Building a Heavy AR-15 Varmint Upper Part 3: Final Assembly & Test Fire

Building a Heavy AR-15 Varmint Upper Part 3: Final Assembly & Test Fire

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.


The EGW tube was loose on the float tube, but was secured with Loctite 242 after the threads on both tube and nut were sprayed with KG Gun-Kote to eliminate the excess clearance.
Norcross drilled and tapped a hole in the rifle ís fore-end tube for a bipod stud, a very useful accessory for the varmint shooter.
The ejection port of the Mega upper receiver had to be lowered to clear the side-cocking handle. Letting it drag would have been a recipe for malfunctions.
The GG&G GS-1 rail is secured to the upper receiver with a full-length clamping plate. Its length means you can secure almost any sort of scope with the proper eye relief.
The extended rail provided proper eye relief for the large Sightron SIII scope. Getting the proper scope height and eye relief are essential to accurate shooting.

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Recommended for You

Customers who purchase any new Mauser or J.P. Sauer and Sohn rifle from an authorized dealer's in-store inventory will receive a coupon code for 50% off the online purchase of one Minox optic. Industry

Blaser USA Announces Special Promotion in Partnership with Minox Optics

Blaser USA - July 19, 2019

Customers who purchase any new Mauser or J.P. Sauer and Sohn rifle from an authorized dealer's...

We at Firearms News hope that you have a safe and happy Independence Day! 2nd Amendment

It's Independence Day NOT Just the Fourth of July!

FAN Online Editors

We at Firearms News hope that you have a safe and happy Independence Day!

Many of the concepts pioneered by Hobart are seen in modern armies across the world and the AVRE platform is alive and well. Historical

Hobarts Funnies and the Battle for the Scheldt

Neal S Shera - July 15, 2019

Many of the concepts pioneered by Hobart are seen in modern armies across the world and the...

See More Recommendations

Popular Videos

Springfield Armory Saint Victor in .308

Springfield Armory Saint Victor in .308

The Saint Victor in .308 features a 16" light weight barrel, M-Lok® free float hand guard and included flip-up sights.

Performance Center M&P Shield M2.0

Performance Center M&P Shield M2.0

From Smith & Wesson, the M&P Shield M2.0 is a great option for a carry gun with optics option.

Shooting the 20mm Lahti

Shooting the 20mm Lahti

David Fortier had the chance to fire the Lahti L-39 while filming the 2018 season of Guns&AmmoTV while on location at DriveTanks.com.

See more Popular Videos

Trending Stories

A $50 Ring Loc-Kit gives AirForce Condor airgunners unparalleled power and accuracy. Air Guns

AirForce Airguns Condor Ring-Loc Kit Review

Tom Gaylord - May 29, 2019

A $50 Ring Loc-Kit gives AirForce Condor airgunners unparalleled power and accuracy.

No one can restore receptivity to another Trump term with these disaffected gun owners but the president himself. 2nd Amendment

2020 is Trump's to Blow with Gun Owners

David Codrea - July 09, 2019

No one can restore receptivity to another Trump term with these disaffected gun owners but the...

Governor Northam is calling a special session of the General Assembly on Tuesday, July 9th, to do nothing but pass more gun control! 2nd Amendment

Virginia Governor Northam Declares War on Gun Owners

Virginia Citizens Defense League - July 09, 2019

Governor Northam is calling a special session of the General Assembly on Tuesday, July 9th, to...

See More Stories

More Gunsmithing

Manufactured by the famous Belgian gun works, Dumoulin, this action is a perfect platform for a Gunsmithing

Sarco's Dumoulin Large Ring Commercial Action

SGN Online Editors - April 14, 2014

Manufactured by the famous Belgian gun works, Dumoulin, this action is a perfect platform for...

I have long been a big fan of Savage 110 series bolt action rifles. They are an extremely good Gunsmithing

Rebarreling a Savage Model 11

Gus Norcross - February 04, 2014

I have long been a big fan of Savage 110 series bolt action rifles. They are an extremely good

Melonite is very resistant to corrosion and is claimed to be a better treatment then hard chrome. Gunsmithing

Sarco Melonite Finished M4 Barrels

SGN Online Editors - April 30, 2014

Melonite is very resistant to corrosion and is claimed to be a better treatment then hard...

See More Gunsmithing

GET THE MAGAZINE Subscribe & Save

Temporary Price Reduction.

SUBSCRIBE NOW

Give a Gift   |   Subscriber Services

PREVIEW OUR CURRENT ISSUE

GET THE NEWSLETTER Join the List and Never Miss a Thing.