DPMS 260 rem load too hot, primers too soft, firing pin hole too big?

[lte82]

Finally got around to doing a full speed work up with the 123 smk for my 24" DPMS 260 rem and these were my results:

H414 44.0gr: 2900 fps - 1.0 moa
H414 44.3gr: 2940 fps - .8 moa
H414 44.6gr: 2975 fps - .5 moa primers look ok
H414 44.9gr: 3005 fps - .3 moa with slightly cratered primers
H414 45.2gr: Two heavily cratered primers, one pierced primer, 1.0 moa. Didn't save the velocity data.

I'm using Rem 9 1/2 primers, and aside from the cratering that starts at 44.9gr, the brass looks ok.

I've read that the DPMS firing pin hole is too big and can cause premature primer piercing, so I'm wondering if it's that, the primers are too soft, or the load really is just that hot even though my brass doesn't have any real signs of pressure.

Should I stick with 44.7-44.8gr, which is right in the node and probably won't blow primers, switch to a magnum cup and go with 44.9, or ??

 

[bigedp51]

Below is the bolt face of my new Remington 700, the firing pin hole in the bolt face is countersunk and not flush and all higher pressure loads cause cratered primers. All large rifle primers have a .027 thick primer cup as far as I know and the magic name "magnum" doesn't make the primer cups thicker or harder. And the CCI so called milspec primers just have a shorter anvil in the primer requiring a hard blow to set off the primer.

Another thing you have to understand is your not crimping your primers like the military does and your primers can back out further on a gas operated semi-auto rifle. Meaning your problem "might" be normal, what does the primers look like on factory loaded ammunition? Do you have ejector marks on the base of your brass? Are your primer pockets tight?

Bottom line, I do not have your rifle in my hands but many primers flow into oversized firing pin holes and testing other primers help your problem.

Below is how a uncrimped primer backs out of the primer pocket when fired and how the brass stretches to meet the bolt face. The more head clearance you have caused by the amount of shoulder bump the further the primer can back out of the primer pocket. On top of this a over gassed AR type rifle will have the bolt moving to the rear while pressure is still in the chamber.

Below is a workup load in a .221 fireball starting on the left and working up in pressure moving to the right. The third case from the left has a cratered primer and the next higher load does not. Primers can fool you about pressure and you should also check your brass for excess pressure signs.

 

[lte82]

Below is the bolt face of my new Remington 700, the firing pin hole in the bolt face is countersunk and not flush and all higher pressure loads cause cratered primers. All large rifle primers have a .027 thick primer cup as far as I know and the magic name "magnum" doesn't make the primer cups thicker or harder. And the CCI so called milspec primers just have a shorter anvil in the primer requiring a hard blow to set off the primer.

Another thing you have to understand is your not crimping your primers like the military does and your primers can back out further on a gas operated semi-auto rifle. Meaning your problem "might" be normal, what does the primers look like on factory loaded ammunition? Do you have ejector marks on the base of your brass? Are your primer pockets tight?

Bottom line, I do not have your rifle in my hands but many primers flow into oversized firing pin holes and testing other primers help your problem.

Below is how a uncrimped primer backs out of the primer pocket when fired and how the brass stretches to meet the bolt face. The more head clearance you have caused by the amount of shoulder bump the further the primer can back out of the primer pocket. On top of this a over gassed AR type rifle will have the bolt moving to the rear while pressure is still in the chamber.

Below is a workup load in a .221 fireball starting on the left and working up in pressure moving to the right. The third case from the left has a cratered primer and the next higher load does not. Primers can fool you about pressure and you should also check your brass for excess pressure signs.

Factory ammo and my low-mid velocity loads have almost no cratering and primer pockets are tight. The upper end loads that have heavy cratering and one pierced primer I havent tested but there arent any visual over pressure signs on the brass.

As far as overgassing goes, I do think that is happening as even on light loads it cycles ammo almost too well.

Ill try to get some pictures and post them and also see if the primer pockets are still tight.

 

[lte82]

Cratering on the left but extremely good accuracy. Full blown primer flow and pierced primers on the right. No ejector swipe or marks on either of the brass though.

