

There is a three-year running thread on Long Range bullshit Hunting about the 338 MAX that more or less matches 338 Lapua velocities...and it's based on a SAUM case. Surprised no one has been hurt yet.

 

[abn31c]

There is a three-year running thread on Long Range bullshit Hunting about the 338 MAX that more or less matches 338 Lapua velocities...and it's based on a SAUM case. Surprised no one has been hurt yet.

I've seen it, no signs of pressure......

 

[BCX]

I run 2 Sherman cartridges. One, a 7 MAX I added free bore and only use N570 I have pushed the 190/195 slightly over 3k fps w 0 pressure on a titanium action. The second, a 30 SM pushing Berger 220 w N570, mid level 300 PRC load at 3089 fps w 0 pressure. I can’t comment on his other cartridges.

 

[Tokay444]

[email protected] pressure.

 

[BCX]

Case full of n570 w the 7 Max doesn’t pressure out so yeah 0 pressure

 

[BCX]

And I’m using a mid level 300 PRC charge weight Berger gave me in the 30 SM so yeah 0 pressure signs

 

[4O6shootist]

Case full of n570 w the 7 Max doesn’t pressure out so yeah 0 pressure

Full case of N570 in my old 7max on a Razor-ti going 2995fps from a 26" proof, primer pocket was done in 3x firings. If you're killing adg in 3x firings, you're pushing retarded psi. Oh i built a 6.5max and 25sst. None of them got the speeds Rich advertised. The 65 max is a 26" bartlein 8tw, with N570 i get bolt lift at 3100 using 156s. My 26" 65prc w/ rl26 runs 156s at 3080 no psi.

 

[BCX]

I added some freebore to my chamber. I’m on 6th shot on some brass. Primer pockets doing good. No abnormal case head expansion.

 

[BCX]

Full case of N570 in my old 7max on a Razor-ti going 2995fps from a 26" proof, primer pocket was done in 3x firings. If you're killing adg in 3x firings, you're pushing retarded psi. Oh i built a 6.5max and 25sst. None of them got the speeds Rich advertised. The 65 max is a 26" bartlein 8tw, with N570 i get bolt lift at 3100 using 156s. My 26" 65prc w/ rl26 runs 156s at 3080 no psi.

I burned out a 6.5 PRC barrel in 800 rounds. How are you getting less velocity w a 7 SAUM improved case w n570 vs a 6.5 PRC case a faster powder? Are you jamming?

 

[Tokay444]

And I’m using a mid level 300 PRC charge weight Berger gave me in the 30 SM so yeah 0 pressure signs

At zero pressure, nothing moves.

 

[BCX]

At zero pressure, nothing moves.

I went back to fix it, you beat me to it… sorry, no signs of pressure.

 

[Defender3]

NVM, ranting while drinking never ends well...

 

[BCX]

I think by the time we see ejector marks (depending on case head hardness), sticky bolt lift, case head expansion your up over 70k psi.

 

[4O6shootist]

I burned out a 6.5 PRC barrel in 800 rounds. How are you getting less velocity w a 7 SAUM improved case w n570 vs a 6.5 PRC case a faster powder? Are you jamming?

The mullerworks 26" prc is very fast! Maybe my bartlein 65 max was just dog shit slow.

 

[BCX]

Idk man, I ran, tested, pressured out a Bartlein 6.5 PRC .188 FB, enough to burn out the barrel in 800 rounds….w Hornady brass. Long before good brass was available. And can’t disclose my findings but I may have or may not have broke 3100 fps in a 28 inch PRC w RL-26 and Berger 156

FYI a 6.5 PRC barrel life is equivalent to a 6.5x284.

 

[BCX]

Find my old thread w Warner Flatlines in the 6.5 PRC

 

[4O6shootist]

FYI a 6.5 PRC barrel life is equivalent to a 6.5x284.

Ones 68.4gr capacity, the other 69.5 so they should be on par. Adg brass with a magnum case head helps hide the psi vs the x284. I have a 26" x284 and 26" prc. I can run 3070 from the 26" mullerworks 8tw and it shoots good there, its very close to pressure and is a cooler weather hunting load. The x284 is a 26" Christensen on a long action, ton of freebore. Found psi in lapua brass at 3010fps, but it shoots best at 2960 w/ 156s. This load is all season stable as I've shot it from 25-85° with only mild velocity changes.

