Earlier this week we posted CMMG’s press release announcing their new MkG45 Guard, a .45 ACP carbine with a “radial delayed blowback” action. A visit to the CMMG booth at the NRA Annual Meetings netted some hands-on time and more details about the gun’s unique action and last round bolt hold-open design . . .
Externally, it’s a modern pistol caliber AR carbine with a billet lower designed to accept GLOCK magazines — via a well-flared magazine well — at the proper angle. The ejection port is enlarged to clear those chubby .45 ACP cases, and the magazine release is oversized as well. Otherwise, it’s AR-15 dimensions and accepts standard AR triggers, grips, charging handles, forward assists, stocks, dust covers, etc.
Over to the BCG, it’s actually very much as we speculated in the comments on the press release post. The bolt lugs are angled at the rear, allowing the force of the case to spin the bolt, unlocking it from the barrel extension after the rotation-caused delay.
The “gas key” is unique as well. It isn’t actually used during the firing process, of course, but is still needed to have a functional charging handle. The design of the MkG45 gas key prohibits the BCG from being inserted into a standard AR-15 upper, which is a good thing because firing a .223/5.56 on a blowback system — radial delayed or not — wouldn’t be pleasant.
Most pistol-magazine AR lowers simply forgo a last round bolt hold-open. Those that have this functionality aren’t exactly lauded for reliability, with many running a flimsy, flex-prone bar all the way from the AR’s bolt catch location to the front of the magazine. On the MkG45, they’ve necessarily used a bar that extends to the front of the magazine, but it’s firmly affixed to the receiver at front and rear and rotates on pins. Rotating to the side, rather than lifting up in the front, provides a more secure, reliable, stiffer interface.