.257 cal Monocoque PCP

[rsterne]

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

[rsterne]

Today I attached the Front Cheekpiece Mount to the back of the Breechblock....



It is mounted below the centerline of the tube so that the inside of the Cheekpiece tube sits against the top of the upper Monocoque tube.... I also machined the front of the barrel to fit the 1/2" ID Bellevilles and threaded it 1/2"-20 NF for the nut that will provide the barrel tension.... The aluminum sleeve that bolts into the front of the Monocoque tube is 9/16" ID to fit on the larger part of the barrel, while the Bellevilles ride on the 1/2" section....



The front of the Monocoque tube is about 1/10" ahead of the muzzle to protect the crown.... If I wish, I can use a Hatsan Air Stripper instead of the nut to tighten against the Bellevilles to provide the barrel tension and also strip the air away from the back of the bullet....



This completes all the parts that fit inside the Monocoque tube.... except to machine the bolt slot which I will do after machining the tube.... I am still undecided if that will be my next step, or if I will make the valve first.... and put off machining the two tubes until the end....

Bob

[rsterne]

Gee, guess what!?!.... I found a beautiful Millet 6-25 X 56 SF scope under the tree this morning!.... Things were quiet around here this afternoon, so I decided to cut the tubes to length and assemble the Monocoque PCP for the first time.... Here is what it looks like....



I still have to add the bolt, fill fittings and barrel stretcher, and it already weighs just over 13 lbs.... without the scope!.... It is 49.25" long, although it looks longer.... It does balance nicely, in the back half of the forestock.... Then I removed the buttplate, and installed the BiPod (actually a TriPod, I guess) and the scope.... I think it looks pretty awesome....



This is pretty much a bench gun, although it could certainly be used prone for varminting, using the BiPod and with or without the buttstock.... I really like the easy adjustability of the BiPod for height and tilt, using the handwheels.... It was a pretty cool Christmas present to put it together and look at it for the first time....

Bob

[rsterne]

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

[rsterne]

I couldn't put it off any longer, so today I did most of the machining on the Monocoque Tube.... all the holes for the various bolts, plus the loading port and transfer port are done, plus the additional 10-32 tapped hole for the front of the Picatinny scope rail.... Here is the top view....



Everything fit really well, all the bolts fit into the holes, but a couple were tight enough to require a touch with a file or Dremel bit to make them easy to assemble.... Careful deburring was the order of the day to make sure that the Breechblock didn't hang up when slid inside.... I was particularly proud of how well the loading port lined up.... Below is the bottom view, and you can see that the transfer port and mounting bolts also line up with the Spacer....



All in all, I am very pleased.... The key to the accurate machining was using the 5/16" hole behind the loading port where the heads of the screws are that take the tension and compression loads of the barrel and tube.... Using a 5/16" pin in the chuck of the lathe through that hole, both for setup (measuring everything from that point), and for indexing 180* for the holes on the bottom, did the trick....

Bob

[rsterne]

Today I drilled the holes in the front of the tubes for the retaining screws for the Fill Fitting and Barrel Tensioner....



Here is a photo of the muzzle, showing the recess for the Belleville washers and nut that will tension the barrel....



That completes the machining on the Reservoir tube.... All that remains on the Monocoque tube is the slot for the bolt handle....

Bob

[rsterne]

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

[rsterne]

Today I machined the top of the PRod trigger group to fit the 1-1/4" OD x 0.095" wall tube.... It normally fits a 7/8" OD x 0.065" wall tube, so one of the concerns is that the sear projects far enough into the thicker tube I am using to safely hold back the hammer.... I used the 1-1/4" diameter router bit I have to cut the larger radius, and at the point it barely touched the center of the recess I stopped and checked the seat engagement.... It was about 1/32", which although I feel was perfectly safe (after all, the hammer can't raise up by more than the clearance in the tube), I decided I wanted a bit more.... I noticed that the boss in the casting that holds the sear pivot pin was about 0.010" below where the router cut, so I mounted the trigger group back in the milling attachment and milled down until I just touched the edge of that.... The arrow in the photo below points to that shiny mark, right beside the sear.....



With the trigger group machined to that depth, there is over 1 mm of sear engagement (I'll get an exact measurement after I cut the slot in the tube), and I'm happy with that.... One other thing that is important is that the slot in the tube to clear the sear must be slightly more than the full length of the sear.... The parts of the sear in front and behind the actual sear now project slightly above the bottom of the tube recess in the trigger group, so if the slot is too short, it would affect the engagement depth, and in fact could even prevent the sear from cocking properly.... All the other parts (the trigger and the intermediate link) are still well below the tube and do not interfere....

The PRod trigger is very strongly built, and I see no problem with it easily withstanding way more than the 20ish lb. maximum load I will be asking it to hold....

Bob

[rsterne]

Yesterday and today I machined the J-slot for the bolt handle in both the Monocoque tube and the Breechblock.... I also installed a Velocity Adjuster that works by choking the barrel port by preventing the bolt handle from withdrawing fully into the lower part of the J-slot.... I consists of a 3/16" rod running in a milled slot, and a 10-32 screw to control the position of it.... In addition, there is a small set-screw which presses a plastic plug against the side of the adjusting screw to act as a brake to prevent it from moving by itself.... Here is the Breechblock, showing the J-slot, bolt handle, and adjuster.... In this photo, it is closed about 30% from wide open....



I'm very pleased with the positioning of the bolt handle and cocking handle.... I was a bit concerned that your hand might hit the bolt handle when cocking the gun, but I can wrap my big mitt around it no problem and my knuckle doesn't (quite) touch the bolt handle during cocking.... Here is a photo of the gun in the cocked position....



You can see that the bolt handle is fully back in the J-slot in that photo, which means that the nose of the bolt is completely clear of the barrel port.... Here is a photo of the gun unloaded, with the cocking handle forward (uncocked) and the breech open for loading.... There is lots of clearance between it and the scope....



I also made a brake for the hammer spring preload position adjuster today, to prevent that from adjusting itself as well.... This completes the major part of the machining on this project.... I can assemble everything now, except I am still waiting for the bolts to secure the Valve and Fill Fitting, so I can't as yet pressure test it.... I measured the reservoir today, and it is a whisker over 400 cc, not including the volume inside the valve, so I should have about 25 CI of air available.... If I can manage to tune for a shot string from 3800 psi down to 3000 (55 bar), that works out to 1375 CI, which would work out to about 7 shots at 200 FPE, if I can get the gun to produce 1 FPE/CI at that power level (which is likely a stretch).... I am more likely to tune it for tethered use at 3800 psi, and to get 3 shots within a very narrow ES off tether.... What the FPE level ends up when tuned like that.... is now the big question....

Bob

[rsterne]

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