View Full Version : Adjustable gas needed with 12.5" AR and SPR/M4 can?

15 June 2012, 10:33
I will be blunt: All of Stickman's pictures of the 12.5" Noveske with an AAC SPR/M4 can, along with other similar rifles, finally pushed me to go NFA. I have the Form 4 for the can, and the Form 1 for my lower all filled out, ready to send when I get the money orders. That means that I now really need to start figuring out what is going into the upper.

The main thing that has me hung up is whether I really need an adjustable gas system (preferably "switchable") or not. I understand that 12.5" has a nice balance in terms of dwell time, muzzle velocity, and suppressor life. What I was looking to find out is whether the switchable suppressed/unsuppressed mode gas block or receiver is functionally essential as the suppressor would likely be going back and forth between the 12.5 and my SPR upper I finally finished (which reminds me, time to resurrect an old thread). Whether I need that or not will decide much in terms of barrel, receiver, and rails. I am mostly concerned with gas blowback and overall function/cycling. The lower will have the Vltor A5 stock & buffer.

The three combinations I was considering are:

Noveske 12.5" Crusader w/pinned Switchblock
9" FSB cutout style rail system (DD, Samson Evo, Centurion, etc)

Something like johnson's rifle here but with a switchblock and SPR/M4:



Innovative W.A.R. receiver
BCM 12.5 SS barrel
Low-pro block
full 9" rail (new Noveske, URX III, Samson or Rainier style Evo, Centurion, etc)


Get BCM 12.5", Noveske Switchblock pinned by ADCO
Vltor MUR
FSB Rails

From what I've seen, heard, read, and have at home, any of the manufacturers above will more than meet my "quality" needs. The only thing that keeps me from 100% just snatching the Noveske and caling it a day is that I usually shoot 55gr ammo to try and keep my cost/fun ratio favorable. I'm not out to shoot quarter size holes at 300 yards, but if the BCM with a 1/8 twist offers a good accuracy balance for 55gr and the occasional batches of 69/75/77gr, I'd like to go that route (not to mention the price difference). I'd also like to know if there is any risk of baffle strike using good 55gr, in any twist?

I'd appreciate personal experience with specifically 12.5" barrels and the SPR/M4, and any other regulated gas or piston systems that would allow clearance between the back of the AAC Miter Brake and still allow removal of the gas piston.

15 June 2012, 10:57
Absolutely NOT needed if you get a good 12.5 like BCM or similar. I'd NEVER run an AR that needed or had an adjustable gas system.

22 June 2012, 21:47
Especially with reflex cans, that differential isn't that huge. Go BCM, Centurion, Noveske, DD for the barrel, I'd consider going VLTOR A5 for the RE/Buffer/Spring setup with the SBR lower, and handguard (DD RIS-2 Mk18, C4 9.0, DD Lite 9.0, TROY/VTAC 9.0 all come to mind) and go from there. Unless you're really trying to minimize gas coming back to the rear when suppressed, I'd just run it with the A5, and it should run until the cows come home.

[Edit] you're already onboard with the A5 - go with that, and an EMOD, ACS-L, or ACS stock. The Mk18 handguard (normal) should be fine, and a fixed rail mounted front sight is a LOT more flexible than FSP since you're going with a custom upper anyway. Alternatively, I think Monty could build that upper with the Centurion 12.5" and C4 9.0; or find a BCM configuration that's already set up that way. The cost savings from either of those options will more than pay for a DD Fixed front sight.

28 June 2012, 19:21
Unfortunately the BCM pre-assembled uppers either have rails longer than 9" or are FSB type (Though I have a dremel handy and could easily shave the FSB). I was also leaning towards a modular tube style rail like the NSR, Evolution, etc. That's not a problem though, I have the receiver block and torque wrench here, and have built at least one upper without any apparent issues. Half the fun for me is building it. Does anyone know if the BCM barrels come dimpled? My White Oak SPR barrel did, however a DD lightweight stripped barrel I got a while back at the gun store was not.

Are there any issues with using a pencil or light profile barrel? I've heard about POI shift with pencil bbl/suppressor, but with the difference in diameter between the pencil barrel and the inside of the MITER mount I would be concerned as far as the mount having room to droop and mis-align the can. Is that an unfounded concern?