 

[Carole Baskins]

I've seen it, no signs of pressure......

Yeah. Most of those folks seem to be conditioned to believe that ejector swipes and heavy bolt lift are acceptable/not of concern.

 

[waveslayer]

Ok guys a little funny update! So on Rockslide I saw a post about 300 PRC +P I asked again about pressures... well wouldn't you know it Broz and Ryan Furman both jump in, the two broke back lovers that banned me on Lomg Range Only, and start to stir the pot. I'm still waiting for an answer to what the pressure is. So they kindly send me this video since I'm "not slaying anything" which made me smile. The smarter guys than me, care to comment ?

 

[Frank Green]

Ok guys a little funny update! So on Rockslide I saw a post about 300 PRC +P I asked again about pressures... well wouldn't you know it Broz and Ryan Furman both jump in, the two broke back lovers that banned me on Lomg Range Only, and start to stir the pot. I'm still waiting for an answer to what the pressure is. So they kindly send me this video since I'm "not slaying anything" which made me smile. The smarter guys than me, care to comment ?

I watched the video twice and even read stuff on his website.

So he is running a double stepped throat from what I gather. He's only cutting a portion of the lands away in the throat before it opens up to the full freebore/throat diameter. He says the o.a.l. of the loaded rounds only gets longer by 002" to .003". OK???? He also says in the video that it's for handloaders only and that if you shoot boxed ammo you will lose a 100fps of velocity. So box ammo guys it's a waste of time.

If it's a truly double stepped throat design (lets call it that) and we all know the first thing that gets fried in a barrel is the throat. With only a small portion of the lands showing to help bullet alignment etc...as soon as this starts to break down and wear...I feel accuracy will be an issue (hard to keep the gun in tune) and you will be also chasing the velocity.

I've used the same 30cal reamer I had custom made for me and I've done a couple of barrels with it. One bore size was a tight .2990" bore and the other barrel was a standard .3000" bore. So a difference of .001" in the land/bore height. My seating depth changed only .010" on the loaded ammo from one gun to the next.

Before I keep typing anymore here I go back to what I've said...

Show a print of what the actual chamber spec's are and get proper ammunition testing done in a pressure and velocity test barrel with that given spec. reamer and get hard numbers. Until that's done.... we're all guessing.

He also says he he a patent on the design as well.

 

[Frank Green]

There is no free lunch guys as the saying goes.

I was talking to one of the ammo companies about this. We agreed the throat if they are reworking it to give less friction on the bullet etc...yes we agree it can help with pressures.

but as he reminded me for every 100-200fps increase in velocity....pressure does go up 5k to 10k psi on standard rifle cartridges.

You might be able to help it/offset it somewhere but it will go up.

Go back to my earlier post and the sample I used with 300WM and seating the bullet out further and dumping more powder in the case.

 

[BCX]

Frank, now wouldn’t barrel and bore diameter effect pressure or even bullet diameter? Let’s say Hornady vs Berger Vs Sierra vs solids? A load that is safe in one barrel may show signs of early pressure in the next. I have seen factory ammo pressure out early and show signs of pressure.

 

[Frank Green]

Frank, now wouldn’t barrel diameter effect pressure or even bullet diameter? Let’s say Hornady vs Berger Vs Sierra vs solids? A load that is safe in one barrel may show signs of early pressure in the next. I have seen factory ammo pressure out early and show signs of pressure.

I think your talking about the actual bore diameter (bore and groove size) correct? Not the outside diameter of the barrel?

If your talking about actual bore and groove size…then the answer is yes! Actual bore and groove size can and will have a big impact on pressures and velocity.

As will the actual chamber reamer spec used as well.

As well as actual type of bullet and bullet diameter will effect it as well. Not just conventional lead core bullets but solid bullets as well. Solid bullets can be a whole other topic and I feel they can be even more temperamental when it comes to the actual bore and groove spec as well as the type of solid bullet it is. I feel the solids are not as forgiving as a conventional lead core bullet.

