Black Rain Ordnance billet upper and Centurion Arms CMR

[Leonidas24]

Yesterday I spoke with a customer I've known for years who was interested in selling a Black Rain Ordnance PG6 in 5.56. I wasn't able to offer him what he wanted because the value of the gun even at 90% condition was less than half what he paid for it several years ago. We came to an agreement to modernize the gun by getting rid of BRO's 7" carbine handguard, and replacing the cheap railed gas block with a low profile one. He hasn't decided what handguard he wants yet, but has gravitated to the Centurion CMR in MLOK. If anyone has any insight to the fitment between the BRO billet upper and the Centurion CMR, it would be greatly appreciated. Midwest Industries Gen III handguards are also an option, as are the BCM KMR-A, and Centurion's C4 picatinny handguard. Any of these will be dependent on proper fitment between the anti-rotation tabs on each, and the BRO upper.

We may also end up replacing the BRO factory barrel with a BA 16" midlength Hanson, and luckily the receivers aren't finished in the gawdy skull decals, but that's for another discussion and I'll likely post up the results once the project is complete. Thanks in advance.

[Leonidas24]

Update: The CMR would not fit this upper, for anyone curious. Not that I expect a plurality of the membership at M4C to be running BRO upper receivers, it's useful to know nonetheless. We ended up deciding on a KMR-A that fit more or less. A few key notes:

The BRO muzzle device (the pineapple one) and crush washer obliterated the threads when removing them. I ended up cleaning up the threads before doing anything else. The Dead Air muzzle brake went on without an issue. Looking cool was definitely a deciding factor in picking a muzzle brake, but considering the customer is considering a suppressor anyway it worked out.

I destroyed a punch trying to remove the gas tube roll pin. Closer inspection revealed the punch had slipped into the inside diameter of the roll pin inside the gas block and got stuck, and would not budge. I wasn't about to lose sleep over a $5.00 punch, the whole thing along with the old gas tube went in the trash. The replacement gas block and tube were both from CMMG, which for a $35 block was good enough for today. The original block was a three piece railed version that did not match with the upper rail height.

The barrel nut was a two piece design (like the old PWS, or Odin handguards) that used a trunnion which was threaded on the ID that allowed a barrel nut to torque the barrel in place, while the handguard was secured with 8 screws to the trunnion. The trunnion was also secured to the barrel nut with two set screws. The assembly was misaligned with the upper receiver gas tube hole by a couple degrees, and made removing the gas tube a pain in the ass. After loosening the two set screws which secured the trunnion/bbl nut assembly, the entire thing untorqued with minimal hand pressure. The threads on the upper receiver were coated with a bronze colored grease.

We retained the BRO factory 16" stainless M4 profile barrel for the time being. Customer will in all likelihood end up with a BA Hanson 16" in the near future. I didn't have any pin gauges with me, but the gas port appeared around .08" or slightly larger. I didn't take a photo but I compared it to a barrel known to have a .072" port and the BRO port appeared larger.

The BCM KMR-A went together as normal until it was time to seat it flush with the upper. The anti-rotation widget wouldn't fit around the upper receiver. Some forceful pressing and light taps with a mallet forced it on to the upper without any visible scratches or flaking from the NiBx finish on the receiver.



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