Which is best for the XCR? We get asked this question all the time so we decided to attempt to answer it here.
First of all both are good rail systems. However, there are advantages and disadvantages of each.
Please refer to all three images as you read this. As you can see from the first image, the M-LOK Upper Receiver is wider and taller. If you prefer a narrower, smoother, and lighter handguard, Keymod is better.
Why is the M-LOK Upper wider? Refer to the 2nd and 3rd images. It’s wider because Keymod accessories mount perfectly flush with the inside of the Upper. In the 2nd image a BCM Gunfighter Grip is attached. Notice how elegantly it attaches! Now look at image 3. Attached there is an Magpul M-LOK accessory. The little nuts and bolts protrude into the upper receiver. This is why the M-LOK Upper is wider. If it were narrower, the nuts would make it so the XCR’s barrel could not be removed without first removing the accessory.
There have been some claims made that the M-LOK is stronger than Keymod and accessories attach to it more accurately. The first claim may be true, but the Keymod is more than strong enough. The second claim is false. Accessories attach to Keymod more accurately than they do to M-LOK. They Keymod mounting hardware simply positions the accessories more accurately than the M-LOK mounting nuts. We hope that this post is helpful. Both systems are very good.