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Thread: .257 cal Monocoque PCP

  1. #1
    Moderator rsterne's Avatar
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    .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
    Last edited by rsterne; Nov 15 2015 at 06:41 PM.
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  2. #2
    Moderator rsterne's Avatar
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    I liked the term Monotube, but since there is an air tube under it (making two tubes), it doesn't quite fit.... The upper tube takes all the loads keeping the critical parts (barrel, breech, and scope) aligned, like an Exoskeleton, so that makes it a Monocoque design.... From now on, I will be referring to it as my "Monocoque Design PCP"....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  3. #3
    Moderator rsterne's Avatar
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    My Christmas present from my dear wife came in today....



    Millet 6-25 x 56mm SF.... Just another scope you say?.... well, yes, but this may give you a hint about what is different about it.... those are 1" rings for comparison....



    Yes, the tube is 35mm, and the scope is 22" long with sunshade and weighs 2 lb. 13 oz. with the rings.... Here is it sitting beside a Leapers 3-9 x 32mm AO with sunshade....



    The scope is very bright and clear.... It focuses down to 15 yards, and has 140 MOA of Elevation adjustment.... I will be machining the Picatinny rail it mounts on to a 20 MOA angle on the bottom, which should put me on target at 100 yards with the scope still on optical center.... The 1/2 MilDot Reticle is calibrated at 25 X, and there is a mark at 12.5 X, which makes each 1/2 MilDot work out to a full MilDot, so that works out to a lot of holdover just using the dots....

    Bad part is, I have to put it away until Christmas....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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    Moderator rsterne's Avatar
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    The bulk of the materials arrived yesterday, and I spent the day finalizing most of the design.... It doesn't look much like an airgun at the moment....



    Plastic pipe is for the forestock and cheek piece.... 1 1/4" OD CrMoly tubing for the monocoque upper tube (0.065" wall) and air reservoir (0.095" wall).... The 1 1/8" 2024-T3 Aluminum round bar is for the breech block, valve, front plug, and all other internal mounts.... The rectangular aluminum barstock is for the external parts; the spacer between the tubes, the adjustable stock mounts, and the front bipod/stock mount....

    Soon to be converted into a pile of chips, and usable parts, I hope....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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    Moderator rsterne's Avatar
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    The block of aluminum from Watertown arrived and I wanted to see how my new 1-1/4" drill bit worked so the first thing I did was square it off to size and drill it to fit on the Reservoir.... That went perfectly, although there is a lot more to do, machine the Forestock Mount into it and a Picatiinny Rail in the bottom.... The Forestock is a piece of 1-1/4" (ID) black plastic pipe 12" long that slides over the reservoir.... and a piece of it 6-1/2" long, split down the middle, will be used for the Cheekpiece.... Here are those parts....



    Inside the Cheekpiece are two "washers" made from 3/8" aluminum, which will form the mounts.... One will be attached to the back of the Breech, and the other to the bracket that mounts the buttplate to the Reservoir Tube.... arranged like this....



    Most of today's work went into making the Stock Bracket, which is shown in the photo below....



    It is machined from a piece of 3/4" x 1-1/2" 6061-T6 aluminum about 5" long.... The large hole fits over the back of the Reservoir Tube and will clamp in place with set-screws.... The screws at the top will mount the rear washer that holds the Cheekpiece.... The three threaded holes at the bottom are for mounting the Buttplate, and an optional 4" wide Baseplate that can sit flat on a bench.... Here is what it looks like in side view....



    The height of the Buttplate will be adjustable in height, tilt, and length of pull, using a threaded rod, locked by a wing nut as shown.... The center position for the front of the Buttplate is 1" behind the bracket, so it should be good and rigid.... By offsetting the threaded rod by 1/8" in the Buttplate, and with the 1/2" spacing on the holes, I will be able to alter the height below the line of sight over a 1-1/4" range in 1/4" increments (3 holes x 2 positions per hole)....

    It is great to finally start making chips.... Next up will be finishing the Cheekpiece mounting washers, and completeing the BiPod / Forestock mount....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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    Moderator rsterne's Avatar
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    Spent most of the day putting up Christmas decorations for the Motel today.... but did manage to get a couple more things done in the shop.... I drilled the two donuts that hold the Cheekpiece in place, drilled the mounting holes in the Cheekpiece, tapped the holes in the rear donut and mounted the back of the Cheekpiece.... I also made a stand that attaches to the bottom of the Stock Bracket instead of a conventional Buttstock to use when shooting from a bench.... It will help hold the rifle level, and act as a compliment for the front BiPod....



    It is 4" wide at the base, so will have lots of stability.... You simply remove the Buttstock and bolt this on instead.... Not a lot of progress, but at least some today....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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    Moderator rsterne's Avatar
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    I got most of the BiPod made today, and did some more work on the Block that will mount it to the gun.... Here is the adjustable BiPod....



