BCM MK2 uppers: Popular 3rd party rails that have fit issues?

[maximus83]

Want to get a BCM Mk2 ELW barreled upper. But saw a few mentions of issues with certain 3rd party rails (like Geissele) that allegedly won't fit the new MK2 receiver. Not sure if that's a real issue. But before the order, wanted to check with guys here who upgrade AR gear more often than me.

• Are there certain quality/popular 3rd party rails, such as Geissele Mk14, that do not fit the BCM MK2 upper? On the flip side, are there quality/popular 3rd party rail brands that folks have verified DO fit, besides BCM's own MCMR?
• If wide compatibility with 3rd party rails is a concern in this case, am I better off sticking with the BCM (or other brand) standard upper, and skip the claimed benefits (I admit at this point, mostly theoretical) of the BCM MK2 upper?

[JediGuy]

From my perspective, why not keep the BCM handguard?

If not doing that, FCD (CMT) also noted their handguards do not fit.

[maximus83]

From my perspective, why not keep the BCM handguard?

Fair Q, most reviews of MCMR sound positive. If it works well I'll keep it. But saw from searching old threads here/elsewhere that some of the other rail systems, like the G MK14 or certain Centurion options, can be more rigid and stronger than the MCMR. Wanted to keep open the option of upgrading the rail, if needed.

[Stickman]

Some rails need tabs filed down a bit to fit the mating section of the original MK2 receiver. Its not anything that is a big deal.

[maximus83]

Some rails need tabs filed down a bit to fit the mating section of the original MK2 receiver. Its not anything that is a big deal.

Ok that makes sense. So the proprietary cut is not that deep. If I got say, a Geissele Mk14 rail later, then it's just a small amount of filing the tabs and it would fit the MK2 receiver? And there's also no issue with the Geissele's longer/proprietary barrel nut?

[1168]

The URX 4 fits the Mk2. I don’t remember for sure, but I think the Larue does not.

[xFREDx]

you could also drill the pin hole in the upper and then get like the drivelock rail and shave off the tabs. or just use the QRF handguard unless you are set on an MLOK variety.

[grizzman]

My opinion is that ELW barrels should be paired with light weight handguards and lightweight stocks.

My MK2 has a QRF attached to it, and it's certainly strong enough for anything I'll be doing to it. My ELW barrels are surrounded by KMR and KMR-A handguards, and I don't expect to bend one any time soon.

[maximus83]

My MK2 has a QRF attached to it, and it's certainly strong enough for anything I'll be doing to it.

Is that the 16" ELW with the 12" QRF? That looks like a solid rail and nice setup, most of my pic rails have been Centurion and never tried a QRF.

What kind of precision have you been able to get with that setup, using quality heavier grain ammo, at 100y or beyond?

[grizzman]

That upper began life as a MK12 Mod 4, with their standard upper receiver, and a pre-A KMR handguard. It got the MK2 upper receiver when they were first introduced, and at that time I swapped the KMR out for a KMR-A. I didn't consider the lighter weight, but less strong KMR to be much of a benefit for this particular rifle. I finally found and installed the QRF about 6 months ago. My only other current full Picatinny rails are both old DD Lite units, and it compares very favorably against them.

I actually haven't shot it for groups with the QRF, but I expect it to continue to deliver the ~.75 MOA it did with the KMR. My standard 16" ELW barrel does really well with 69 grain FGMM and random loads with 69 and 77 grain SMKs. FGMM is sub MOA and everything else I've put through it does less than 1.5 MOA. I was impressed enough with it and my gov profile 16" BFH barrel (that shoots almost as well as the stainless MK12) that I assembled a near twin of the ELW with a BFH 16" and KMR-A, but haven't shot it for groups. It's my primary 5.56 HD AR, so I don't care much about seeing what it'll do at distance. I've got others set up for that.