Building a Heavy AR-15 Varmint Upper Part 3: Final Assembly & Test Fire
January 27, 2014
In Part 2 we left off with our gas system installed, ready to screw on the fore-end tube. Most AR builds involve no fitting of parts, but this one is a case study in things that can go wrong when piecing a rifle together.
We had a barrel nut that wouldn't fit over the barrel, a gas block that had to be opened up for proper fit and now as we test fit the EGW float tube, we find a loose tolerance between the external threads on the nut and the internal threads in the tube.
It wobbles around. While it should be a class 3 fit, it feels like a class 2. We are going to secure the tube with Loctite but thread lockers aren't designed to fill gaps and we need to tighten the fit so the tube is straight when installed.
My refinisher came up with a solution. We painted the threads with KG Gun-Kote and baked it on. The paint filled the gaps and the tube threaded on to the nut with little movement. It was secured with Loctite 242 threadlocker.
If we ever need to remove it to gain access to the gas system, the barrel nut can be warmed up with a propane torch and the tube will screw right off.
A bipod stud was installed at the customer's request.
Another problem cropped up when I test fitted the bolt carrier. When the side cocking handle was installed, it rubbed hard on the bottom of the ejection port. The receiver was set up in the mill/drill and the ejection port was lowered .040-inches for clearance. Problem solved.
The extended scope rail was installed next. The GG&G GS-1 rail attaches to the upper receiver with a full length clamping plate secured with two screws. This is a rigid, high quality rail selling for around a hundred bucks. The added length allowed us to mount the large scope securely with proper eye relief.
For scope rings we chose 30mm low TSR aluminum rings from TPS Products (Brownells 100-000-957). When mounting a scope on any AR we want the center of the optic to be 1.4- to 1.5-inches above the top of the receiver for proper head position. The GG&G rail added .5-inches and the TPS rings measure .925-inches to the ring center, so scope height is about as low as you would want it. It looks lower than it actually is due to the added height of the extended rail.
After mounting the scope, all that was left to do was test firing the upper and zeroing the scope. Our customer is mainly interested in 77-grain bullets and we will zero the scope at 100 yards with ammo in that bullet weight. The lower receiver assembly designated for this project is still awaiting a backordered trigger, so I grabbed a spare lower out of the rack and headed to the range.
Range conditions this time of year in Maine are frigid at best. I didn't spend a lot of time shooting but I zeroed the customer-supplied Sightron SIII 8-32x56 scope and fired a few preliminary test groups. 5-shot 100 yard groups with a 3-pound trigger varied between .4- and .5-inches with handloads (Nosler 77 BTHP and N-140) producing the best targets.
I suspect with a lighter trigger and some load development, we can get it down under .4-inches without too much trouble. Black Hills 77-grain BTHP ammo was a very close second to the handloads, producing consistent groups. The Rock River barrel didn't seem to like 69-grain Federal Gold Medal match ammo, although most rifles thrive on it. Each barrel is different.