I figured the readers here would be more accustomed to a question of this nature. I have a wildcat round in .338 cal that is very similer to the 338-06 only slightly shorter. When I fill each case with 2460 ball powder up to the base of the neck and tap it down so it's consistant the wildcat round holds 2.4 gr less powder than the 338-06. My question is can one safley get a potential velocity estimalion from this knowledge based on the same barrel lenth, powder and bullet used in each. Thanks all.

quote:

Originally posted by NBHunter:
" My question is can one safley get a potential velocity estimalion from this knowledge based on the same barrel lenth, powder and bullet used in each. Thanks all.

"

Yes! A CLOSE ESTIMATION

V1 squared/capacity1=V2 squared/capacity2. The error, however will probably be greater the larger the differential is in the two capacities.

There are a number of features that come into play such as short fat vs. long and shoulder angle, both affecting burning rate, and a host of others but for a close rule of thumb this should do roger

A fairly accurate ballpark estimation is to figure velocity will increse/decrease at 1/4 the rate that powder capacity increases/decreases. So say your case holds 4% less powder then a currently produced round, you could expect to loose 1% of the velocity. So you're loosing say 2.5 grs from a parent that holds a nominal 60 grs of powder. That is a 4.2% decrease in capacity. If your 338-06 is say launching 250 gr @ 2500 fps, then your cat should be capable of say 2475 fps.

Thanks Paul,
Those numbers are pretty much in line with what I was expecting. It's not so much a need for velocity, I'd use the WinMag for that, But I wanted to know the velocity limit to help me monitor potential pressure problems when working up loads for it.

For help in the load data I'd planned on reducing starting loads (for a given bullet weight) for the 338-06 and working up until I come close to the target velocity based on the percentage formula of capacity.

I see that as a very prudent approach.

The reason I re-chambered my 35 whelen ackley to a 350 Rigby, is it just wasn't reasonable to expect 250 gr @ 2700 fps at sane pressures. And the barrel refused to shoot when I dropped my charges. Now I get the same 250 gr @ 2700 fps, better accuracy, and I don't worry about straining the action.

Paul H's approximation is the correct one. I first read about it in the writings of John Barsness, in the Wolfe magazines. You can satisfy yourself as to it's correctness by comparing .308 to .30-06 to .300 Win Mag in various loading books.

bartsche's approximation is equivalent to % fps change = 1/2 the % case change. This approximation comes from a simple balance of available energy. A case with 10% more case holds about 10% more powder and so might have 10% more chemical energy, which in turn might give 10% more kinetic energy. Since KE rises with the square of fps, 10% more KE is about 5% more fps.

However, this simple analysis falls short by failing to account for the loss in efficiency of the larger case. To keep the efficiency the same, one must have (1) the same peak pressure and (2) the same expansion ratio. To have the latter, a 10% larger case needs a 10% longer barrel.

In other words, for a typical 24" sporter barrel, rechambering to a case 10% larger will give an efficiency similar to chopping over 2" off the barrel of the original chambering. It is this loss that drops the % fps gain down to only 1/4 the % case increase.

(If it was not obvious, the figure of merit is not the capacity of the empty case but the net capacity, under the seated bullet.)