Building a Heavy AR-15 Varmint Upper Part 1: Parts Selection

Building a Heavy AR-15 Varmint Upper Part 1: Parts Selection

A customer of mine recently expressed interest in building a heavy varmint upper receiver assembly for his AR-15. We discussed buying one of the excellent Rock River varmint uppers rather than building one, but finally decided to go the custom route to accommodate special features the customer had in mind.


He is a target shooter who shoots off a bench rest and sometimes a bipod. Weight was not a concern. Accuracy was our main goal, with cost also a consideration. We wanted to build the best rifle for the money. In this first article of a series we will list the parts we selected for the build and the reasons we chose them.

The Barrel: The customer requested a heavy barrel with a fast twist and 18 to 20-inch finished length. A muzzle device (flash suppressor) wasnít necessary. I briefly considered using a post-ban (crowned muzzle) DCM service rifle barrel. They are readily available from numerous manufacturers at a reasonable cost.


White Oak would have been a top choice based on my personal experience, but they were heavily backordered and I really wanted a heavier barrel than would normally be encountered on a black rifle.


Rock River produces extra heavy varmint barrels in lengths from 16 to 24 inches. I have always thought shorter barrels are more rigid and probably more accurate, all other factors being equal, and Iíve had good luck with the 18-inch length, so we went with a 18-inch RR stainless steel heavy varmint barrel in 1:8 twist.

These barrels are about as heavy as is practical, measuring 1.051 inches behind the gas block shoulder, .936î at the gas block and .923î from the gas block to the muzzle. The naked barrel weighed 3 pounds, 11 ounces. Dealer cost including shipping was $257, which I thought was very reasonable.

A gas block was supplied with the barrel, but it sported a rail that would interfere with the free float tube, so we didnít use it. My preference would normally be a rifle-length gas system on an 18-inch barrel. This one was mid-length, but I went with it.

Upper Receiver: If we're going to hang a super heavy barrel on an aluminum upper receiver, we ought to at least consider a stouter than average upper. Browsing the Mega Arms website, I found the Mother of All Uppers. The super heavy billet upper sports sidewalls 3/8î thick and does away with the forward assist and ejection bump. It defines massive. Weight of the Mega upper is about twice that of a standard one at 13 ounces.

Bolt Carrier Assembly: Since this is primarily a bench gun, I chose to go with a side-charging bolt carrier. The side-charger is simply more ergonomic for this style of shooting and for a shooter accustomed to bolt-action rifles, is probably more natural.

I could have drilled and tapped a standard carrier, but they are quite hard and it wasnít a job I relished, so I looked to parts vendors for a solution.

The Young Manufacturing NM side-charging carrier assembly is very nice and tempting but the cold reality of staying on budget steered me toward Fulton Armory. The stripped carrier with cocking handle was $98.96, but Iíd have to provide the gas key and screws to complete it. Total cost around $115.

My fiscal discipline faltered when purchasing the bolt itself. I chose the JP Enhanced Bolt Assembly from Brownells (452-000-111) hoping for better than average quality in this critical part. At $122.99 retail, it costs several times what a mil-spec unit does and only long term testing will determine if my choice was frivolous.

Free Float Forend: We needed a fairly stout forend to support this heavy rifle on a bipod. There are lots of choices. We didnít need any rails, just a tube. I chose the EGW rifle-length free-float tube. Internal diameter was large enough to hide the gas block, installation was simple and it is very strong and reasonably priced.

Gas Tube & Gas Block: The gas tube is standard for mid-length gas systems. The gas block needed to be low profile to hide under the float tube with an internal diameter of .936î to fit the fat barrel. I used a steel Troy gas block.

Optics Platform: My customer likes big scopes. The scope going this rifle is a Sightron SIII 8-32x56 with 30mm tube. Obviously, the standard M4 top rail isn't going to be long enough to support it, so I ordered a GS-1 extended top rail from GG&G. Iíll have to test fit some rings to see what will work best.

In the next segment of this article we will assemble the upper.

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