Fire Forming Question

[Jess]

I have been fire forming 300 weatherby cases for some time now and have found that 15 grns of Unique powder the fill to the neck shoulder with cream of wheat then just a piece of papertowel to keep the contents in place produces a nice crisp shoulder to 300 ackley. New project requires fire forming 243 winchester brass to a 243 Ackley shoulder. A unaltered weatherby case will hold 101.5 grns H2O and a 243 win case holds 55 grns of H2O. Mathmatical extrapulation and hocuspocus reveals that approximately 8.1 grns of Unique should match by case volume. Can it be assumed that this will appropriately produce the sames results as with the weatherby fireforming.
Or can anyone who has fireformed the 243 win yo Ackley give me their info.

 

[Winchester 69]

I'll give you a little different answer than what you're anticipating.

The 243 AI should not require fire-forming the brass. The standard brass should chamber just fine and will be formed on the first firing. On a properly cut AI chamber (the barrel is set back when cutting the new chamber), the distance from case neck/shoulder junction to the base is the same as standard, and standard ammo functions as it would in a standard chamber.

BTW, a full-pressure load will form longer-lived brass.

 

[Ackman]

Forget the extrapolation stuff and don't mess with fillers. Just work up an accurate load with bullets then go shoot. A couple manuals give max loads for the std. 243 and W760 powder as 45.4-46gr with a 70gr bullet, velocity about 3300 to 3400 tops. They're also using 22" factory barrels, different powder lots from yours, etc. In other words you can't be going by that data as gospel, it's just a place to start. My fireform load through 2 - 243AI's (one 26" barrel, one 25", both .273"nk and 0 freebore) was a 70gr. TNT and 48.3-48.7gr W760 powder. Velocity was anywhere from 3770-3860 depending on the charge and the primer and which barrel. Each case (about 2,000) formed perfectly. The loads chosen shot 3/8" or less and accounted for a whole bunch of prairie dogs and rockchucks in weather that was on occasion 100 degrees. Those loads and velocities are pretty much the same with formed brass. You really don't need to be fireforming with fillers, it's a waste of time.

On a properly cut AI chamber (the barrel is set back when cutting the new chamber), the distance from case neck/shoulder junction to the base is the same as standard, and standard ammo functions as it would in a standard chamber.

This part of your statement is not true. I've measured quite a bit of variation in .243 brass. With an AI chamber the the distance to neck/shoulder junction should be at least .010" shorter than with a standard chamber. Mine are considerably shorter than that.