Special Purpose Rifle (SPR) Upper Receiver Build

We decided to take a crack at building a .223/5.56MM Special Purpose Rifle style upper receiver for an AR15. We wanted to end up with an accurate rifle, low recoil impulse that is extremely shooter friendly.

Rather then go specialized tool crazy. we decided to minimize what we used to make the project easier to assemble. Brownells $99.99 AR-15/M16 Critical Tool kit provides most of the specialized tools we use here. The kit, consisting of an armorers wrench, lower receiver block, receiver action block, detent depressor, pivot pin installation tool an bolt catch pin punch provides a builder the essential rifle specific tools needed to work on their AR-15/M16.

To build this rifle (we already had a CMMG lower), we ordered the following items: (Note, we included the lower receiver parts and will be adding an article outlining complete assembly of the rifle in the near future.)

DMR build parts — SPR upper build parts have been gathered.

AR upper in block — Using the receiver action block and insert from our AR-15/M16 Critical Tools Kit, we secure the upper receiver. The insert is a key piece, preventing the aluminum upper from bending under torque. The clam shell stye block is secured in a vise.

SPR barrel inserted into upper — We gently slide the White Oak Armament Barrel into place on the upper.

PRI gen 3 forearm — This is the PRI Gen 3 forearm we will be installing. The forearm, equipped with a long top rail, and the shorter side and bottom rails, is secured via a barrel nut (bottom left) and forearm lock ring (bottom right).

photo (92)barrel nut — We lubricate the upper receiver threads, insert the barrel, and place the the forearm lock ring and then barrel nut on the barrel in the manner shown.

Smith Enterprise Armoer's Wrench — Smith Enterprise Armorer’s Wrench included with the AR-15/M16 Critical Tools Kit. Note the three metal dowels (right) on the end of the wrench. These will secure the the PRI barrel nut.

torquing the barrel nut — A torque wrench attaches to the Smith Enterprise Armorer’s wrench to ensure the appropriate amount of torque is applied. Per PRI’s instructions, we tighten the barrel nut to 45 ft/lbs and loosen the barrel nut three times. On the third time, we tighten it to 45/ft/lbs and then index the barrel nut so the gas tube can pass through without exceeding 65 ft/lbs of torque.

PRI front sight and gas tube — The PRI flip up front sight is machined to tight tolerances and requires steady pressure to insert the gas tube. To accomplish this, we use our gas tube wrench. The wrench gently clamps onto the gas tube, preventing damage, allowing it to be inserted and properly indexed.

PRU flip up front sight — The rifle length gas tube is secured to the PRI flip up front sight gas block via a roll pin. Using a roll pin starter punch and a bench block, we drive the pin flush.

Tightening PRI Gen 3 forearm — We place the free float tube over the barrel and then slide the front sight assembly into place, guiding the gas tube int the receiver. We then secure the forearm lock ring to the forearm using a strap wrench. PRI makes a spanner for this task, but we has good results with our strap wrench.

PRI flip up front sight — We apply blue thread locker to the cross bolts on the flip up front sight and tighten them in place. Note the bright stainless steel finish of the barrel was scraped when the sight was slid into place, we will polish this out later but will deburr the leading edges of the sights in the future.

With the barrel carefully secured in a vise, we install the Surefire brake using our armorer’s tool.

surefire indexing wheel — The Surefire brake includes an indexing wheel (top) that allows the installer to use the different thickness washers (bottom) to properly index the brake.

We selected the PRI gas buster charging handle for this build. The over sized latch doesn’t protrude too far from the rifle, yet still allows positive manipulation of the charging handle when optics are in place.

EXO BCG — A Fail Zero EXO coated bolt carrier group completes the upper receiver. This proprietary coating adds lubricity to the moving parts and makes them easier to maintain.

With our upper receiver complete, we grabbed a CMMG lower we had lying around. Installing a Magpul Gen-2 PRS stock and Geissele DMR trigger, the rifle was complete. To top the upper, we selected a Leupold Mark 4 2.5-8×36 M2 TMR scope in an ADM recon mount. Initial testing was quite positive, with three different loads coming in sub-MOA. A complete review will be following shortly.