Fire forming 260 AI with surplus 7.62x54R powder and cast bullets

[atl5029]

Hi yall,

I am in the process of building a 260 AI rifle, and I have been pondering how I can fire form brass, get some trigger time, but not waste barrel life fire forming brass. I was thinking a good solution would be to fire form the cases using a GC cast boolit. I also have a canister of pull down powder from Bulgarian heavy ball 7.62x54r. I have used this powder to load new x54r brass with new bullets using starting charges and working up in small increments. The powder seems to be in the 4064 - 4895 range. I was thinking about starting with a 30-31 grain charge and a 135 grain cast GC bullet to fire form my brass. Any thoughts?

Anyone know what pressure is needed to fire form brass?

Thanks!

[runfiverun]

40-k should do it, depending on the brass.
I can do it with 40-k in my 7x57 Ackley if I use Remington brass.
it takes 2 shots at 50-k with Winchester to get the shoulder fully formed.

[Larry Gibson]

"Anyone know what pressure is needed to fire form brass?"

It takes a minimum of 7k to 11K to expand brass cases. How much psi depends on the thickness, hardness and elasticity of the cases in question. The time pressure curve also influences how well formed the cases are. A medium burning powder psi with a longer time pressure curve will fireform better than and extremely fast burning powder with the same psi, especially when no filler is used with a cast bullet.

Since you want "trigger time" also I'll assume you want a semblance of accuracy also(?). I suggest you start at 24 gr of the pull down powder under the 130 gr cast bullets. That should provide 30K +?- psi and should fireform the cases nicely. However you can work up from there if necessary depending on the cases as mentioned. You may also be able to work down for better accuracy.

I base that on I have found the Bulgarian pulldown 7.64x54R powder to be comparable in burn rate to a slow lot of milsurp 4895 or 4064 as you mention. I've used 26 gr of that 4895 under 130 gr cast and the psi measured 37k in my 6.5-308. The work required to fireform the cases into the larger AI chamber will soak up sum of the measureable psi.

Larry Gibson

[atl5029]

Thanks for the great advice Larry! I'm glad someone has done something similar to this before and can post actual experience. I'll start with the charges you recommend. Were you shooting a 260, or some other 6.5 cal cartridge? Also wondering how you were able to measure pressure for that load.

[Larry Gibson]

I was fireforming formed 6.5-308 from .308W cases. I measured the pressures via an Oehler M43 PBL in that rifle. I also measure the psi of numerous other cartridges in numerous firearms.

Larry Gibson

[TCLouis]

Anneal and fire form?