Also these are only .3gr away from each other. 44.9 and 45.2. 44.9 seems to be the sweet spot for the load however 44.6 also produces good nearly as good results with almost no cratering. The primer pockets seem ok on all of them too.

 

[bigedp51]

Below is from the Sierra manual, excessive shoulder bump or over resizing the case creates excessive head clearance. When you pull the trigger the firing pin pushes the case forward until the shoulder of the case contacts the shoulder of the chamber and then the cartridge goes bang. Then the chamber pressure builds and pushes the primer out of the primer pocket, as pressure increases the brass stretches to meet the bolt face. Meaning the primer takes a double hit and the firing pin acts like a cookie cutter and punches the center out of the primer.

Bottom line, you "may" be over resizing your cases and creating excess head clearance and your firing pin dimple in the primer is turning into a "outie belly button" and then it falls out.

If you train your primer anvils to plug the hole you've solved the problem.(holy primer humor) There is a reason why the military crimps it primers, so how far are you bumping your shoulders back when sizing?

 

[lte82]

Below is from the Sierra manual, excessive shoulder bump or over resizing the case creates excessive head clearance. When you pull the trigger the firing pin pushes the case forward until the shoulder of the case contacts the shoulder of the chamber and then the cartridge goes bang. Then the chamber pressure builds and pushes the primer out of the primer pocket, as pressure increases the brass stretches to meet the bolt face. Meaning the primer takes a double hit and the firing pin acts like a cookie cutter and punches the center out of the primer.

Bottom line, you "may" be over resizing your cases and creating excess head clearance and your firing pin dimple in the primer is turning into a "outie belly button" and then it falls out.

If you train your primer anvils to plug the hole you've solved the problem.(holy primer humor) There is a reason why the military crimps it primers, so how far are you bumping your shoulders back when sizing?

Thanks for all of the good info here. As far as bumping the shoulder back, I actually just switched over to a small base die instead of a standard FL die, though I've never measured shoulder bump on either. I've just given the die a 1/4 turn past hitting the shell holder and let it rip. Maybe the small base die is bumping it back a lot further than I need? I will try resizing less and see if that helps.

Also, do you see any visual signs on the brass? I can usually read brass pretty well and I just don't see anything jumping out at me.

 

[bigedp51]

The center right case might have an ejector mark on the number "2", meaning it might just have started to flow into the ejector.

You need to get a Hornady cartridge case headspace gauge and measure your fired cases and then bump the shoulders back approximately .003 when full length resizing. My small base die only makes the diameter of my .223/5.56 cases .0005 smaller in diameter than my standard die. BUT the small base die sizes further down the case to bring the the case to minimum SAAMI dimensions. The small base die has nothing to do with shoulder bump, BUT the up and down adjustment of the die controls shoulder bump.

Below is a Hornady cartridge case headspace gauge and it is a thousand time better than using the Wilson type case gauges for die setup.

Below on the left is a Redding competition +4 shell holder and it is .004 taller than the standard RCBS shell holder on the right. The +4 shell holder makes the case .004 longer and prevents over resizing the case and still allows the the press to cam over and the die to make firm contact with the shell holder. This takes the press flexing out of the equation and gives a constant amount of shoulder bump.

Below is an exaggerated drawing of a case being full length resized, "LOOK" at the blue, red and green lines and "shoulder setback". The general thumb rule on shoulder bump is .001 to .002 on a bolt action rifle and .003 to .004 on a semi-automatic rifle. To do this you need to measure your fired cases and adjust your dies accordingly. I simply put the +010 shell holder in the press and work down in size until I get the desired shoulder bump.

 

[lte82]

Ed, I finally got around to sizing and priming all of my brass, and I came across something interesting. The "hot" loads I made with the heavy 139 scenars and 140 amax (41.6gr H414) all had heavy ejector marks and the primer pockets definitely lost some tightness, but primer cratering was minimal. The fastest 123gr loads (44.9 & 45.2gr) have virtually non-existent ejector marks and still have factory tight primer pockets, but they have deep primer cratering and even a pierced primer.

As your post states above, this is definitely not an overpressure issue. Tonight I sized all of my brass with the standard die and just enough shoulder bump so that the bolt will close. I'm going to try to retest tomorrow if the weather allows.

Thanks again for all of your help.