.308 high pressure?

[markspring1978]

I started to play with some .308 loads this weekend. I'm using 43.5 grains Ramshot Tac and pushing a 168 Sierra Matchking. I chrono'd the loads and came up with about 2800 FPS. This seems a little hot to me and is significantly faster than the speeds published on Western Powders website with 44.5 grains of the same powder. Anyone have any thoughts on this? No signs of high pressure on the cases.

Thanks in advance for your thoughts.

 

[One_Man]

Re: .308 high pressure?

Barrel length? Brass? COAL?

 

[Clark]

Re: .308 high pressure?

Quickload think that 168 gr SMK 2.8" OAL, Ramshot TAC, 24" barrel is:
a) 43.5 gr 58 kpsi 2681 fps
b) 44.5 gr 58 kpsi 2736 fps
c) 48.4 gr 76 kpsi 2942 fps
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Scott Sweet has calculated and divulged to me the failure mode and pressure for the following case heads; Mauser, H&H magnum, Rem Ultra Mag, 460 Weatherby, Rigby, 7.62x39 small Boxer, 7.62x39 large Boxer, and .222 Rem.

I have only verified consistency with Quickload in 223 and Mauser case heads.

Scott calculated von Misses stress in the 1889 7.65x53mm Mauser case head when built with a large Boxer primer [.308Win, 30-06, 243, etc] fails with a loose primer pocket at 77 kpsi +/- 7%.
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Per my experimental overloads I find that the 1889 7.65x53mm Mauser case head when built with a large Boxer primer [.308Win] fails with a loose primer pocket at 75 kpsi +/- 3.5% QL with with IMR4895, H335, and Varget. I have not validated the Quickload library constants for TAC.

I find that measuring any changes around the extractor groove is a better pressure indicator and precursor for loose primer pocket than either waiting for the primer to fall out or measuring the primer pocket with pin gauges.
That is because the spent primers can stay in there like hanging chads, and the expansion is often not symmetrical. The non symmetrical expansion can be found by turning the calipers relative to the outside diameter of the extractor groove, but round pin gauges are blocked if any part of the primer pocket maintains the original diameter. When I developed this system, ~ 5 years ago, it was suggested that I find "D" shaped pin gauges, but I have not done that.
--------------------------
Here is an example of my tests:

Hodgdon max book load:
308 WINCHESTER, CASE: WINCHESTER, BBL: 24", PR: FEDERAL 210M, 168 GR. SIE HPBT COL: 2.800" H335, 42.0 gr., 2631 fps, 49,300 CUP

My test:
Pacific .308 Win reamer, VZ24 trued action, A&B fluted stainless 24" F54 barrel, H335, CCI200 primer, 2.9" OAL, Speer 168 gr. HPBT Gold Match, brass: Win308Win:

1- 10 order left through right

0) 42 gr. QL= 2565 fps & 46 kpsi, 0% overload, did not load 42 gr.
1) 43 gr. QL= 2618 fps & 49 kpsi, 2% overload, ok
2) 44 gr. QL= 2670 fps & 52 kpsi, 5% overload, cratered primer this and higher
3) 45 gr. QL= 2722 fps & 56 kpsi, 7% overload
4) 46 gr. QL= 2774 fps & 60 kpsi, 10% overload
5) 47 gr. QL= 2825 fps & 64 kpsi, 12% overload
6) 48 gr. QL= 2875 fps & 68 kpsi, 14% overload, mark on brass from bolt face extractor this and higher,
7) 49 gr. QL= 2925 fps & 73 kpsi, 17% overload
8) 50 gr. QL= 2974 fps & 78 kpsi, 19% overload, extractor cut on brass expands .0020"
9) 51 gr. QL= 3024 fps & 84 kpsi, 21% overload, extractor cut on brass expands .0020"
10) 52 gr. QL= 3073 fps & 90 kpsi, 24% overload, extractor cut on brass expands .0110", primer fell out,
------------------------------

What does it all mean?
a) Expect short brass life at 75 kpsi ~ 77 kpsi with that case head.
b) Yes, you have high velocity.

 

[jet_lagged]

Re: .308 high pressure?

Clark,

Thank you, that is great info.

Since barrel strength is really seldom an issue theses days, I had been looking a for a rough way to gauge what the brass would actually take.

 

[vman]

Re: .308 high pressure?

Clark thanks for the write up very informative

I have a question about winchester .308 brass... As i have only ever used lapua .308 brass, im not too sure if the win brass has the little recessed area around the primer pocket, such as that on the lapua brass.

Is this a feature that the winchester brass doesnt have, or have your primers just flattened so much they have filled the recess?

Im trying to determine the level of primer flattening I currently get.

Im using Ar2208 (supposedly Varget). Book loads says 44.0gr starting to 47.0C Max when pushing a 155. To be honest I have taken the loads to 47 and have seen very little difference in primer flattening... if any, at 47.0gr.

Could you please further explain what you mean by "mark on brass" and "extractor cut". I think i have an idea but would just like some clarification.

As i approach higher loads I can see some very slight polishing on the case head caused by the extractor pin rubbing against the case and the bolt is lifted and the case extracted... No difficult bolt lift during any of this

Do these look normal? 44.0gr Left, 46.0gr Mid, 47.0gr Right... Bolt lift felt normal. Crattering on all of them but i think thats remingtons bolt design causing that.

