Greg,
Is it not simply the case that the TMK projectiles are longer/slimmer at the "sharp end" than, say, the equivalent weight SMK and others?
I was put off using them in my .308 as the OAL increased to the extent that they wouldn't fit the mag if powder space was to be preserved. Presumably the same would apply to your .308 too. Is it a Remington by any chance? In other words, if you seat the TMKs to mag length you risk losing case volume and having a compressed load.