Cratering 7 1/2 primers

[HighUintas]

I shot some 147 eldm today out of a factory 24" Tikka barrel. 6.5cm
Starline brass 0.002 bump
Remington 7 1/2 SRP
44.5gr staball 6.5
2793 fps avg
2.820 ish col

Yes, I know this is over Hornady's book max.

I got significant cratering on the primers on some rounds and none on some. Probably at least a little cratering on most. There were zero other pressure signs. No sticky bolt, no ejector marks whatsoever. All rounds shoulder bump were +/- 0.0005.

The firing pin clearance is fine and there is no bevel on the hole. I've shot hot loads with this bolt in other cartridge and no cratering.

No I haven't backed off and tried lower charges yet. However, with a low charge of h4350, 140 match burner, and cci450, no cratering.

I know this load is extremely likely to be over book pressure, but what is strange is that there's no sign of pressure other than the cratering, which is also odd because I've never had cratering in my Tikkas even with loads that do cause sticky bolt lift.

I'll probably back it down 1/2 grain, but just wondering what's the cause of this odd occurrence.

Any ideas?

 

[TaperPin]

Sounds like a stiff load, but maybe not too bad - not every chamber gives sticky bolt lift.

Brass life may be somewhat short, maybe not, but the biggest downside will be blanked primers if a cratered primer lets go - the little disk can jam up the firing pin.

A CCI primer has a tougher cup and won’t crater as much.

 

[HighUintas]

Although all the cup dimension charts show cci450 and rem 7 1/2 having same thickness I was thinking possibly softer primer cup material as well.

Going to back off a bit next time I load that bullet and I'll see what it does

 

[Castle Rock]

Close to 2800 is on the ragged edge, expect some issues
Ammo left to heat soak in a hot barrel will show considerably more pressure

 

[ResearchinStuff]

Rem 7 1/2s are a magnum srp, I'd believe your rifle that the load is too hot.

 

[HighUintas]

Rem 7 1/2s are a magnum srp, I'd believe your rifle that the load is too hot.

I'm pretty sure it's over pressure. Just trying to figure out why some primers cratered but there were zero pressure signs

 

[TaperPin]

You might check the firing pin spring against a new one next time it’s apart - a soft spring can bounce back allowing the cup to enter the firing pin hole more.

 

[MT-nuffgun]

We’re the rounds that cratered shot when the barrel was warm?

 

[Vern400]

Cratered primers but no pressure signs would be puzzling, if it were a thing at all.

You're redline. Cook a round for 60 seconds in a hot barrel. Blow a bit of primer into the bolt and you could be down over what? 30~40 fps for that last half grain? Your rifle is trying to talk to you.

You'll probably be okay if you shoot with glasses on. A blown primer or worse, a blown case is pretty unfriendly.

It's a free country, but I'd back off and be happy.

 

[HighUintas]

We’re the rounds that cratered shot when the barrel was warm?

Very first, cold barrel and ammo cratered. Some that sat in the warm barrel for 30 seconds looked perfectly fine. Temp didn't seem to matter

 

[Mulyhuntr]

2800 with a 147 through a "slow tikka" is pushing it.

 

[BrBa]

If you know the case capacity of your fired cases in gr H20, you can plug that info into Pmax Ballistics and get a reasonable approximation for pressure. You'll need to subtract the volume of the seated bullet from the case capacity (Pmax calls it "powder space") but it's fairly simple. Multiply seating depth (in inches) by 0.05473911039614 and convert that value (cubic inches) into grains of water. Should be between 4 to 6 grains of water to subtract. I'm betting you're well above 70000 psi.

P-Max internal ballistics