 

[Clark]

Re: .308 high pressure?

Here is a pic of some 223 brass that went through one of my AR15s.
On the case head there are two marks, a semi circular one and a chord [line across a circle].
These two marks were caused by the brass pushing against the bolt face, but the bolt face was not flat. There were two cuts in the bolt face, one for an ejector plunger, and one for the extractor claw. The brass flowed into these two features of the bolt face, leaving a mark on the brass.

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I am sorry for using inconsistent terms.
I used to write "extractor cut" [in old data from my old range reports], but now I write "extractor groove" or "extractor groove expansion" or "egg".

The Mauser case heads in cross section with large Boxer primer pocket on top and small primer pocket on the bottom.

The max pressure for the top case will be ~ 76kpsi when the primer pocket grows and new primers fit too loosely.

The max pressure for the bottom Mauser case head will be ~ 90kpsi when the primer pierces, depending on firing pin to firing pin hole fit.

The failure mode for the top case is the inside diameter of the primer pocket increasing. That failure is difficult to measure directly. By measuring the outside diameter of the case directly over the primer pocket, one can get an early indicator on changes in the primer pocket. Directly over the primer pocket is the extractor groove. Measuring the extractor groove with dial calipers is best done by aligning the direction of the caliper jaws bevel with the extractor groove bevel. This means dial face toward the case mouth. The case is then rotated in the jaws while jaw pressure is kept consistent, and the operator looks for a minimum and a maximum on the dial face. That is repeated after the shot and the data is compared.
Large rifle Boxer primer pockets are between .2085" and .2100".
Large rifle Boxer primers are between .2105" .2130".
The primer to primer pocket fit is and interference fit of between .0005" and .0045".
Measuring a change of less than .001" in the inside diameter of a primer pocket is not easy, but measuring a change of less than .001" of the outside diameter of the extractor groove is fairly easy.
A large Boxer primer pocket and a deep extractor groove make for a weak case head. The thin cylinder of brass is under axial compression from the case pushing back against the bolt face, and under tangential tension from the pressure inside the pocket with no supporting pressure against the outside of the case against the extractor groove.

If you watch this 10 second GIF from Varmint Al, you can see the Mauser case head large primer pocket inside diameter grow AND the extractor groove outside diameter grow. When the elastic limit of growth is reached and there is plastic deformation of the primer pocket, the case is soon to be thrown away.

 

[SPAK]

Re: .308 high pressure?

I'm still relatively new to all of this but I'm learning quite a bit. Anyway, I've found that cases subjected to over pressure will not fit properly in a shellholder. I don't know what the estimated pressure would be, but cases that have a slight extractor and ejector marks with some shine to them i can feel the edge of the edges of the extractor groove rub the sides of the shell holder.

Cases that have been subjected to significant overpressure will not fit in the shell holder at all. The ones that don't fit in the shellholder, at least the ones that I've seen have also had pretty severe burrs from the ejector and/or extractor.

These same cases will also not hold a primer in the pocket, or the primer will be extremely easy to seat with little to no resistance.

I don't know at what point or how much expansion to the extractor groove has to occur before it doesn't fit in the shellholder but I would imagine that if it gets that far, you're well beyond the safe load mark.

 

[MitchAlsup]

Re: .308 high pressure?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: vman</div><div class="ubbcode-body">I have a question about winchester .308 brass... As i have only ever used lapua .308 brass, im not too sure if the win brass has the little recessed area around the primer pocket, such as that on the lapua brass.</div></div>

Those indentations are a sign that those cases have been crimped at their first factory loading. This crimp is either cut or swaged out when the reloader gets his hands on them.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Im trying to determine the level of primer flattening I currently get.</div></div>

The picture shows you are just fine.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Im using Ar2208 (supposedly Varget). Book loads says 44.0gr starting to 47.0C Max when pushing a 155. To be honest I have taken the loads to 47 and have seen very little difference in primer flattening... if any, at 47.0gr.</div></div>

I use 47.8 gr of Varget and I am getting 25+ reloads per case with Win brass.

 

[vman]

Re: .308 high pressure?

Mitch, they are virgin lapua cases not loaded at factory... Im not 100% but i dont think they are any sort of crimp as the lapuas need basically no prep work besides potentially FL sizing.

I can easily seat Fed210M primers in the cases without the need to remove a crimp such as that found on mil surp brass.

I was just wondering why that indendation/groove exists in the first place and if its found on other brass. Also, if these cases are subject to overpressure will the primer flow out into this cavity easily clearly defining a point of primer flattening.

Clark you are a wealth of information thank you very much

 

[Texar2]

Re: .308 high pressure?

Thank you clark. That was just what i needed.

 

[badbot]

Re: .308 high pressure?

Win brass is about the lightest weight or higest internal volume.
this extra room allows more powder with lower pressures.
your loads look good Vman

 

[markspring1978]

Re: .308 high pressure?

Thanks everyone for their comments. Clark you are an amazing resource. By the way, I have found that TAC due to temp sensitivity is not the greatest powder for precision reloads. I have found several other interesting things out in this process, will share in a future post.