denton said:
John Barsness' rule, which seems to work well, is that if you get X% more case capacity, you'll max out at 1/4X% more muzzle velocity. So by that rule, 10% more case capacity gets you 2.5% more MV. Estimates of 2-3% are right on in the case of the 7x57. All other factors equal, the 7x57 will always exceed the 7-08, but not by enough to make much practical difference.
The rule he uses is called the 4-to-1 Rule. Here's a link to the article where he describes this rule:
http://findarticles.com/p/articles/mi_m0BQY/is_4_55/ai_n31392893/
If you applied his rule to this debate, you would have to use the 7-08 published MV since he's using factory & reloading data (assuming identical pressures in 2 modern rifles). According to Remington's
factory ammo ballistics table, the 7-08 pushes a 140 gr at 2860 fps. The case capacity of a 7-08 is 55 gr. The 7x57 capacity is 60 gr. By dividing 5 (the difference in capacities) by 55 (the smaller 7-08 case), you get 9.1% more capacity in the 7x57. Applying his 4-1 rule, you divide .091 by 4 and get 2.3%. If you multiple 2.3% by the 2860 MV of the 7-08, you should see a MV increase of approx 65 fps in the 7x57 (everything else equal). If you wanted to use reloading data and used 3000 fps as your starting point on the 7-08, you would get a 69 fps increase in the 7x57 (everything else equal). Increasing the MV of the smaller round increases the difference in fps between the 2 but the percentage stays the same of course.
I'd still like to see the math this guy used to come up with his 4-to-1 rule. It would seem to me that all cartridges would be slightly different. Plus if the
smaller of the two cartridges you're looking at has a
lower SAAMI max pressure, then this rule won't work. Example of this is the 300 Savage versus the 30-06 Springfield. If you apply his rule, you come up with 7% increase in MV for the 30-06 over the 300 Sav. If you multiple that by the published 300 Sav Remington factory load (2630), you get 184 fps. Add that to the 2630 and you get 2814 fps. But then reference the 30-06 (same table) and you'll see a published MV of 2910. That's almost a 100 fps difference. This is because the 300 Sav max pressure is 47,000 psi versus 60,000 psi in the 30-06.
One more example and then I'm done. If you look at the 30-06 versus the 300 Win Mag and apply his rule here, you end up another 100 fps shy of the actual Remington factory table data. I'll show the math again if someone wants to see it. The SAAMI max pressure differences between these two cartridges is 5,000 psi (much less than the 13,000 psi diff between the 300 Sav and the 30-06). Interesting...
How does the 1/4 rule apply to the overbore cartridges? It would seem that using just one rule of thumb for projectile motion would be rather obtuse. It might get you pretty close but it doesn't take into account different max pressures. In college physics I remember there being 4 different equations for linear motion. You had to examine the specifics of the experiment in order to determine which one or which combination of them you needed to use. I would think this to be no different.