varmintsinc,
Actually it is a rare "Ackley Improved" round that will gain 100 fps, with everything else being equal, namely pressure and barrel length. And this normally involved cases with sloping bodies and relatively small bores compared to case diameter. To gain 100 fps in a modern round capable of 3000 fps or, powder capacity must be increased at least 13% or so, since potential muzzle velocity only increases at 1/4 the rate of powder capacity.
For instance, two AI rounds that gain 100 fps or maybe a little more are the .250 Savage AI and the .300 Weatherby, which is essentially the .300 H&H Ackley Improved. However, each of these cases APPEARS to gain a lot more velocity than 100 fps, because the factory ballistics (and pressures) for the .250 Savage and .300 H&H are so low. Which is exactly why so many AI rounds supposedly gain a lot more than 100 fps.
Most handloaders also load them pretty warmly. They also tend to get custom AI rifles built with longer barrels. Put together a slight gain in case capacity, higher pressure, and a longer barrel and there are some gains. But load both rounds to the same pressure in the same barrel lengths, and it ain't near as much.
All of this is why the .35 Whelen AI doesn't gain much over the standard round, certainly not 100 fps. There isn't much gain at all in case capacity, because the case itself is already prestty straight, and there's almost no shoulder to blow out.
When I fooled around with the .35 Whelen AI, I found it gained about 3 grains in powder capacity, or about 4.5%, over the standard version. This translates to about a 1.1% gain in muzzle velocity, everything else being equal, or about 28 fps if we base out estimates on a 250-grain bullet at 2500 fps.
This doesn't mean the .35 Whelen AI is a bad round. Some people claim it headspaces more precisely than the standard version, and thus is more accurate. Maybe, but I suspect the real reason is that .35 Whelen AI rifles tend to have custom barrels, and are owned by meticulous handloaders. But it is still a fine round, certainly at least as good as the original.