    It doesn't look like much, but I think it should function well.... The crossbar, which will bolt to the mount on the gun, is made from a piece of 3/4" x 1" wide 6061-T6 barstock.... It is drilled and tapped for two pieces of 1/2" threaded rod.... The pieces shown are 8" long and 12" apart, and should allow for an elevation relative to the bench of about 6 degrees (360 MOA).... I can make longer rods, or they can just sit on a block of wood to raise the muzzle further relative to the bench if necessary.... The wheels are 2-1/2" OD pulleys with a 1/2" bore, setscrewed to the shaft, with a nut tightened up against them and secured with green Loctite.... The ends of the nuts are radiused so that when you rotate the rods, using the pulleys as handwheels, they won't hang up on the bench, allowing easy height adjustment in very small increments.... There are wingnuts to lock the height, which increases the rigidity of the assembly as well....It is remarkably stable, and I think with the easy adjustment should prove an asset to shooting from the bench.... It will bolt to the mount below....



    There is still more work to do to this (you are looking at the bottom of it).... I still have to cut the slots in the Picatinny rail, and drill and tap either side of that for the screws that will bolt the BiPod above to it when shooting from the bench.... The boss on the side facing you fits inside the plastic pipe I'm using for the Forestock, and keeps it positioned so that the top of the pipe is touching the top of the reservoir, offset downwards.... As you can see, the boss is not concentric with the hole for the tube.... This will be attached to the reservoir with setscrews.... I hope to complete these parts tomorrow, and then it will be time to start on the internal parts....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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    Moderator rsterne's Avatar
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    I finished off the Mount for the Forestock and BiPod today.... Here it is with the Forestock in place.... The Picatinny Rail is done now....



    I also completed the BiPod.... It now bolts to the mount with two #10 SHCSs and has a built in level to check the cant.... which is easily adjustable with the handwheels on the threaded rods.... which are just above the bench....



    Used in combination with the plate on the bottom of the rear Stock Mount, which completes the TriPod, the rifle will be easy to adjust to any required elevation by using the handwheels, and then a final set of the cant using the bubble....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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    Moderator rsterne's Avatar
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    Made up a mount for the Scope Rail to hold it for machining to cut the radius on the bottom and put in 40 MOA of angle.... This was a time consuming and nerve-wracking job.... Here is the results....



    You will notice that the height of the lower portion of the rail is less at the front (left) than at the back.... The difference in the 6" long rail is just under 0.070" which works out to 40 MOA and should put me pretty close to the optical center of the scope at 200 yards.... I had to add one more mounting screw at the back....



    I machined the taper into the bottom using a 1-1/4" diameter carbide router bit I got from China for less than $15 including postage.... It cuts very smoothly, and the radius fits the tube perfectly.... Setting the scope rail up so that I could machine it was tricky, though.... I had to bolt it to a backing strip of aluminum (1 x 1.5") with screws in 2 of the 3 holes and move the screws along so that I could machine the entire length.... I was JUST able to manage it without having to move the jig in the milling attachment....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

  10. #10
    Moderator rsterne's Avatar
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    This is one of those days you almost dread, and then feel great when it's over..... provided everything works the way it should.... The spacer between the two tubes is 10" long and concave on top and bottom to fit the 1-1/4" diameter tubes, and keep them parallel and 1/4" apart.... It could have been made from a piece of 3/4" x 1" aluminum bar stock, but the only piece I had long enough was 1-1/2" wide.... so the first thing I had to do was saw it lengthwise and then mill it to 1" wide.... Since it was 10" long and I can only machine a bit over 6" of length, that required two setups, and they were critical to match up parallel and exactly 1" wide.... Fortunately, that setup of the vice on the milling attachment on my lathe was also necessary for milling both sides to a radius using the 1-1/4" diameter router bit I just bought.... but again it meant two setups per side, critical that the part be square to the machine in both axis to end up with the same thickness at the center of the web from end to end, to end up with the tubes parallel in height.... and also with the grooves centered in the width of the part, to make sure the tubes end up parallel side to side.... I'm pleased to say that it went very well.... Here is the finished blank....



    It fits the tubes perfectly, in fact better than previous breeches where I have machined the bottom with a ball-end mill.... I would definitely use a carbide router bit for this job again, especially with them being a fraction of the cost of a high-speed steel ball-end mill.... It needs to be run fairly fast, with not too deep a cut.... I was running 1270 RPM and removing 0.020" per pass once the full width of the router was engaged.... and using the power cross-feed, that was smoother than going by hand.... Including shallower finishing passes, that meant for about 15-17 passes per setup, times 6 (including milling the width to 1"), so about 100 passes.... It took me 5 hours to produce the piece you see above.... There is a bit more machining to do to this part, it has to be drilled for the transfer port and the three bolts that fasten the upper and lower tubes together.... plus a recess in one end to hold the back of the Forestock in place.... However, that won't happen until the tubes are finished and this will be machined to match.... Besides, I've had enough for one day.... It's time to have a drink and relax.... It's been a very productive few days, and it's time for a break....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal: Too many to count!

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