fire forming and pressure

[dragman]

I am fireforming 300 H&H brass into 7X300WBY and while it is blowing out nicely I am getting a little pressure signs in the primer. Load I am using is Reloader 22 with 70 grains with a CCI 250 primer and a 168Berger VLD hunting set just on the lands. It is shooting nicely while fireforming but speed is only @ 3070fps I think it should be a little faster. after the brass is formed since it is adding case capacity do you think the pressure will go down letting me add more powder letting me pushing it faster. I don't need the speed just a little concerned.

 

[TonyR]

Have you checked the headspace of an unformed case? I have had to use a hard jam or false shoulder when fireforming some cases with excess headspace. Could be giving you what look like pressure signs.

 

[dragman]

thanks I will try that tomorrow. I was a little worried about seeing pressure that soon.

 

[TonyR]

Theoretically, a belted magnum case should headspace off the belt and not give the sort of headspace problem that can require the hard jam or false shoulder but that might not be true with some reamers. It would be worth checking.

Another possibility is excessive case length so the case mouth is getting pinched in the throat. Is the overall length of the case longer after it is fireformed?

You are right about pressure going down once the case has been formed. When I fireform 284 Shehane cases from Lapua brass, it takes 1.5 grains more powder to get back to the MV of an unformed case. So it probably makes sense to use a load appropriate for something like a 7mm/300H&H to fireform than a 7mm/300Weatherby load which would be fine after fireforming.