76/760 vs K mag fitting...
Brian:
First, I have never had the PLEASURE to handle a K. So I am only guessing here. Shooting a K is one of my little goals in life...
Yes, the front-to-back dimension of the mags is the key to fitting in a 76/760. I have heard some Suomi mags will fit in some 760s without filing.
The channel you refer to, is that channel on the back side of the magwell (since the mags have a raised section there)? Does your K have the removable magwell cover? If so, with the cover removed can you still insert & fire a stick mag (like the M31 Suomi)?
Please post some close up pictures of inside your K's magwell.
SubGunFan