11.5" and 12.5" with the help of John Thomas
Mods: the upper receivers were assembled and modified by John Thomas with parts that acquired and sent to him. Lower receivers were assembled by me. Feel free to move the thread out of the custom build forum if needed.
With that said, the following two builds are SBR's. Build number 1 consists of:
Lower:
-Centurion Arms CM4 stripped lower receiver
-Centurion Arms 2 stage AST trigger
-FCD spring kit for all springs in the LPK except for trigger springs
-FCD pin and detent set for all pins and detents in the LPK except for those in the trigger and action spring
-FCD ABC/R serrated Bolt Catch unbiased
-FCD serrated magazine release button
-FCD EMR-A standard length serrated magazine catch/ambi mag release
-FCD PF-040 take down pins
-FCD REF carbine buffer tube
-FCD Castle nut
-FCD H1 endplate
-Sprinco white spring with Colt H2 buffer
-Magpul K2 grip
-Magpul CTR stock
Upper:
This started out as a complete Centurion Arms 11.5" FBI spec barreled upper receiver that consisted of:
-11.5" FBI spec HF CL barrel with 0.069" gas port
-Centurion Arms pinned lo pro gas block
-10" Centurion Arms C4 quad rail
-PRI Gasbuster CH
-Centurion Arms BCG
-A2 Flash hider
-Centurion Arms forged upper receiver
I sent this upper to John Thomas of Retro Arms along with a FCD 6315XE FH, SF shims, and a Daniel Defense RIS II handguard and had him time the muzzle device for use with a Dead Air Sandman S.
To make a long story short, I was on a time crunch due to having to be out of town - combined with USPS delays in getting the upper to John in the first place - but he sent the upper to me 2nd day air. Of course I had to pay the difference in the original shipping quote, but John sent the upper back my way before I could even mail him the Money Order for shipping payment. The upper made it with one day to spare.
On top of that, the work on the upper was as good as you can ask for.
Once it came back to me, I added:
-Scalarworks Peak fixed front sight
-Magpul VFG
-Magpul MBUS 3 flip up rear sight
-Scalarworks 1.93 LEAP Aimpoint mount paired with Aimpoint T2
-FCD ATA caps on Aimpoint T2
The purpose of this rifle is for mostly dedicated suppressed use (once the can clears). I chose the FBI spec barrel mainly for the gas port size (0.069 was the description on Centurion's product page, but I never measured it to verify). I also liked that the FBI spec barrel was a heavier profile which at least in theory would do better with a relatively heavy suppressor hanging off the front.
Build number 2:
Lower:
-SOLGW stripped lower receiver
-Centurion Arms 2 stage AST trigger
-FCD spring kit for all springs in the LPK except for trigger springs
-FCD pin and detent set for all pins and detents in the LPK except for those in the trigger and action spring
-FCD ABC/R dimpled Bolt Catch unbiased
-FCD dimpled magazine release button
-SIONICS magazine catch
-FCD PF-040 take down pins
-FCD REF carbine buffer tube
-FCD Castle nut
-FCD H1 endplate
-Sprinco white spring with Colt H2 buffer
-Magpul K2 grip
-Magpul CTR stock
Upper:
-SOLGW stripped upper receiver
-12.5" Centurion Arms Lightweight Carbine gas HF CL barrel
-Centurion Arms pinned lo pro gas block
-FCD 6315XE Flash hider
-Centurion Arms Carbine gas tube
-Larue 11" Gooseneck quad rail
-Magpul VFG
-BFG picatinny sling loop
-SOLGW BCG
-FCD Ejection Port Cover
-Colt CH
-Scalarworks Peak fixed front sight
-Unity FAST Aimpoint mount with Aimpoint T2
-FCD ATA caps on Aimpoint T2
-FCD Low snag dimpled forward assist
I sent all of the above parts to John Thomas for assembly. I don't currently have a garage and the necessary tools to assemble uppers myself, and for how fast John gets it done for a very reasonable price, it's just easier. Like the above upper, turnaround was very fast, and the work was done perfect.
The purpose of this upper was for a GPR inspired by the One Gun solution thread. I went with the Unity High Rise mount over a 1.93 for a couple of reasons. First, I was able to get the bundle consisting of the Unity mount + Aimpoint T2 for less than the Scalarworks Leap bundle. Second, I wanted to see what the hype about the taller than 1.93 mounts was about - being that I really liked the 1.93. I have had neck pain issues it makes a noticeable difference for me in shooting comfort.
I found that shooting at RDS distances, the higher mounts were much more comfortable. I do also like the built in rear sight in the Unity mount for a BUIS, I was surprised to see they include a front sight post that can be mounted into the sight as well, but I omitted that since I had the Peak front sight already. The plan is to pair the Aimpoint with a magnifier. I was going back and forth on whether to try and LPVO on this set up, but after going down a rabit hole of which LPVO to get I decided to try the Aimpoint T2 and magnifier first.
I don't currently live anywhere that I would have to shoot past 50 yards anyway. The 12.5" upper will host a Dead Air Sierra 5 once it clears.
The last weekend I took both rifles out and put 1500 rds through the 11.5" FBI spec barreled upper. It was only lubed before the trip and then run non-stop over two days with about 1000 rounds the first day and 500 the second day. I had only one FTF which looked like the result of a short stroke at about 800 rounds. I am pretty sure it's because I threw a Sprinco Blue spring to try out and the rifle was pretty dirty by that point anyway. I threw in the Sprinco white back in and it ran without hiccup right after that.
The 12.5" barrel upper had 800 rounds put through it in the same trip.
Both rifles were consistently hitting steel at ~180 yards.
