I took apart some 5 round 450b mags last night and cut the spring to leave one full coil at the bottom and they will hold 7 rounds. It was necessary to stretch the spring some to extend 2 1/2″ – 3″ above the mag body before installing the bottom plate to provide sufficient pressure to lock the bolt open after the last round; however, loading and feeding felt better after replacing the spring with a standard 20 round magazine spring.
I also discovered that 20 round BMI 5.56 mags with the grey no-tilt follower will hold 7 rounds and feed fine in my rifle without the single shot follower.
Go get’em Jager, you’re well into the good stuff now and seem to have a great handle on the concept. Keep the case sealed, by keeping the pressures mildly up. As for the last round feed, early on Bushy used that extra follower to cure that, but of course it eliminated one round from the mag. They have since come up with a dedicated follower, simplifying the overall mag dynamics and the mags are still feed as impeccable as ever, especially in the newer stainless steel model magazine. To remind everyone, using a GI mag is completely a-ok, but the last round can be problematic. Running a ½” ball nose mill cutter down the center of the follower will assure the last round feeding, in GI mags and give you 12rds total. But if you want to feed all the different types of bullets then you really need the Bushmaster mag, especially for the pointed bullets. Of course, if you feed that 275gr well with the Magpul, then who can argue with success, others however may need the Bushy mag.
For years we used the GI mags without fault. The cartridges in the stack tend to tilt up a little when one lifts the bullet point, but feeding was and is never a problem. BM are prefectionist (thank God) and wanted a clean system, hence the deacated mag.
Running a half inch ball nose mill cutter down the center of th GI follower will yield a mag that has never given me any problems and 11 to 12 rounds, depending on the manufacture of the GI 30 rd mag.