Lets stick with actual bore and groove size of the barrel. Anytime you tighten up the overall bore size of the barrel lets say both the bore and groove…you will drive up pressures!!!! I’ve seen actual pressure test barrels where the bore was .0005” undersize and the groove was also a .0005” undersize. So let’s say a min spec. 30 cal is .3000” x .3080”. By making the bore and groove size both a .0005” undersize that will drive up pressures right around 10k psi! Making the bore and groove size bigger…..it will drop pressures faster as well as you can lose velocity.

That’s why I tell guys…unless you know actual bore and groove size of a barrel to the 4th decimal place and you know what actual chamber reamer spec was used to cut the chamber….you cannot compare or say a barrel from maker X‘s barrels produce faster velocities than barrels from barrel maker Z! Unless you know the actual sizes…you are not comparing apples and apples!

I’ve also seen where the throat freebore was .0001” to .0002” under min spec. Then you stuff a bullet in it that ends up being right at the min spec…that .0001” or .0002” tighter freebore can drive up pressures around 6k psi and drive up velocities as well.

Tighter isn’t necessarily better! Too loose isn’t good either!

 

[waveslayer]

So Rockslide banned me after calling out Broz and Ryan Furman regarding pressure testing. Ha ha ha I'm making friends everywhere

 

[BCX]

Well explained Frank! I believe all these variables play a part in a cartridge getting a bad reputation with early signs of pressure along w shady loading techniques.

 

[supercorndogs]

So Rockslide banned me after calling out Broz and Ryan Furman regarding pressure testing. Ha ha ha I'm making friends everywhere

I can't help but wonder if silencing this information, which is backed by experts in the industry, would make them legally liable in the event someone is injured, and a legal team does some testing and finds the ammo is over what is considered safe pressure levels.

 

[Carole Baskins]

So Rockslide banned me after calling out Broz and Ryan Furman regarding pressure testing. Ha ha ha I'm making friends everywhere

You're better off not being a part of that circle jerk. Those guys live in their own reality.

 

[DocRDS]

There is no free lunch guys as the saying goes.

I was talking to one of the ammo companies about this. We agreed the throat if they are reworking it to give less friction on the bullet etc...yes we agree it can help with pressures.

but as he reminded me for every 100-200fps increase in velocity....pressure does go up 5k to 10k psi on standard rifle cartridges.

You might be able to help it/offset it somewhere but it will go up.

Go back to my earlier post and the sample I used with 300WM and seating the bullet out further and dumping more powder in the case.

Came in here to see the fuss--got free advice from Mr. Green....

Another reminder to all of us giddy newbies STOP CHASING VELOCITY (yer gonna get yourself kilt!)

Dear Doc,
Stop chasing high velocity in 300 WM with 220 gr bullets.

Best Regards

Your Rifle, Fingers, and Face.

 

[4O6shootist]

So Rockslide banned me after calling out Broz and Ryan Furman regarding pressure testing. Ha ha ha I'm making friends everywhere

Ryan Furman was at one time, a really cool guy. He had posted a video about bedding a particular carbon tikka stock years back. I asked him a question about it, he took the time to call me ans discuss everything at legnth, prob a half hour of his busy day. Jeff Broz has always been a jerk. He makes all the Christensen reviews where CA sends him hand picked rifles that shoot small groups in testing. I personally know a few of the other editors for LRO and Rokslide and they're genuinely good dudes. Shame to see Ryan falling into Jeff's path of attitude.

 

[waveslayer]

Ryan Furman was at one time, a really cool guy. He had posted a video about bedding a particular carbon tikka stock years back. I asked him a question about it, he took the time to call me ans discuss everything at legnth, prob a half hour of his busy day. Jeff Broz has always been a jerk. He makes all the Christensen reviews where CA sends him hand picked rifles that shoot small groups in testing. I personally know a few of the other editors for LRO and Rokslide and they're genuinely good dudes. Shame to see Ryan falling into Jeff's path of attitude.

