I'm going from memory here but in a factory 24 inch Winchester barrel and using 4831 powder, the standard 270 Winchester would do just over 2900fps with a 150 Hornady. The Gibbs would do just over 3000. The 270 Weatherby about 3150. This was in the same barrel at similar pressures (judged by miking the case head).
My grandfather fireformed 270 Winchester factory ammo in his Gibbs until a case separated and blew the FN extractor out and away. He took the rifle back over to Rocky Gibbs for repair and got a lecture on headspace which he never forgot.
Gibbs did load to the limit of the brass and specified different maximum loads for different brass. He cautioned shooters that the top loads were only usable in some lots of brass. Many of his clients ignored this instruction and, consequently, many ran into trouble. In many cases, it seemed like Rocky over-estimated the intelligence level of his clients!
I have, from time to time, thought of chambering a Gibbs for myself but have always resisted the urge. It just hasn't seemed worth the effort. GD
