A 100 grain .277 bullet has a sectional density of only .182 while a 6mm 100 grain is .242. Most 100 grain .277 bullets are thin jacketed bullets designed for varmints which means they tend not to hold together very well. If you insist on hunting with them, get yourself a box of wet phonebooks and shoot into it then look at the bullets and make sure they are breaking apart too much. I find a 130 grain bullet for a 270 hard to beat.