Broz was cool until I posted that I had an Extra Sig Kilo 8k for a smoking price, he sent me a message asked who I worked for... I never replied. I work for Aspen Healthcare by the way. Ryan turned to douche when I challenged him on his reloading advise about all sorts of BS. Funniest part the try to gang bang me after I simply asked to see pressure testing. Pretty classic. Anyways I honestly don't care, just find it hilarious . And your comments are dead on! Broz arrogant piece, I did say they push CA because they are one of their sponsors, also Dedensive Edge is as well, hence why you can't ask for data! Can't hurt their little sponsors

 

[Carole Baskins]

Broz was cool

Many locals near "his" ranch would not agree.

 

[abn31c]

I can't help but wonder if silencing this information, which is backed by experts in the industry, would make them legally liable in the event someone is injured, and a legal team does some testing and finds the ammo is over what is considered safe pressure levels.

I've been wondering this about Sherman for a long time. The dude was a forestry agent or something and all of a sudden he's an engineer. SMDH, Someone is gonna get hurt.

 

[4O6shootist]

I've been wondering this about Sherman for a long time. The dude was a forestry agent or something and all of a sudden he's an engineer. SMDH, Someone is gonna get hurt.

Far from his self learned engineer, just a hotrod wildcatter pushing crack powder snake oil to the masses. Kinda like the miracle healers traveling the Frontier at turn the 20th century selling a miracle elixir, that was just opium lol

 

[waveslayer]

Far from his self learned engineer, just a hotrod wildcatter pushing crack powder snake oil to the masses. Kinda like the miracle healers traveling the Frontier at turn the 20th century selling a miracle elixir, that was just opium lol

Like John Lazzeroni. I do use his Warbird, it's awesome hunting round with the right bullet.

I've always wanted to start a thread asking about the old Lazzeroni rounds

 

[BCX]

Far from his self learned engineer, just a hotrod wildcatter pushing crack powder snake oil to the masses. Kinda like the miracle healers traveling the Frontier at turn the 20th century selling a miracle elixir, that was just opium lol

I guess we can say P.O. Ackley did the same? What’s your hate w improved cartridges? Any time we hand load any cartridge we take the responsibility in are own hands!

 

[4O6shootist]

I guess we can say P.O. Ackley did the same? What’s your hate w improved cartridges? Any time we hand load any cartridge we take the responsibility in are own hands!

Po ackley took anemic old school cases that had a lot of room for improvement, that was a good idea, as the parent had a lot of taper and shallow shoulder angle. Taking a saum, prc, and changing the case geometry to an improvement of 1-2 grains, sometimes his cases don't increase capacity at all, comparing a ss/sst to the saum parent. He uses 0.068/" body taper with a 40° shoulder, and same taper with 30° shoulder on the sst. When you increase a cases capacity by 3% you generally gain 1-1.5% velocity, that's it. All he's doing is creating a case that has geometry for more chamber grip and less shoulder expansion, this allows a lot more pressure to be run before traditional bolt lift/swipe hard extraction. His chambers are oversized at web and tight body apex dies to keep getting clickers. What he's done does give more performance, but it isn't coming from more capacity improvements, it's coming from higher pressures being tolerated by the cartridge/chanber.

 

[jpndave]

Po ackley took anemic old school cases that had a lot of room for improvement, that was a good idea, as the parent had a lot of taper and shallow shoulder angle. Taking a saum, prc, and changing the case geometry to an improvement of 1-2 grains, sometimes his cases don't increase capacity at all, comparing a ss/sst to the saum parent. He uses 0.068/" body taper with a 40° shoulder, and same taper with 30° shoulder on the sst. When you increase a cases capacity by 3% you generally gain 1-1.5% velocity, that's it. All he's doing is creating a case that has geometry for more chamber grip and less shoulder expansion, this allows a lot more pressure to be run before traditional bolt lift/swipe hard extraction. His chambers are oversized at web and tight body apex dies to keep getting clickers. What he's done does give more performance, but it isn't coming from more capacity improvements, it's coming from higher pressures being tolerated by the cartridge/chanber.

