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rsterne
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
rsterne
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Well, I finally got back to working on this project…. The reservoir was empty, I had a very slight leak in one of the O-rings (there are only two in the reservoir), so since it was empty I took the opportunity to pull it apart and replace the 70D O-rings with 90D…. It has been sitting at 4000 psi for 24 hours, so I guess I cured the leak…. I measured the land and groove diameter, finished up my reamer drawing, and today I made the chamber reamer…. I’m very pleased with the way it came out…. Each time I do one of these, it gets just a little nicer and more accurate….
I used a three flute design, like the one I did a couple of months ago for my .22 cal Regulated Disco Double…. It is machined from a piece of 3/8″ O1 Drill Rod, hardened, and then polished, and tempered in my wife’s oven at 465*F for an hour…. While it was baking I did a bit more reading on tempering, and discovered that it is the thickness of the transparent oxide layer on the polished surface that causes the colour…. It starts at yellow, through dark straw, brown, purple, blue, then a lighter blue, as the temperature is increased…. I was aiming for a dark straw to brown colour, which is about 490*F…. Once the oven was up to temperature, after about 10 minutes, it looked just right, but I decided to let it heat soak for an hour, and it darkened to brown with a few purple spots…. I was a bit worried that I had gone too far, but apparently even though the part isn’t getting hotter, the oxide layer gets thicker with time…. One recommendation I found was to use 1 hour per inch of thickness of the part, so I really don’t need to heat it for as long as I did…. If anything, it might be a little tougher, and not quite as hard, but I think it will cut just fine….
For those of you not familiar with chamber reamers, the part on the left, with no cutting flutes, is the pilot that guides the reamer by running on the lands…. The reamer is then necked down a bit, and has a 1 degree taper for about half the length of the flutes, and then a parallel section to cut the chamber…. I make my chambers about 0.001″ over the groove diameter…. It then steps down again at the end of the flutes…. The only part that really does any cutting is the last half of the taper and the first part of the chamber…. You just keep running it deeper into the barrel until the chamber is the length you need for your bullets…. I will be cutting the chamber on my Monocoque to fit both the 100 gr. and 113 gr. BBTs, which means it will also fit the 98 gr. and 109 gr. HP versions…. I have sized a bunch of them (both lengths) to use for checking the chamber depth, most to 0.257″, but a few to 0.256″ and 0.258″ just so I can judge the fit…. The aim is to have the nose of the shorter bullet starting into the leade, but not have the longer one too difficult to chamber…. 113 gr. on the top, 100 gr. on the bottom….
It’s really nice to get back to work on this project after a year of waiting for bullet moulds and a chamber reamer (which never arrived)…. It won’t be long and I will be able to test fire it for the first time….
Bob
Yesterday I reamed the chamber, and arrived at a length where the shorter bullet stopped against the leade about 1/16″ before being chambered fully, and the long bullet was about 3/32″ from being chambered…. This meant you could just feel slight resistance on the bolt handle to chamber the 100 gr., and a slight push to chamber the 113 gr…. That was the last bit of machining I had to do, other than machine the transfer port to final length…. This morning I gave a final lapping to the crown, and then polished the bore with JB Bore Paste, followed by Bore Brite, cleaned the gunk out of the bore, and the gun was ready to assemble and test fire for the first time…. That happened this afternoon, nearly a year later than I had hoped….
My Great White was at 4000 psi, which was about where I hope to run the gun, so I just tethered it and ran the initial tests…. I used the lightest and heaviest bullets I will be using, the 98 gr. HP version of the 3.5 caliber long BBT, and the 113 gr. FN version of the 4.0 caliber long BBT…. With the velocity adjuster bottomed out I couldn’t quite cock the gun, but with it backed off 1/2 turn the gun would cock fine, and the cocking effort was still quite reasonable…. This gun has a separate cocking handle on the hammer, all the bolt does is load the bullet into the chamber…. There are so many things different about this build, although I had high hopes, I really didn’t know if they would be realized…. It was with those very mixed emotions I pulled the trigger for the first time…. and was rewarded with a velocity of over 1000 fps…. I did the typical testing I always do with a new gun, find out where the maximum is (the velocity plateau) and then work back from there…. I then changed to the heavier bullet, and worked my way back up to maximum…. I only shot 1 or 2 bullets at each 1/2 turn preload adjustment, and then plotted the curves, as shown below….
The maximum velocity is reached at this pressure with 1.5 turns of adjustment left with the heaviest bullet, and 2 turns left with the lightest one…. The peak performance was 1003 fps (219 FPE) with the 98 gr. bullet, and 950 fps (227 FPE) with the 113 gr…. This is basically right what I had hoped for, what I really didn’t know was the pressure it would take to achieve those numbers…. Now I know it takes 4000 psi…. I shot a few more rounds, and did a bit of playing around to see what I could get for a shot string…. I’m not interested in detuning this gun for a long string, the gun will primarily be shot tethered to a regulator, and tuned for a short (3-5 shot) string off reg…. Tuning in this way keeps the efficiency reasonable, and yet enables you to disconnect for a short string if necessary…. Once the tank was down to 3800 psi, and I had a rough idea of where to adjust the preload to get a short string, I shot the following off reg….
