Gunsmithing Pressure signs.

[desertrat1979]

Ok, so my buddy and I have been doing some load development on his AR10 and we have been noticing some big pressure signs with reletively mild charges. Pressure signs include cratered, flat and popped primers and some really loose primer pockets plus ejector swipe. So heres the specs.
DPMS AR10 upper, lower and BCA.
24" Kreiger heavy barrel with an Obermeyer chamber
I am using standard Lee .308 Win dies
Full length sizing
We have used both Varget and Norma 202 and 175 SMKs and 178 AMAXs all with the same issues.
Today we had pressure signs on the 178AMAX with 41.0gr of Norma 202. We had similar results with 44.5 grains of Varget. The gun is accurate as hell and I know these loads are far from hot. I Load the COAL to 2.775, its about all the mags let me get away with. I appreciate any ideas you can toss my way.

 

[MAGUA]

Start with a lower charge of powder and work up in 1/2 gr increments.

 

[LawnMM]

The Varget load is on the warmer side, I wouldn't call it mild. I think your problem is COAL. I'm not at home but as I recall COAL for 308 is 2.800. If you are loading shorter you are reducing case capacity and increasing pressure at the same powder charge.

DPMS lower should take a pmag. Try loading a few to 2.800 and single feed them, see if it gets better.

 

[Brasscow]

44.5 of varget is not a mild charge with a 178 amax, especially if you're using lake city brass.

 

[z7.jled]

45 grains of varget is the published max load according to hodgdon with a 175 smk at 2.8 col. That is compressed so when you load shorter you compress it more and could be raising the pressure.
I would reduce the load some and try to adjust col to figure out your max col that will feed, then work up to a date load for your rifle. Hot loads reduce brass and barrel life and that sounds like a nice barrel, so I would reduce the load, but that is me.

 

[Layton]

With QL it looks like you are exceeding max pressure with the Varget at that COAL. Real close to it on the Norma.

L

 

[desertrat1979]

I was wondering if COAL was going to be an issue, just not in a pressure sense. Didn't even think about that. Problem is the rounds are seated to a hair under mag length. Trying to avoid having the tips drag on the mag. (And thats with a Pmag). I will back off on the charges a bit and do a work up from there. Thanks for giving me a place to start guys.

 

[LawnMM]

I've never had trouble getting cartridges loaded to 2.8 to load from a pmag, why are you another 0.225 shorter? what's your muzzle velocity?

 

[Layton]

Even at 2.8 COAL you are still over on the Varget . 43g is around the max you may want to go. Norma at that COAL should work up to around 42g before you hit max pressure. Hope that helps solve your problem. Also PRI mags while not cheap do have more space and will allow for a slightly longer COAL.

L

 

[desertrat1979]

I've never had trouble getting cartridges loaded to 2.8 to load from a pmag, why are you another 0.225 shorter? what's your muzzle velocity?

I dunno if my math is off, but the difference in 2.8 and 2.775 is. 025. that ain't a whole lot of room there. Im willing to bet not all pmags are held to such a high tolerance. Regardless, its making sense now. i think ill do a new work up with the 202, set the max around 41gr and start off at 38 and go from there.

 

[LawnMM]

I dunno if my math is off, but the difference in 2.8 and 2.775 is. 025. that ain't a whole lot of room there. Im willing to bet not all pmags are held to such a high tolerance. Regardless, its making sense now. i think ill do a new work up with the 202, set the max around 41gr and start off at 38 and go from there.

You caught my math error. 0.025 deeper in the case is plenty to cause you problems at the upper end of the powder charge.

The pmags are actually larger than 2.8 and they feed flawlessly. You can do a whole workup if you want but what I'm suggesting would cost you 3 rounds. Load them to 2.8 and see if things improve, if they do all you need are some mags and you don't have to start all over with load development when you have an accurate load already.

It's your gun so do as you wish.

 

[Jungle George]

The Varget load is on the warmer side, I wouldn't call it mild. I think your problem is COAL. I'm not at home but as I recall COAL for 308 is 2.800. If you are loading shorter you are reducing case capacity and increasing pressure at the same powder charge.

DPMS lower should take a pmag. Try loading a few to 2.800 and single feed them, see if it gets better.

I'd bet money on this! COAL is critical. My DPMS GII likes a bit of free bore.

Cheers,

George

 

[Deadshot2]

Of all the pressure signs mentioned, flat primers is one of the least reliable. Primers can flatten and give false signs merely from pushing the shoulder back too far when sizing. If not using a case gauge or headspace measuring tool it's all to easy to do this. When firing with a "short headspace case" the primer pops out slighty and is then smacked back into place when the pressure peaks in the case, expanding it to full chamber size.

Cratering can also be caused by the bevel that some firearm manufacturers put on the opening of the firing pin hole. "False Crateriing" is a real pain as it can stop someone from continuing up in load development charge weights and miss out entirely on what might be a great "accuracy node".

I'd take note of the flattening and cratering but the ones that stop me in a load development are the bolt face wiping, hard bolt lift, and the ejector horseshoe shaped mark. I also do my load development over a known quality chronograph and if I see speeds that greatly exceed those published with max loads I get real cautious.

I shoot with a guy who never stops until he has to put some real effort into opening the bolt. Seems to work for him but I tend to set up a ways away from him when he's working up new loads

 

[dmike]

202 & 178 AMAX...I got good results with 39.8-39.9gr in a GAP-10 (1:10). I know that's slower (guessing 2500 in your DPMS?) but it's accurate and low pressure. I've run up to 42gr in my 700 but that's seating at 2.88-2.90.

I've always run pmags but I was recently told that the metal front on the DPMS mag allowed you to go over 2.80 COAL?