Well said!. I think this is where at least partly the problem is stemming from. The "new" straightened case wall/steep shoulder angle has more chamber grip which means less force on the bolt lugs. Couple that with tough brass, seems like most often it's ADG, (but Alpha, Lapua, etc. can do the same) and the normal pressure signs don't show as early. That does mean the action isn't seeing as much pressure force back. But it is still there in the barrel. The tough brass shifts pressure signs up in pressure where they show. Do all these improvements mean it's ok in that action, with that brass and that load? Maybe, but the barrel and chamber are still seeing the higher pressures. Is the Sig Fury 277 ok at the 80k pressures they run at? Kind of the same idea and probably near where a lot of these "no pressure sign" loads are running. A lot of these guys double down on the stupid rather than own what's actually going on. Some of the pissing matches are just comical. Without a proper pressure test as @Frank Green has mentioned multiple times it's all an educated guess at best.

 

[Eostech]

The "new" straightened case wall/steep shoulder angle has more chamber grip which means less force on the bolt lugs. Couple that with tough brass, seems like most often it's ADG, (but Alpha, Lapua, etc. can do the same) and the normal pressure signs don't show as early.

This is the conclusion I came to after looking into things like the 7SS. There is no magical efficiency to be had by changing body tamper slightly and shoulder angle, especially newer case design, (ie 260 will see more benefit than a 6.5 creedmoor from these changes.)

The higher velocity is because of higher pressure which is fine (I don't really care.) if people want to load and shoot in their own guns knowing this however passing it off as normal or safe is disingenuous at a minimum.

Another factor to consider is the amount of energy in the bigger cases, I'm sure most people here have had cheap .22LR blow out a rim at least once before and it wasn't a big deal, not near the ballpark of igniting 80-100 odd grains of powder next to your face.

 

[Emerson0311]

There is a lot of snake oil in any “performance” industry. The laws of physics don’t change though.

 

[Eostech]

Can't wait for the 7SS+P, 3300FPS with a 190grn Berger in a short action, es of 2 and more barrel life.
NoO PreSsuRE, iT JuST WOrkS.

 

[4O6shootist]

Can't wait for the 7SS+P, 3300FPS with a 190grn Berger in a short action, es of 2 and more barrel life.
NoO PreSsuRE, iT JuST WOrkS.

You should probably "*******" Rich Sherman, because that's the exact diarrhea that runs from his mouth.

 

[bax]

... The "new" straightened case wall/steep shoulder angle has more chamber grip which means less force on the bolt lugs ...

I do not understand how this will work. I looked up the yield strength of 316 stainless steel - about 30,000 PSI and cartridge brass - about 75 MPa (megapascals) or about 10,000 PSI. Given the yield strength numbers, the steel will accept about 3 times more pressure before yielding than brass. If I am doing this right, the brass will flow like butter compared to the steel - explaining head separations. The idea that the brass will grip the chamber walls and because of this grip (friction?) and tougher brass, bolt face pressure will be reduced. That does not make sense to me. I confess to nearly complete ignorance here, can someone who actually knows something about this explain how this will work?

 

[Eostech]

The idea that the brass will grip the chamber walls and because of this grip (friction?) and tougher brass, bolt face pressure will be reduced. That does not make sense to me.

Ever shoot a rifle with oil in the chamber?

 

[Frank Green]

Ever shoot a rifle with oil in the chamber?

I had to chuckle reading your post!

That's why I tell guys to make sure the chamber is dry. Liquids don't compress...all the pressure will get directed to the bolt which in turn puts it to the receiver.

Had a local P.D. guy a couple of years ago bring the rifle in for rebarrel job. Showed me a case (box Federal match ammo) and the primer was flat as a pancake. He said the bolt was really hard to open etc....I asked was it the first round fired for the day? He said yep...after cleaning. When I took the barrel off the receiver solvent/oil literally dripped off the barrel threads. Inside the action by the bolt lugs the oil/solvent was all caked/built up. Not good!

Later, Frank
Bartlein Barrels

 

[jpndave]

I do not understand how this will work. I looked up the yield strength of 316 stainless steel - about 30,000 PSI and cartridge brass - about 75 MPa (megapascals) or about 10,000 PSI. Given the yield strength numbers, the steel will accept about 3 times more pressure before yielding than brass. If I am doing this right, the brass will flow like butter compared to the steel - explaining head separations. The idea that the brass will grip the chamber walls and because of this grip (friction?) and tougher brass, bolt face pressure will be reduced. That does not make sense to me. I confess to nearly complete ignorance here, can someone who actually knows something about this explain how this will work?

