The result when this occurs is the same about 90% of the time. When the bolt is released, the magazine is thrust downward in the magazine well/lower receiver, runs past the mag catch, and falls out of the rifle. No round is chambered as a result.
Upon examining the lower receiver, it became apparent that the bolt catch does not limit the upward movement of the magazine sufficiently to prevent it from overriding the mag catch. As a comparison, the design of the AR bolt catch physically limits the upward travel of the magazine when inserted to prevent mag catch override.
The upward travel limit of the ACR bolt catch is quite generous, and the magazine can physically move farther up the magazine well/lower receiver than it should.
In addition, the mag catch spring on the ACR is rather weak, in comparison to the AR mag catch spring. With the ACR, this acerbates the issue by not "catching" the magazine on the way in, nor "catching" it on the way down when the bolt goes forward. This accounts for the magazine falling out of the rifle when the bolt is released - the mag catch spring was stronger, the magazine would likely be held in place and the top round would chamber.
I think Bushmaster should take a look at both issues, and perhaps upgrade the mag catch spring to one of greater force. The bolt catch issue might be too much to address with a redesign, but a slightly stronger spring might do the trick.
I think this rifle has great promise, as it is accurate and ergonomic in most other respects. But this magazine/bolt catch issue and the selector lever need some retooling.