That is 4 shots, averaging 928 fps (209 FPE) within a 14 fps ES (1.5%)…. The pressure started at 3790 psi and ended at 3320 psi, and the reservoir on the Monocoque is 24.7 CI (405 cc), so that works out to an efficiency of 1.04 FPE/CI…. Let me tell you, it’s about twice as loud on the fourth shot as on the first, so when tethered it will be quite a bit better than that four shot untethered average…. Now that I know the gun is a success, I can work on the last piece I need…. an inline regulator that I can adjust from 3000-4000 psi by changing the shims…. That will allow me to fine tune the velocity sweet spot with the pressure, while setting the hammer spring reload to give me that 3-5 shots off tether that I desire….
During this build, I had a lot of negative comments about the 7″ twist being too fast, that it would prevent me from ever achieving my goal…. and the rifling being too deep (it’s 0.001″ deeper than a normal .257 barrel), which would likewise ruin the performance…. I think these initial results should be enough to put the nay-sayers to rest…. I’m just grinning from ear to ear that I achieved my goal, without having to exceed 4000 psi….
PS, I dropped a 34 gr. JSB Heavy pellet into it…. 1362 fps (140 FPE) on 4000 psi…. :
…. fastest I have ever driven lead with an airgun….Bob
Today, I took apart a Ninja regulator and rebuilt it for inline use…. It looks like these other ones I have done previoulsly….
The modifications I made are as follows…. Do this only if you are competent working around HPA, and at your own risk.
1. Completely disassemble the regulator, removing both burst discs, the male Foster and the gauge from the reg. body.
2. Cut off the threaded stem that screws into the tank just below the safety vent hole.
3. Drill the bottom of the body to 11/32″ until you just touch the three cross holes that pass air to the fill, gauge, and HP burst disc, and tap 1/8″-27 NPT.
4. Deburr and use compressed air to blow all debris and chips out of the regulator body, and install a plug, using Teflon tape to seal it.
5. Reinstall the male Foster (with NO check valve) and gauge, and install burst discs of the appropriate rating for the inlet and outlet pressures.
6. Remove the pin valve from the bonnet, and drill through 11/32″ and tap 18″-27 NPT. Remove all debris and chips with compressed air.
7. Install a female Foster in the bonnet. I used a close hex nipple because I didn’t have a female Foster with male threads. Both Fosters should be rated for 4500 psi.
8. Install new O-rings on the regulator piston. I would recommend using a Ninja rebuild kit from Mac1 Airguns, which comes with 90D Mil-Spec Urethane O-rings.
9. Install a Belleville washer stack appropriate to the pressure range you need, with appropriate shims, on the piston.
10. Install the piston in the reg. body and screw on the bonnet.
11. On the output side, attach an accurate gauge equipped with a bleed valve to the female Foster.
12. Fill slowly through the male Foster using a tank or pump, to 1000 psi, and check for leaks. Repair if necessary.
13. Slowly increase the pressure at the inlet, while watching the gauge. If you increase the pressure too close to the rating of the outlet burst disk it will fail.
14. If the burst disc fails, change your shorts, then change the disc, reduce the shimming in the regulator, and repeat.
15. If the indicated output pressure is too low, shut off the tank, bleed the system, and increase the thickness of the shims. If too high, remove shims.
16. When you have the regulator adjusted to the setpoint you want, “burp” the bleed valve a few times to settle the needle, and the reg. and double check the pressure.
17. Once you have the regulator adjusted the way you want, close the tank valve, and bleed the system. Record your Belleville stack and shims for future reference.
18. You can now tether your gun to the output side of the regulator with a HP hose, and when you turn on your tank, your gun will fill to the setpoint pressure.
19. If your gun has a pressure gauge, fill slowly and double check that its gauge is close to what you read on your quality gauge. They may not agree exactly.
20. Enjoy shooting your gun tethered to your tank. Ideally, the burst disc on the output side of the regulator should be close to, or only slightly above, the MSWP of your PCP, in case the regulator fails.Although you have to change shims to alter the regulator setpoint, this makes a great inline regulator at far lower cost than the $300 plus adjustable variety…. The regulator for my Monocoque is set at 3800 psi…. This required a very unusual Belleville stack to achieve that high output pressure, consisting of five pairs of Bellevilles, with each pair consisting of a 0.047″ nested inside a 0.032″…. and then two thick Ninja 0.020″ (red) shims, arranged like this…. The flat shims are inside the regulator body, with the opposite end of the stack against the large end of the piston….
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There are two BIG warnings about using this setup…. There is very little total travel, only 0.060″ from loose to fully collapsed…. The pairs of Bellevilles take a huge amount of force to drive them flat, about 860 lbs., and that only takes 0.012″ of movement per pair…. This means that EVERY 0.001″ of shims changes the output pressure by about 90 psi…. Just a few thou of shims can make the regulator go from 3000 psi to not regulating at all, and passing the full tank pressure through to the output…. Most Belleville stacks are not this harsh to adjust, but when you need to go to high pressures, they get very fussy…. My regulator has a steel bonnet, I don’t know if the aluminum bonnets found on some regulators are strong enough…. but Ninja sell regulators with the output set as high as 3000 psi, and having a 5K burst disc on the output side…. If they failed, they would pass the 4500 psi tank pressure through to the output side, and I trust Ninja to know what they are doing…. so I personally have no problem with such a high setpoint…. but I am NOT recommending that you do the same….
Bob
davidgoldstein
HI Dave…. Sadly, no…. I just don’t get any time to shoot in the summer when we are running the Motel, and snow on the ground the rest of the year…. The good news is that we are closing the Motel in Feb. 2021, so that summer I hope to have enough time to do LOTS of testing…. and a LabRadar to assist me…. Thanks for your patience….
Bob
davidgoldstein
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