The Serbu Firearms RN-50 is a minimal break action rifle chambered in .50 BMG. It is an attractive option for a rifle of its caliber at a reasonable price point. Though the RN-50 accepts a standard AR-15 pistol grip and buffer tube, the relative position of these components is not the same as a MIL-Spec AR-15 lower receiver. Determined to install some unique wood furniture on the Serbu RN-50, I began the search for a compatible wood thumbhole stock. It turns out that such a stock simply did not exist for the Serbu RN-50… until now.
I designed this stock to be manufactured with a small format 3D printer using wood-plastic composite PLA filament. The stock has adjustments for both the length of pull (13.5″-14.25″) and a vertical comb offset. The stock is composed of 5 pieces – the body, rear adjustment block, pistol grip, butt plate and cheek rest. Each component is rigidly attached to the lower receiver about a 1.25″ diameter aluminum AR-15 pistol buffer tube to provide additional strength.
Each part must be finished before assembly to ensure proper clearance for all hardware. Before assembling any components, the buffer tube channel of the body is lightly sanded and cleared to allow proper fit to the pistol buffer tube. With the buffer tube attached to the lower receiver, the stock body must fit tightly and requires sufficient pressure to slide in place. The pistol grip is attached after the stock body is in place.
The pistol grip is bolted to the lower receiver and then bolted to the stock body at the base of the pistol grip with a 20mm M3x0.5mm bolt. The buffer tube channel of the rear adjustment block is sanded and then fixed to the stock body with two 20mm M3x0.5mm bolts. With these three parts properly fit, all five parts can be sanded with 150 grit sandpaper to achieve a smooth wood-like finish.
Finally, running some 60 grit sandpaper over the parts in a uniform direction can be used to achieve the appearance of wood grain. The parts and hardware are removed and are now ready to be stained. To achieve the depicted finish, four light coats of Minwax PolyShades Bombay Mahogany were applied and sanded smooth with 600 grit sandpaper between each coating.
The stock body is fitted to the buffer tube before attaching the pistol grip. The adjustment block is then attached to the stock body. Two 3/8″x2″ flat head bolts with lock washers are installed to the butt plate with lock washers and hex nuts. Without over-tightening, the hex nuts are rotated to align with the recess in the adjustment block. Two 20mm M3x0.5mm bolts with washers are used to attach the recoil pad to the butt plate. With the two 3/8″ thumb nuts in place, the butt plate assembly is fitted to the adjustment block and adjusted to length. The butt plate must be kept parallel to the adjustment block to prevent binding.
The adjustable cheek rest is attached with two 5/16″x2-1/2″ bolts using steel and plastic washers where contact is made with the surface of the cheek rest. A lock washer and 5/16″ thumb nut are used to adjust and tighten the cheek rest in place.
Notes on Production Model:
The production model varies slightly from the pre-production test unit depicted above. After testing the pre-production model, the stock proved to be robust and strong enough to handle the impulse of a .50 BMG. A number of internal improvements have been made to aid in the manufacturing process and further improve strength. Initial tests showed no catastrophic points of failure. After 20 test fires, a cosmetic fracture began to form at the junction of the stock body and pistol grip.
The production model has been improved to prevent this from occurring and no longer requires a bolt to secure these pieces together. The junction is now constrained with an expansion joint to prevent fractures from impacting the stock or bipod. The finger grooves have been made deeper and the length of the cheek rest extended.
Bill of Materials:
|KAK BUFFER TUBE||1||$20.00|
|PACHMAYR F325 RECOIL PAD||1||$28.99|
|AR-15 CASTLE NUT||1||$3.99|
|AR-15 RECEIVER END PLATE||1||$3.49|
|5/16″ THUMB NUT||2||$6.04|
|3/8″ THUMB NUT||2||$6.04|
|1/4″-28 1″ CAP SCREW||1||$0.28|
|1/4″ LOCK WASHER||1||$0.04|
|5/16″ STEEL WASHER||4||$0.25|
|5/16″-18 2-1/2″ SCREW||2||$1.20|
|5/16″ LOCK WASHER||2||$0.22|
|5/16″ PLASTIC WASHER||8||$0.63|
|3/8″-16 2″ FLAT SCREW||2||$1.26|
|3/8″-16 HEX NUT||2||$0.58|
|3/8″ LOCK WASHER||2||$0.31|
|M3x0.5mm 20mm SCREW||2||$0.20|
|M3x0.5mm HEX NUT||2||$0.11|