O.D. of RL 15

[Lemonaid]

During today's range session I had three pierced primers. The data: 6PPC RAP (Bolt action ) 1-7 twist 24 inch long Shilen barrel.
Hornady 6.5 brass sized and prepped for 6PPC, 2x fired.
CCI 400 primer, Hornady 105 BTHP, 2.250" oal (15 thou jump), compression just starting at 2.250 at 27.0 grains.
Reloader 15, 26.6 to 27.0

Hornady/Grendel case, RL 15, CCI 400, 2.250“, Hornady 105 HPBT
27.0 2697,2744 Pierced Primer!
26.8 2704,2718,2750,2738 4=1.028 3=.265 SD 20.5 AVE 2728 P. Primer(barely)
26.6 2694 Pierced Primer!

Now I am aware (now) that these loads are over pressure despite the listed max charge of RL15 for 6mm ARC of 27.6.
To get to the published velocity max of 2614 for a 24 inch barrel it should max out at around 25.6 for my setup.

Finally my question is this: If I had used CCI 450 primers instead of 400's and had close to the same velocities but no serious primer pressure indications would the loads be considered over pressure? Would you switch over to CCI 450's with the possibility they are masking over pressure conditions?

[Es 350]

If I understand correctly your rifle has a 6PPC chamber and you are sizing 6.5 Grendel brass down to 6 PPC?

If the loads are in fact over pressure primer change will not matter as the primer pockets will be trash anyways.

While the CCI 400 does have a thinner cup vs a 450 over pressure is over pressure.

Things to check for..... 6PPC used for utmost accuracy , reamer's/chambers most times are designed with small neck requiring neck to be turned for consistent neck wall thickness and dia.
If your rifle was chambered with such a reamer and you are not neck turning your brass you do not have enough neck clearance. This will drive chamber pressure up as the case cannot expand to release bullet.
Try to drop a bullet into a fired case, it should drop in freely , if it will not this is propably your problem.

Also anytime you neck up or down a case it is easier to form a donut (build up of brass)at base of neck and will cause same pressure increase.
A carbon ring in throat can also cause pressure increase
A bore scope will be needed to check this out.

I would switch to 450's as they have a thicker cup and withstand pressure better . Although being a hotter primer they will increase pressure/velocity so back off charge weight .

The thing with pierced primers is the damage they do to the Bolt face/firing pin.
Everytime you pierce a primer the hot gas act same as acetylene torch cutting metal. This of course is bad as it weakens the Bolt face.
Inspect your firing pin as well, gas cutting may have made a sharp spot (or could be from factory) this will pierce primers as well, if so smooth up with fine sand paper.

[Lemonaid]

ES 350, thanks for your advise. I left out a bit of relevant data in that loads using CCI 450 primers and converted 7.62x39 brass with Fed 210 primers would show extractor swipes but the primers only looked a bit flattened when hot loads were used. No such marks on the brass were observed with the CCI 400's.
For the exact reasons you stated about damage to the bolt face and firing pin tip I will go back to the 450's or other primers that have thicker cups.