Range Report Jackets and RPM's

[MarkLeupold]

Some background: I'm planning on advancing my rifle this summer and I'm planning on going with a 6.5mm, 1-7 twist, 22 inch finish length. It'll be a 6.5x47mm Lapua with a bullet in the 123, 130 (most likely), or 140 class. The bullet selection is going to be 95% match-type with a probability of the occasional expanding hunting bullet and a possibility of some lightweight varmint bullets depending on how they perform and what comes out of this discussion.

Where do bullets and jackets start to fail? The physics student in me says that RPM isn't quite enough to give a hard limit, that bullet diameter should play a relatively large part in the force observed at the jacket. Is it typically the jacket that fails, or is it the core being separated from the jacket when starting into the lands that happens sooner? I'm almost sure that varmint bullets will fail at their velocities through the 1-7 twist, but it's not a huge concern.

 

[lash]

I'm curious as to why you chose 7 twist over the more standard 8 to 8.5 twist for 6.5mm. Especially since you don't seem to be planning to use the heavies or solids in it. If your goal was to use 147, 150, 155g or monolithic solids in it, then I could see the faster twist. I don't claim to be any sort of expert, so am interested in that I might learn something.

 

[MarkLeupold]

I've seen a lot of people use the 1-7 on shorter barrels and gain a lot in the BC of lighter bullets. Frank has one on his 20" .260 Tikka and sees the same thing. It's along the same lines as over/hyper-stabilization except that I can't spin them quite as fast as the monolithic solids. With shorter barrel, I don't think I can get to high enough velocities to destroy jacketed bullets, but that's where this thread is coming from: I want to know where the edge is. I'd actually prefer a 1-7.5, just to back off a little bit, but the barrel manufacturer I'm going with doesn't have that option in 6.5mm.

 

[flyer]

I understand the concept of using a fast twist on a short barrel to get a higher rpm with low muzzle velocity.

You are not going to have a short barrel or low muzzle velocity so I don't think it's going to do anything positive for you.

If you had a 12" barrel it might make sense.

 

[THEIS]

Hi,

@MarkLeupold .... the short answer is that there is no definitive "point" as to where jacketed bullets "fail" due to spinning too fast. Not only will it differ from each manufacturer; it can differ in different projectile weight of same caliber by same manufacturer.

Shit.... the "edge" your looking for can be so fine that your combination works great at your local shooting area but you get ghost bullets when you travel to another shooting area....I am pretty sure you do not want to deal with that crap

If you are wanting to "make up" BC due to short barrel then why not just switch over to solids? You gain BC immediately (typically) due to their design AND you can spin them as fast as you want to " make up" for your shorter barrel?

Sincerely,
Theis

 

[MarkLeupold]

@THEIS Thanks for the feedback. I guess "making up BC" shouldn't really be the term I use to describe what I want to do. I just want to stabilize my projectiles as well as I can, and I would spin them even faster if I could. I shouldn't really worry about it because I'm switching from a .308 and anything 6.5mm is going to run circles around it. I would shoot solids if they were more economically viable for me. Broke college student can't afford to put a $1.50 or $2.00 projectile downrange every trigger pull...lol

 

[lash]

Mark, the 123s and 130s do very well out of a shorter 6.5 barrel, even without an "over spin" twist rate. Plus 22" isn't really that short. My Tikka TSR1 is a 20" stock barrel and it shoots everything just fine, including the old standby 142 SMKs. Sure, they are going slower, but 1000 yard shots still hit like they should.

I'm not saying that you will be wrong to go with the 7 twist, but you may be over thinking this.

 

[MarkLeupold]

Mark, the 123s and 130s do very well out of a shorter 6.5 barrel, even without an "over spin" twist rate. Plus 22" isn't really that short. My Tikka TSR1 is a 20" stock barrel and it shoots everything just fine, including the old standby 142 SMKs. Sure, they are going slower, but 1000 yard shots still hit like they should.

I'm not saying that you will be wrong to go with the 7 twist, but you may be over thinking this.

Probably, as I tend to over think everything! Sometimes you just need someone to tell you to "keep it simple, stupid."

 

[winniedonkey]

Gain twist!!!!

 

[MarkLeupold]

Gain twist!!!!

You did a little digging to get here, but since you are and have mentioned it, I have a question about gain twist.

If you have a standard twist barrel say at 1-8 and you have a gain twist that ends in a 1-7, the bullet fired from the gain twist has a higher RPM in flight, yes? So, I'm assuming that the problem with blowing up bullets happens inside the barrel, with the rapid rotational acceleration of engraving into the rifling, instead of it being caused by pure RPM's. How much can I use that to my advantage? My main concern is match bullets, but if I wanted to run some speedy, light for caliber varmint bullets, would gain twist allow me to do that without damaging bullets, or is that still limited by physics and RPM's?

 

[winniedonkey]

You did a little digging to get here, but since you are and have mentioned it, I have a question about gain twist.

