Pressure question…virgin vs fire formed

[sacklunch]

Quick question on pressure…I’m sure it’s a math problem and my gut tells me the answer is yes, but wanted to ask the masses…

Given identical charge weight, seating depth, bullet, neck tension, etc..all variables the same except virgin vs fire formed brass.

If an initial load was a tad hot for my liking, bolt swipes but no dimpled primers, with virgin brass, will the slight increase in case volume with fire formed brass shoulders bumped back 1 thou or just brought to chamber, will pressure at same charge weight be less?

I’d like to just shoot this load and be done with it, as speed, ES/SD all looked great and it was what I needed for accuracy out to distance. I’d be ok with it if pressure will drop on the 2nd loading, but don’t want to push the rifle this hard if it’s a negligible decrease.

 

[Doom]

Just the opposite. At least usually. The fired brass when resized is larger than new and so it absorbs less energy due to the stretching of the case to fill the chamber. Hence more pressure. How much? I can only say I've seen and heard of up an increase in velocity of 30fps.

 

[sacklunch]

Glad I asked

 

[straightshooter1]

Quick question on pressure…I’m sure it’s a math problem and my gut tells me the answer is yes, but wanted to ask the masses…

Given identical charge weight, seating depth, bullet, neck tension, etc..all variables the same except virgin vs fire formed brass.

If an initial load was a tad hot for my liking, bolt swipes but no dimpled primers, with virgin brass, will the slight increase in case volume with fire formed brass shoulders bumped back 1 thou or just brought to chamber, will pressure at same charge weight be less?

I’d like to just shoot this load and be done with it, as speed, ES/SD all looked great and it was what I needed for accuracy out to distance. I’d be ok with it if pressure will drop on the 2nd loading, but don’t want to push the rifle this hard if it’s a negligible decrease.

I think a lot depends on just how much the brass is being stretched to form into the chamber. My data indicates the virgin brass tends to be slower than when it's sized; not by a great amount, but some. For example, my virgin .308 Alpha brass averaged 2703 fps and after firing forming velocity is at 2721 fps.

As Doom suggests, it appears that the forming that's going on absorbs some energy during that process.

 

[whatsupdoc]

Virgin brass vs fired brass may or may not cause a rise in pressure, remember that is only ONE variable in the pressure
equation. Your loads should never be on the edge of high pressure.

 

[918v]

What is the shoulder length of your new vs fired cases?

 

[sacklunch]

Virgin brass vs fired brass may or may not cause a rise in pressure, remember that is only ONE variable in the pressure
equation. Your loads should never be on the edge of high pressure.

I’m t’s not dangerously high, from my experience, but it is stout. But this is a hunting rifle and I’d like to push it close to max…but based on the concedes here I’ll probaly back it off .2gr for the 2nd firing

 

[sacklunch]

What is the shoulder length of your new vs fired cases?

I don’t recall off hand. But I accidentally bumped them back 30 thou so there was quite a bit of fire forming going on…I was planning on just bumping 10 though, my mistake.

 

[918v]

I don’t recall off hand. But I accidentally bumped them back 30 thou so there was quite a bit of fire forming going on…I was planning on just bumping 10 though, my mistake.

You mean 3 thou? So you took virgin brass and bumped the shoulders 3 thousandths?

 

[sacklunch]

Fire formed (once fired) brass was bumped .003 for 2nd firing. Virgin brass was virgin brass.

Over bumped a bit, plan was .001, forgot to adjust my die from a separate 6 creed rifle and just rolled with it on all 50. 003…walking the dogs and typing is conducive to typos, my bad.

 

[918v]

Fire formed (once fired) brass was bumped .003 for 2nd firing. Virgin brass was virgin brass.

Over bumped a bit, plan was .001, forgot to adjust my die from a separate 6 creed rifle and just rolled with it on all 50. 003…walking the dogs and typing is conducive to typos, my bad.

Depending on how short your virgin brass was, that could have caused your ejector marks, not necessarily because the pressure was too high but cuz the case was slamming against the bolt.

 

[sacklunch]

Ejector swipes were on the untouched/never sized/no shoulder bump virgin brass after first firing. Bolt lift just a smidge heavy too.

I’ll just back the load off .2gr and call that a max. Don’t feel like doing much load dev on this one, it hammers and will be a low round hunting rifle.

Was mainly hoping the answered would say less pressure on 2nd firing, so n which case I’d just keep the original charge weight. But based on the feedback, I’ll back it off a tad expecting pressure to build on fire formed brass, vs pressure decreasing

 

[912173]

I had to back off from 42.6gr to 41.8gr 4064 in my .308 to get back to my new brass pressure and velocity.