.257 cal Monocoque PCP

For some time now I have been contemplating what my winter project should be, now that the Motel is slowing down, and the snow is fast approaching…. This is the time of year when I get to play in the shop, and bring to life things I have been thinking about all summer…. This year’s project is very ambitious, a scratch built rifle using only a few purchased parts…. the barrel, trigger group, scope, the fasteners, and a few small items like the gauge, picatinny rail and mounts, etc…. Here is a sketch of the general arrangement…. 

This gun is a monster…. It’s 48″ long, with a 33″ barrel, and will weigh an estimiated 12 lbs. plus scope…. It’s heart will be a 7″ twist, .257 cal TJ’s barrel from a new mandrel I am having made…. The fast twist is necessary to tame the new 113 gr. Bob’s Boattail bullet I designed, which is basically a scaled down version of the 200 gr., .308 cal “Whiteout” BBT that won the 200 yd. EBR in October…. This is its intended fodder…. Molds will hopefully soon be available from NOE….

In order to push that bullet into the mid 900s, I will be using 3800 psi, normally supplied by tethering to a regulator on a 4500 psi Great White tank…. However, the reservoir, which is made from 1.25″ OD x 0.095″ wall CrMoly, will be 400 cc, so I should be able to get a few shots off tether within a 1-2% ES…. The main tube runs the full length of the rifle, and houses the valve, hammer, hammer spring and preload adjustment…. and provides a mount for the PRod trigger group and a home-made adjustable stock, to be machined mostly from aluminum, and a short Picatinny rail up front to mount a BiPod…. The hammer has it’s own cocking knob, and it’s free-floating, ie when the gun is uncocked it rattles around in the ~0.1″ space between the valve stem and the captive spring guide…. The 1/4″ diameter steel spring guide slides through an adjusting screw mounted in a block bolted into the tube…. It carries the long hammer spring, preloaded to between 5-10 lbs. of force when uncocked, which maximizes the hammer velocity without requiring more than 20 lbs. of force when cocked…. I have used this arrangement before, and I really like it…. It makes for a lighter and more constant cocking effort, and not having the hammer spring in constant contact with the hammer, and yet lots of preload, reduces or eliminates hammer bounce…. The preload adjustment is made through the end of the main tube with a long socket wrench….

The exciting part of this design, to me, is what has been deemed the “Monotube” upper structure…. This is a 1.25″ OD x 0.065″ CrMoly tube that runs nearly full length…. The breech block, which will be machined from 2024-T3 round bar for a sliding fit inside the tube, extends from about 4″ ahead of the loading port to the back of the tube…. In that one piece, it mounts the barrel, serves as the chamber, transfer and loading port, and bolt carrier…. It will be solidly bolted inside the tube, and the barrel threaded into it securely…. On top of the tube will be a Picatinny rail for the scope, bolted to the tube and breech block, ahead of the loading port…. At the front of the tube is a short aluminum plug, which is a close fit on the barrel, also bolted into the tube, 1″ from the front…. The muzzle of the barrel (recessed just inside the tube) will be threaded 1/2″-20 NF, and a stack of Belleville disc springs will slide over it, and be tensioned by a simple 1/2″ nut, tightened by a socket wrench…. By using a stack of five, 0.073″ thick Bellevilles in a series arrangement, I can achieve an adjustable tension of up to 1300 lbs. on the barrel in two turns of the nut, with a corresponding amount of compression in the outer tube…. Once the tension is applied, the barrel, tube, breech block and scope mount will work as one solid piece…. The object is the maximum possible rigidity, approaching that of a solid 1.25″ diameter barrel with the breech machined into it, but at a fraction of the weight…. Not a lightweight, for sure, but light for how solid it is…. There is a simple, double-concave aluminum spacer at the back, between the two tubes, holding them 1/4″ apart (so the barrel tube is free floating), with through bolts securing the breech block in the upper tube to the valve and hammer spring carrier in the lower tube…. holding the two tubes parallel….

The most expensive part of the whole thing is the Millet 6-25 x 56 SF Scope (obtained from Eric at http://www.scopesandammo.com) which my wife will be giving me for Christmas…. It is massive, 22″ long with sunshade, and weighs 2.2 lbs., built around a 35mm tube…. The MilDot-with-bar Reticle is calibrated for the maximum magnification (25X) which means the dots are far enough apart to allow sighting at 100 yards and have the top of the lower post be the point of aim at 200 yards…. The distance from the dot-to-bar becomes 1 Mil at 12.5X, and that point is marked on the zoom ring, so at that setting you have a MilDot reticle, but with 10 dots available in every direction…. The Elevation turret has 140 MOA of travel because of the huge 35mm tube, which should allow enough to reach out about 1/4 mile…. I will be machining a 20 MOA slope in the Picatinny rail so that the trajectory intersects the line of sight at 100 yards with the scope centered…. Hold under, using the MilDots, will be used at ranges under 100 yards…. Funny thing is that the scope doesn’t look at all out of place on such a long rifle….

I’ve ordered the barrel mandrel, and the CrMOly tubing and 2024-T3 bar stock…. and Mrs. Claus ordered the scope…. so this project is off and running…. and I can’t wait to start making chips….

Bob