Question about effective pressure

[.22gongzilla]

Have a question for an experienced reloader about effective pressure for a target barrel: I seem to get excessive pressure with certain powders that may be related to barrel length. It's been pointed out that peak pressure occurs close to the chamber, but does duration contributes to effective pressure also? I mean, with certain slower powders I get pierced primers (as well as excessive velocity) at charge rates that are well within standard load data parameters - can an oversized barrel really do that?

 

[SheepDog]

I think by "oversize" he is referring to the length not the bore diameter.
Pierced primers can be caused by a defective firing pin or excessive pressure. A longer barrel is going to give higher velocities (normally).
Check your firing pin first for a well rounded tip and protrusion. If that checks out OK then regardless of the load data you could be exceeding the maximum pressure in your gun.

 

[T-shooter]

Case volume and bullet seating depth will alter pressure. I was working up a load for a 5.56 using CFE223. With a COL of 2.260" the velocity averaged (over 5 shots each) 3,045 fps. I like running mine out to 2.300" and with the increased volume the average velocity slowed to 3,014 fps. A had to go from 28.4 grains to 28.8 grains to get the velocity back, 3,050 fps. (All were Federal cases and the jump on this rifle is over .100")

Also had a bad overpressure using Hodgdons' suggestions. The limit was 45.9g. At 44.5g I was seeing ejector marks on the cases (didn't notice until later) At 45.0g, it recoiled much harder, was louder, and a very large flow into the ejector hole. The case exanded so much that the primer fell out. I was using CBC cases and I found out they are smaller. I later measured and the internal volumes averaged 3% smaller than Lapua or Hornady .308 brass.

 

[.22gongzilla]

Case volume and bullet seating depth will alter pressure...

...indeed, and I generally seat against the lands for all my loads so your point is well taken but I suspect that my question may revolve around PSI vs Cup pressure. All I have is anecdotal evidence but it seems that slower powder is much less forgiving than the load data suggests when used with a barrel that's significantly longer than the 24" test barrel, even when the twist rate is the same.

Thanks for responses, and FWIW I would urge new reloaders to use caution with slower powders when reloading for a target-length barrel for the first time.

 

[Zero333]

Barrel length will not change the peak pressure of a load.

Pierced primers are due to the primer cup being soft along with too much clearance around the firing pin in the bolt head.

You can try a harder cupped primer like cci or especially the milspec cci primers.

 

[.22gongzilla]

Pierced primers are due to the primer cup being soft along with...

...right, but after consulting a past issue of the Hodgdon Annual Reloading Manual I've always preferred to use regular primers with Varget - especially with softer bullets that have no business being run at extreme pressures. With 55 grain Nosler spitzers everything comes together at the max 27.5 grains for my rig, even if the primers are a little flat and the starting velocity is about 60 fps higher than the load data predicts. No problem there.

But the 64 grain Bergers are another story, they've presented a real challenge. My latest anecdotal data suggest that 27.5 grains of H414 should put me in the ballpark at around 3180 fps start velocity and this is what I'll try this week. Whether it's just coincidence if this turns out to be another 27.5 grain load achieving another max published velocity I can't say. But regardless of barrel length max velocities are not to be taken lightly - I found that out. I still suspect that max velocity in this case is related to the 12" twist rate, but I can't even begin to explain it satisfactorily.