If you have a standard twist barrel say at 1-8 and you have a gain twist that ends in a 1-7, the bullet fired from the gain twist has a higher RPM in flight, yes? So, I'm assuming that the problem with blowing up bullets happens inside the barrel, with the rapid rotational acceleration of engraving into the rifling, instead of it being caused by pure RPM's. How much can I use that to my advantage? My main concern is match bullets, but if I wanted to run some speedy, light for caliber varmint bullets, would gain twist allow me to do that without damaging bullets, or is that still limited by physics and RPM's?

Aren't varmint bullets thin jackets? I'm not much of a "lota data logged hunter" so I will hope others can chime in. I just mentioned the fact of a gain twist since the 1:7 is a little extreme for 123-130. Personally in a similar boat and playing numbers it's hard to define the rpms on calculations without the input for it. I have a general idea what to shoot for but I'm keeping it a secret if it doesn't work. Same 123-130 is my only zone.

 

[MarkLeupold]

Aren't varmint bullets thin jackets? I'm not much of a "lota data logged hunter" so I will hope others can chime in. I just mentioned the fact of a gain twist since the 1:7 is a little extreme for 123-130. Personally in a similar boat and playing numbers it's hard to define the rpms on calculations without the input for it. I have a general idea what to shoot for but I'm keeping it a secret if it doesn't work. Same 123-130 is my only zone.

Yeah, typically they're thin jackets.

I'd get a gain twist and just experiment with stuff if I was in the financial spot to do it. Hell, if I was in a good financial spot, I'd probably have three or four gain twists laying around in various combinations to test! There's not a whole lot of info about it, but it probably is the direction we're going to see barrel technology go.

 

[XLR308]

As has allready been stated a 22" Barrel on the 65 Creedmoor is far from short ( I believe that is actually the optimum leangth originally speced ) and the bullets you listed are proven performers in all conditions from the 8 twist.
I personally went with a 1-7.5 on mine but my main focus from the start has been based around the 147eldm and 150smk and had it finished at 23" as it seamed the optimum length for balance and little sacrifice in speed.

 

[McCrazy]

Gain-twist barrels are usually used to spin heavy bullets faster in order to stabilize them before they leave the barrel, but start at a slower twist rate in order to try to keep pressure spikes down which allows velocity to be increased.

You are correct that the projectile would have the faster rifling spin rate as it leaves the barrel as that is the last length of rifling it touches.

The issue I see with using a gain-twist on a thin jacketed varmint projectile is that the rifling would inscribe and deform even more of the jacket than a constant-twist barrel and possibly further increase the chances of the projectile disintegrating beyond what the higher RPMs would.

 

[308pirate]

@THEIS I just want to stabilize my projectiles as well as I can, and I would spin them even faster if I could. I shouldn't really worry about it because I'm switching from a .308 and anything 6.5mm is going to run circles around it. I would shoot solids if they were more economically viable for me. Broke college student can't afford to put a $1.50 or $2.00 projectile downrange every trigger pull...lol

I would spend more time dry firing and shooting and less time worrying about esoteric minutiae. Or you might find that someone else with an "inferior" caliber will run circles around you and your 6.5

 

[MarkLeupold]

I would spend more time dry firing and shooting and less time worrying about esoteric minutiae. Or you might find that someone else with an "inferior" caliber will run circles around you and your 6.5

Believe me, I shoot as much as I can and I dry fire every day I have access to my rifle, but sometimes looking at the minutiae on the side is fun. I'm just getting started in precision rifle, and there's no substitute to putting in the work. That I know.

 

[Ledzep]

Bullet jacket failures have two main causes.

1) Rotational velocity- Get the bullets above 350,000rpm and you're going to start seeing issues. The lead isn't strong enough to hold itself together, and the core isn't strong enough to hold the lead in. These types of failures almost always happen inside of 50 yards. This is usually what kills lightweight varmint bullets.

2) Core failure/melting- This can happen with even a "normal" rate of twist rifling, and modest velocities. The cause is comprised of several factors, but rotational speed doesn't appear to be the primary one. Tight bores with deep grooves and square-cornered rifling seems to be the trend. Additionally, dirty, rough, erroded bores can develop this issue sometime in the life of the barrel, where the particular bullet might have been A Okay to start out. These can happen anywhere from the muzzle out past 100yd. What happens is the core gets hot, melts, destabilizes the bullet, then rips apart when it starts flying sideways. 10 will fly true and accurate, one will blow up or fly sideways through the paper. This usually happens to heavy-for-diameter match bullets (147 eldm, 175 7mm smk, some older Bergers suffered, 180 7mm eldm, etc...). Very hit or miss, hard to replicate.

Across the board, thinner jackets, faster speeds, faster pressure curves, rougher bores, sharper, deeper rifling, etc.. all play a part. If you're blowing up thin-jacketed match or varmint bullets, try cleaning the snot out of your barrel, or switching to bullets with known thicker jackets (go from an ELD-M to an ELD-X for example).

