rummer, I use a 250 grain cast bullet (actually ~245) in my whelen at 1995 fps. This IS a chronoed velocity. I do NOT feel under gunned by this velocity. I bet you take any 3 factory 35 whelens and chrono them, you will get a variance in speed AND accuracy! have you really calculated how much difference that 250 fps less affects the "killing 'power" of the Whelen? Hornady 4th ed,page 425, only shows 2400 fps in a 22" model 700. Using the 250's ,with the 200's they show 2650fps with only ONE powder
From your account of shooting results I don't see a problem with the .358 dia bullets. IF your barrel IS smaller than it should be ,you should be getting HIGHER velocity BECAUSEof higher pressures! You would be seeing more evidence of flat primers and expanded pockets too. IF YOU ARE HOT RODDING the loads to get that extra few fps you are heading into dark waters. 1.25 groups from a 250 grain slug will kill ANY deer in America and most anything else you want to hunt with it!
When I had that nasty 25-06 mentioned before, I got LOTS of signs that some thing was wrong. Blown primers were just one,since this is not your problem and the bolt is not "sticky" ,WHY SPEND THE MONEY ON A GOOSE CHASE? USE IT TO BUY SOME hORNADY BULLETS!