I'm going to take a stab at this...

First, tougher brass does NOT change what the bolt face/lugs sees pressure wise. It masks the signs of pressure we are using to know when we are approaching (or more likely past) limits. Tougher brass doesn't flow as easily as softer so the signs occur at a higher pressure. That is why I used the Sig Fury example with the hybrid case design intended for higher design pressure. "I think" that we can get close to that performance using ADG/Alpha brass, etc. The rifle still sees the pressure though so just like an action for that Fury round needs to be able to handle 80K, so would this example.

As to the straighter walls having less bolt thrust force. Have you ever wedged between two walls, say a stairwell or hallway, back to one wall and feet to the other? You can support your weight with the force applied to the two vertical surfaces. Same idea with the straighter cartridge walls but the forces are much higher than a 200lb man/woman/? Another example is the cams rock climbers use to ascend. Maybe others have some examples.

 

[bax]

I'm going to take a stab at this...

First, tougher brass does NOT change what the bolt face/lugs sees pressure wise. It masks the signs of pressure we are using to know when we are approaching (or more likely past) limits. Tougher brass doesn't flow as easily as softer so the signs occur at a higher pressure. That is why I used the Sig Fury example with the hybrid case design intended for higher design pressure. "I think" that we can get close to that performance using ADG/Alpha brass, etc. The rifle still sees the pressure though so just like an action for that Fury round needs to be able to handle 80K, so would this example.

As to the straighter walls having less bolt thrust force. Have you ever wedged between two walls, say a stairwell or hallway, back to one wall and feet to the other? You can support your weight with the force applied to the two vertical surfaces. Same idea with the straighter cartridge walls but the forces are much higher than a 200lb man/woman/? Another example is the cams rock climbers use to ascend. Maybe others have some examples.
View attachment 7832343View attachment 7832369
View attachment 7832375

I stopped rock climbing a few years before cams hit the market. I was using Chouinard nuts - I still have some and some blades and bongs, some of those postage stamp pitons, and similar stuff. I was more into ice, rigid boots and crampons, and Salawa ice screws. I am very familiar with wedging up a chimney but I don't think it applies.

I think your concept is that pushing your feet against the side walls is pressure that locks the brass into position and the brass tensile strength restrains the brass at the base and this reduces the pressure on the cartridge base. But does it? Compared to barrel and receiver steel, cartridge brass is soft and has a yield strength one third of steel.

If you shoot a piece of brass many times you will see the shiny ring of incipient case head separation or, perhaps, a partial or full head separation. What is happening? The barrel is screwed into the receiver. The pressure inside the chamber holds the the upper part of the case against the case walls - unless, as Mr. Green says, there is some incompressible liquid in there. The same pressure pushes the case base against the bolt face and the bolt head lugs push against the receiver lugs. And the pressure against the shoulder pushes the other way. When the round fires those expanding forces elastically stretch the receiver a little, the chamber gets bigger, and the brass stretches just a little just above the case base. When the pressure drops the receiver elastically pops back into its original dimension forcing the brass back together. The receive and barrel steel return to their original dimension because their deformation was elastic. But the brass does not return to its original shape because the brass is less elastic than steel - I think because it has lower yield strength.

I don't have a mental picture of what happens to make that shiny ring. I keep thinking about a 16 ounce water bottle when you squeeze it along its long axis - one hand on the base and one on the cap, now push your hands together. It sort of collapses like a accordion. I'm thinking that something like happens to the brass after the stretch induced by firing.

 

[BCX]

Perfect explanation, I run 90% improved cartridges and hardly ever have to trim brass. You will get min brass growth running improved! As with any cartridge I load for, I do keep a close eye on the case head expansion at the .200 line and primer pocket depth. Pressure tested NO but a caliper and constant measuring will tell you when your into some excessive pressure.

300 NM has some of the worst brass flow or case growth I have ever seen. More case taper = less grip on the chamber walls. More bolt thrust perhaps.

I believe Jim Boatright has written a article on this matter.