As far as gain twists, the final rate of rotation at the muzzle X the muzzle velocity is the rotational speed (with unit conversions to get it to rpm). The bullet will not continue to rotationally accelerate after leaving the muzzle because nothing is forcing it to do so (f=ma). Because of air resistance it will immediately start losing rotational velocity/acceleration once it's out of the bore.

IMO, gain-twist is nice only for one thing-- ensuring the rate of twist never decreases down the length of the bore. In that regard I would say very mild gain twists are okay, like 1:8.5 or 1:8.25 ending up at a 1:8 at the muzzle. The more aggressive you change the pitch, the more jacket distortion you induce (think threading a 1/4-20 bolt into a 1/4-24 hole-- though obviously not that extreme the same thing is happening to your jacket).

More aggressive twists could be done with driving band bullets and/or solids.

 

[moaflat]

Bullet jacket failures have two main causes.

1) Rotational velocity- Get the bullets above 350,000rpm and you're going to start seeing issues. The lead isn't strong enough to hold itself together, and the core isn't strong enough to hold the lead in. These types of failures almost always happen inside of 50 yards. This is usually what kills lightweight varmint bullets.

2) Core failure/melting- This can happen with even a "normal" rate of twist rifling, and modest velocities. The cause is comprised of several factors, but rotational speed doesn't appear to be the primary one. Tight bores with deep grooves and square-cornered rifling seems to be the trend. Additionally, dirty, rough, erroded bores can develop this issue sometime in the life of the barrel, where the particular bullet might have been A Okay to start out. These can happen anywhere from the muzzle out past 100yd. What happens is the core gets hot, melts, destabilizes the bullet, then rips apart when it starts flying sideways. 10 will fly true and accurate, one will blow up or fly sideways through the paper. This usually happens to heavy-for-diameter match bullets (147 eldm, 175 7mm smk, some older Bergers suffered, 180 7mm eldm, etc...). Very hit or miss, hard to replicate.

Across the board, thinner jackets, faster speeds, faster pressure curves, rougher bores, sharper, deeper rifling, etc.. all play a part. If you're blowing up thin-jacketed match or varmint bullets, try cleaning the snot out of your barrel, or switching to bullets with known thicker jackets (go from an ELD-M to an ELD-X for example).

As far as gain twists, the final rate of rotation at the muzzle X the muzzle velocity is the rotational speed (with unit conversions to get it to rpm). The bullet will not continue to rotationally accelerate after leaving the muzzle because nothing is forcing it to do so (f=ma). Because of air resistance it will immediately start losing rotational velocity/acceleration once it's out of the bore.

IMO, gain-twist is nice only for one thing-- ensuring the rate of twist never decreases down the length of the bore. In that regard I would say very mild gain twists are okay, like 1:8.5 or 1:8.25 ending up at a 1:8 at the muzzle. The more aggressive you change the pitch, the more jacket distortion you induce (think threading a 1/4-20 bolt into a 1/4-24 hole-- though obviously not that extreme the same thing is happening to your jacket).

More aggressive twists could be done with driving band bullets and/or solids.

Hi Ledzep, the 350,000 RPM top end - as a figure of merit not to exceed, is pretty terrific for me with a few rifles shooting a 7 Twist- 22-250, 6CM, 6.5CM, 6.5 PRC and a 31” 7.5T 264WM. Was that number published anywhere? Is it for most manufacturers like Hornady, Sierra, Nosler, Berger etc ?
My GOAL is to stabilize high BC heavies for sub.5 MOA .
Cheers Joe

 

[Ledzep]

I don't know if it's formally published anywhere; it just seems to be the region where things start happening pretty repeatably. North of 300,000rpm and you start getting fairly reliable blowups with certain guns with certain bullets.

Even at super-high RPM it's barrel-to-barrel case by case. Some rifles can sling them at 375,000 rpm. Others will blow them at 275,000. It's a hard thing to test because each barrel damages the jacket differently. Multiple factors involved and I don't know if anyone necessarily knows how much of each factor (combined with the others) will make a bullet let go.

It's amplified with the small caliber stuff because they both go faster, and require faster twist rates to stabilize. A 250gr 30 cal A-tip, for example will never get pushed fast enough to worry about an 8.5-9 twist getting it over 300,000rpm. A 90gr Berger 224, you bet your ass you can spin them to death in a plethora of calibers.

Wish I knew more but I don't. Just reporting trends I've seen in person and reported on the net.

 

[308Gunner]

I have a 22” 1-7 twist on my 6.5 slr and I have shot 130 Norma and the 130 RDFs as well as 140 Hornady and it shoots all very well. The 130 RDF is running 2920 and hammers. You will be fine as far as gain twist I just got my new Bartlein that a left twist starting at 1-9 and will finish at 1-7 as soon as my current tube is gone it is off to WTO so Clayton can work his magic on it