Pressure on once fired brass?

[MagnumManiac]

Hi all,
Have just recently run into an issue I cannot explain.
I have a long throated match chambered 300WM that is showing excessive pressure on loads that worked no problem on unfired cases.
I’m getting loose primer pockets and hard bolt lift where it wasn’t present previously. Haven’t run them over the chrony yet on the once fired loads, will do on the weekend.

Any thoughts on this?
All help appreciated.

Cheers.

 

[Texas10]

How much did the cases expand after first use? Did the OAL shrink considerably? What was base to datum measurement before and after? If a good amount of energy was spent moving brass on the first use, your load was probably on the cusp of too hot to begin with, but just now showing definite signs.

 

[Lone Hunter]

Different lots of powder?

 

[FeMan]

I see it like Texas does. New brass is probably under-sized and springs back more. On the second firing it is likely a lot closer to fitting the chamber and if pressures are high, it won't spring back as much. I have had (HOT) loads in my STW where the cases were a bit tight-fitting before firing. The bolt was tough to open on those buggers, when the same load with generously sized brass didn't produce the same tough bolt lift.

 

[FURMAN]

Many people have experienced this although I never have. If you have any new brass I would load your load up and run it over a reliable chrono then work your once fired brass up to that velocity from a lower charge. This should fix your problem.

 

[MislMan]

Temperature sensitive powder perhaps. Couple that with leaving ammo in the sun and that could drive up pressure quite a bit.

 

[dkhunt14]

You can get away with more powder on a new case. They are small for the chamber. It takes up energy to expand the case. Also hard to tell if you are too hot with new brass becuase it's small.

As an example a 308 Bear fireforming from 340 Weatherby brass to 308 Baer, once formed, it takes 1/2 grain less powder to total the same velocity. Thats why when i break in a new gun, I take one or two cases and keep going up in powder. I want to see if i can size and have them function. If you get sticky bolt on a new case, you are way too hot. Usually it never shows up till the second or third firing. That way i only lose one or two cases and can find out where I can go to in powder. Matt

 

[MagnumManiac]

Thanks guys, it’s winter here, so the temp isn’t a problem.
I ran all loads over the chrony and pressure trace this morning. The unfired brass is right at 3166fps and 62,890psi. The once fired brass is at 3186fps and 63,909psi.
Well within the 65,000psi max limit...... so I’m thinking my Norma brass is soft?! New primers fall out of the second fired brass now. The Win brass I have is much harder and handles 8 reloads with the same load and components.

Cheers.

 

[dcali]

Sometimes it takes a couple shots to ruin the brass. I've done the same thing with hot loads in some brand new Lapua brass (ouch $$$). Seemed ok after one firing. After two it was toast. It could have something to do with some of the energy going into forming the brass, but I suspect that it has more to do with the amount of time that the brass is under what is too high a pressure. I'm just guessing, however.

 

[243winxb]

Norma vs Win brass , weight & volume?

modulus of elasticity- Cartridge Brass-
Material is 70 copper/30 zinc with trace amounts of lead & iron , called C26000. Material starts to yield at 15,000 PSI when soft (annealed), and 63,000 PSI when hard.
Material yields, but continues to get stronger up to 47,000 PSI when soft, and 76,000 PSI
when work hardened. Modulus of Elasticity is 16,000,000 PSI. This means to pull a 1.000 inch long strip to 1.001 inch long induces a 16,000 PSI stress.
So if you pull a 1.000 inch strip to 1.005 inch long, you get about 76,000 PSI, which is the max obtainable.

 

[Clucknmoan]

Had a similar issue with a .223, not to this extent though. Tracked it down to the wet tumble cleaning solution I was using. It was leaving an almost tacky residue on the brass.