The .223 Remington has a taper on the body of the cartridge of 1° compared to the .450 BM which has a taper of .90° i.e. essentially the same. Comparing the feedlips of the BM .450 BM magazines that I have they look a little different than standard AR15 GI magazines. Perhaps BM made a change in recent years to improve feeding.
I would be interested to know if BM is using a proprietary mag body now with different feed lips... from your picture is doesn't look like it though.
I found the OD of the BM cartridge was just
slightly larger than the gap between the mag lips. When more than a couple rounds were loaded, the spring pressure would force the lips to spread, allowing the front of cartridge to assume a nose up attitude. How far up depended on how many rounds were in the mag... so essentially, the orientation of the top round in mag was different for every shot.
Looking at your pictures, if you were to lift with slight pressure on the nose of the cartridge straight up, the cartridge would slip right between the lips and orient upwards. With a fully loaded standard mag (without a BM round limiter installed) the rounds could do this by themselves, particularly if the mag was handled roughly.
The problem was aggravated by the poorly compatible taper angle of the cartridge. Regardless of follower (I used the BM supplied mag followers), due to the taper angle, when the rounds are stacked they wanted to naturally assume a nose up attitude, which caused slipping through the lips. The more ammo loaded in the mag, the more the spring force and the more the compounding taper angles aggravated the tendency for mag lip spreading and random nose up orientation of the top round in the mag.
I found these problems to exists both with my factory supplied BM mags and with GI mags with or without BM followers installed. Now this was a few years ago so they may have finally made a proper specialized magazine by now... I would certainly hope so.
The cartridge itself seemed great though. I was dumbfounded that any engineering department of a major gun manufacturer would put such a gun/mag/round combination of the market. ANY reasonable product testing or engineering should have uncovered those problems and I can not believe BM was simply unaware of the obvious design shortcomings... rather I think they tried to instead make do by supplying round limited mags (5 rounds IIRCC) - and if only those round limited mags were used, the guns did function reliably inspite of the fact I never felt I could count on them to do so.
I would have felt a lot better about the whole deal if BM had a warning in their specs that ONLY downloaded / limited capacity magazines should be considered suitable for use with the rifles. To this day there is no such notification on their website page for the rifle.