Of the rails we / I have used and tested, other than a monolithic or semi-monolithic design (e.g., Cobalt, where the rail is technically a separate part but is designed for a matching proprietary upper with a “sleeve” that the rail locks into), older Geissele designs like the MK 4 / MK 8 (or my personal favorites, the “OG” 416 SMR and MK 1s) as well as the KAC URX4s are among some of the best rails to mount lasers to in terms of rigidity, stability, and minimal deflection, particularly when using barricades, cover, etc.

Doesn’t mean they’re the only ones and there aren’t others, or even potentially ones that are better, but of the ones I’ve tried, they’ve been the best.

That being said, unfortunately for most folks unless they happen to be a real KAC fanatic and have all the URX-specific tools, the URX4 is going to more or less be an armorer install, versus a rail most folks with basic AR tools can swap out at home.

As for the Geisseles, “Bendy Boi” memes aside, while still good, from a laser mounting-only perspective, the newer MK 13 / MK 16-style Geissele rails are a little less rigid than the older MK 4 / MK 8-style rails, which contributes to them being so light weight and the MK 8s only have four M-LOK “facets” as opposed to the MK 16’s six—but again, if you’re looking at only this one factor in isolation, the older design rails are probably “better.”

Something to consider about HK 416-derived and “inspired” handguard designs like the Geissele, is that the 416’s rail and the 416 in general is one a very very few fighting rifles built with handguard laser zero retention as a primary design factor / consideration, because the handguard was initially designed to be removed and replaced during routine servicing of the piston op-rod, for an organization that placed / places an extremely high priority on night vision fighting tools, and especially at the time, IR aiming lasers, which is part of what makes the HK 416 so interesting from a “night fighting” perspective.

And while some users may debate the degree of success HK was able to achieve in terms of zero retention during frequent removal and reinstallation, it doesn’t change the fact that the general design is / was inherently designed to optimize aiming laser mounting and maintaining zero, which appears to carry over even into HK 416 “inspired” handguard mounting designs (the Geissele barrel nut is essentially an HK 416 barrel nut adapted to a standard AR upper’s thread configuration and Geissele barrel nut wrenches can be used for the 416 and vice versa, and many many other manufacturers have since come out with rail designs utilizing very similar barrel nuts).

~Augee