Z1r, You are right, but the o.p. said "custom" FN. As an example, let's say it was a military contract action for 8x57 and later sporterized to say, a .257 R...then the feed rails are trying to contain a shorter, reduced diameter, increased body taper cartridge, and this can create feeding problems. Where the original 8x57 will feed flawlessly, slow, fast, and upside down 4 ways from Sunday...a large divergence from the parent design may not feed flawlessly. Barreling a military Mauser in .284 Win. is a classic example, I've done 3 or 4 and they all needed tweaking before the customer took delivery.
