I bought a Mega Arms Billet GTR-3H stripped lower receiver today with the intention of using it for an SBR build. I love the Geissele MK5 rail and want to use it on my gun, but I've been told that the MK2 doesn't fit very well with some billet receivers. Is this a problem with the MK5 and Mega Arms receivers? I didn't think it would be an issue since the receiver is mil-spec. And I can't seem to find any information on such a specific problem. If anyone knows anything, replies would be much appreciated. Thanks.

The fit issue with the Geissele SMR rails is generally with the upper receiver, not the lower.  Specifically, billet uppers that are not milspec width at the bottom of where the rail attaches will not fit in between the anti-rotation tabs.  This requires filing down the tabs (or the receiver, if possible) to mate the two.  I just went through this with my ASA upper and a Mk4 M-Lok:

Before:


After:

Mega has two different billet lower receiver models: one that fits forged upper receivers and one that is machined specifically to fit the company's upper billet receiver. I bought the one that fits the billet upper receiver; thus, I'm locked into buying the billet upper receiver down the road. But the upper is mil-spec and I assumed that its mil-spec for the entire unit. I didn't think that some had dimensions that weren't.

Thanks for the pics. I can see how that might be a problem. What did you use to file it down with?

Mega Arms product description says that their upper accepts the anti-rotation tabs! Sweet!

A ... um ... file.  

the geissele tabs are different, MEGA redesigned their uppers for handguards that has anti-rotation tabs on the top, not the bottom

The "H" lower is machined to match the hexagonal shape of their billet upper, but it will still accept a normal forged upper.

Aside: I'd say that it's octagonal (side are perpendicular to sides):



I'm considering to match the SMR/ASA profile that I have going, which are both octagonal.

I just picked up this upper and hand guard. They do not work together out of the box, I took a dremel with sanding wheel to the hand guard tabs and knocked them down a bit now it fits like a glove.