primers backing out on near-starting loads VArget /308

[wayno1]

Rem700 PSS, 1/12, 26" bbl .308, OAL case length 2.80, once fired cases, primer pockets uniformed and flash holes deburred, case length 2.005-2.008, FC match cases - 1x fired.
Fed 210M (match) primers

Working up some loads for the .308. Hodgdon site shows 41.0-42.0 for a starting load. Using rcbs powder scale and dispenser.
At 42.0-42.9 70% of the cases fired have the primer backed out a tad. When I seated them the were seated slightly below the level of the case. Working load up in .3 grain increments.
The backing out stopped at about 43.2 grains through 46.0 grains, although the primers showed some flattening but no cratering as the powder charge increased.

Never had that happen before so am curious. Maybe over did the primer pocket cleaning, but used a Sinclair tool just for that purpose.

Groups tightened up around 43.2-43.8 very nicely. I may just reload the batch from 42.0 in different cases and try again.

Thanks for any input,

Wayne

 

[PDXGS]

Thats odd....What primers have you tried?

 

[wayno1]

oops, guess that was important - left it out.

Fed 210M primers - edited original post and added it.
Thanks!

 

[wayno1]

Looks like I found an answer on 'cartridgeloaders.com'
My cases had the shoulders back a touch too far than the .002 recommended for resizing. More like .005 which brought them back to or under what an unfired case would meaure with a shoulder comparator.
Will resize these just slightly per the recommended and try again.....!!!

shm*lmafinga said: My first try at loading 60gr Hornady Vmax for my XCR: CCI 450 primers, Remington brass, full length resized, trimmed to 1.75", COAL 2.26". Started at 25.0 grains of Varget and moved up to 26.5 grain compressed loads of five rounds each. The first five rounds were the most accurate, but the spent cases all had their primers backed out slightly. (1" group) In increments of 0.5 grains I increased the powder charge. Everything higher than 25 grains had no problem signs, but had drastically worse accuracy. (2" groups and worse) Why would the lightest load back my primers out, yet be the only decently accurate load of the bunch?

*agleye said: This light starting load backs out your primers because there is insufficient pressure to fully obturate the case. The firing pin drives the case forward as far as it can go into the chamber, the pressure builds sufficiently to cause the case walls to adhere to the chamber walls, but there is not enough pressure to stretch the case slightly so the case does not back right up to the boltface. However, the pressure pushes the primer out till it touches the boltface, leaving it slighlt protruding from the case. As pressures increase, the case is forced back against the boltface, and the problem disappears. As for accuracy, you should be able to find another "node" of good accuracy closer to max pressures somewhere. Work up carefully in .5 grain increments. The accuracy should improve somewhere during this process. Regards, Eagleye.

 

[Fuzzball]

Try a little experiment. Prime two-three sized cases and fire them. See if you still think excess chamber pressure is what's making your primers back out.

 

[wayno1]

appreciate the reply, but I think the post I found kinda told the story. I was bumping the shoulders back too far and with a 2.80 OAL the primers were backing out at the light loads. will bump only the minimal amount next time.