Backed out primers/Reloading newbie

[Ctdrummerguy]

Hey guys, this is my first post, just looking for advice.
I haven't been reloading that long, but I recently decided to go with a different bullet weight for my Remington 700 ADL. I switched from 168gr SMK HPBT to Bergers 175gr HPBT long range match target projectiles. I decided to do some testing to see which load my rifle likes best. I tested two powders (RL15 and IMR 4064) at 1grain intervals to see which groups the best. Charges started at the starting load in the Bergers reloading manual and ended at the max load. All cases were once fired, same head stamped brass, all FL resized and trimmed to the same length.
I headed to the range with all my loads, and fired them from a stand to ensure consistency. After testing, I noticed on the lower charges, the primers were backing out slightly. They weren't falling out, and weren't loose during seating, but after firing they were protruding a few thousandths from the primer pocket. There's no evidence that gas escaped from the primer pocket and no other signs of overpressure on the casings. Problem is that these lower charges grouped far better than higher charges. The lowest charge (37.5 gr of IMR 4064) grouped the best at 0.672" at 100 yards.
My question is, how can I keep this load without having primers backing out, or should I go with the higher charges at the sake of accuracy?

 

[Deadshot2]

This is pretty much standard for lower charges, especially if you have a headspace closer to the max length.

When the primer is struck, the pressure that builds up between the cup and flash hole pushes the primer back out of the pocket slightly. When the main charge fires the pressure in the case then pushes the primer back in, providing it's high enough to stretch the case to take up the extra room in the chamber.

This also is a frequent occurrence if you are oversizing your case. If you intend to shoot these cases in the same rifle, then invest in a "Hornady Headspace Gauge". This is a device that merely fits on your caliper jaws and measures from the case datum point on the shoulder to the case head. First measure a few cases that you have fired and the primers are pretty flat across the case head. Back off your sizing die and size a case. Measure the sized case. Your die should be "pushing" the shoulder back around .002" or so. Any more and you are stetching the case every time you fire it, eventually risking a case head separation. Any shorter and you may encounter difficulty in closing the bolt. Just adjust the die up or down until you have this .002" difference between a case you've fired and the one you've sized.

FWIW, my problems with primers is the opposite of yours. I have a hard time finding the edge of some primers after I fire a cartridge. I like nice hot loads. If I can still open the bolt easily, it's not too hot

 

[Big Z]

Your brass prolly didn't expand to push the head all the way back to the boltface. Try backing your sizing die away from the shellholder when sizing your fireformed brass so it just barely moves the shoulder (or not at all). It'll keep your brass alive longer as it doesnt have to stretch to fill your chamber each time. The sticky on brass prep shows pics, but basically, mark a fireformed case neck and shoulder up with a sharpie or smoke from a candle. As you move your sizing die towards the shellholder you'll see where you're moving brass. When you see some smudge on the shoulder, set the locknut and rock on. These custom-fit brass may take liking to a different charge weight than your FL sized stuff.