scope-eye is absolutely correct with his answer - it can't be explained any better than that.

Is the ammunition to spec, and not someone's reloads?

If the FGMM is crimped as is sounds that it is, I would pull down the rounds using a collet puller, resize the necks to remove the crimp, and re-assemble the rounds with the bullet seated deeper in the case.

If this is a constant problem with your rifle, it may be worthwhile to slightly increase the freebore of your rifle, after consulting with a couple different qualified gunsmiths, of course.