They have them (when in stock that is) for 5.56 also. If it is removed from the front or the back, I wouldn't think that would make much difference. Point was, take a PRI mag and have the front milled wide enough and deep enough to load up longer than normal rounds. I don't think you can cut the slot deeper than 10 rounds on the 5.56 magazine, and maybe not that deep. You might want to try it on cheaper CPD mags before cutting up a $40 PRI magazine. Cutting out the mag really does not gain you a whole lot of length anyway. I did cut a magazine for one of my 6.8 cats to see what would happen and I think I could only gain about .025 inch over the normal PRI length of 2.300 inch, which would be the same for a PRI 5.56 magazine also.