I can't stand it anymore. The .270 Gibbs has at most 10% more case capacity than the standard .270, which does NOT equate to 150-200 extra fps at the same pressure. Instead it would result in about 75 fps. This is according to the 1/4 rule: Any increase in case capacity with cartridges of the same caliber results in a potential increase of 1/4 as much velocity AT THE SAME PRESSURE.
The "wildcat" syndrome rarely follows this rule, because when most people obtain a wildcat rifle, they load it up until the first "pressure sign" shows up (usually an ejector-hole mark on the case head) and then back off the load a grain. Since ejector marks usually don't show up until presure is at least 70,000 psi, backing of a grain might bring the load back down to just under 70,000. NATURALLY this increases velocity considerably over the typical 60-65,000 psi of factory ammo or handloads using tested data.
To get another 150 fps AT THE SAME PRESSURE as the standard .270 Winchester, we have to go to the .270 WSM case, which has about 20% more powder room. To get 200+ fps more we have to use the .270 Weatherby, which has 30% more case capacity.
I have seen these "comparisons" of factory rounds and various wildcats over and over again. Some people even compare the fairly wimpy ballistics of some factory ammo with that they get by super-stuffing an improved version. One guy even published an article about how he got 250 fps more by "improving" his .35 Whelen. This was a comparison between the 2400 fps Remington 250-grain factory load and what he got by loading the hell out of the AI to get 2650 fps.
The thing he missed, however, is that the .35 Whelen Improved only results in about a 4% increase in powder room, because there isn't much shoulder to blow out. This translates to a 1% increase in potential velocity when (again) the two rounds are LOADED TO THE SAME PRESSURE. And 1% in any .35 Whelen load is about 25-30 fps.
