Author Topic: .257 Monocoque PCP  (Read 2718 times)

rsterne

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.257 Monocoque PCP
« on: February 27, 2017, 08:42:36 PM »
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.... Here is a link to the original thread on the GTA.... http://www.gatewaytoairguns.org/GTA/index.php?topic=99759.0

Bob
« Last Edit: February 27, 2017, 08:48:08 PM by rsterne »


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lee545

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Re: .257 Monocoque PCP
« Reply #1 on: February 27, 2017, 09:55:42 PM »
I did read the original thread some time ago and was wondering where it went. That's some beautiful work there! Nice job on the reamer. I have been heat treating so much steal, I finally took the time to build a PID controlled heat treat oven. Best $200 you could spend on any shop tool!! So much better than guessing with a torch. Do you doing any final grinding after heat treating or just hit the edges with a stone and oil?   

rsterne

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Re: .257 Monocoque PCP
« Reply #2 on: February 27, 2017, 10:09:51 PM »
I just use a small flat diamond file (the finest one I have) to stone the lip created by the end of the mill when I cut the flutes.... I don't touch the outside radius of the reamer.... Plunging the end mill 0.010" past the centerline creates the necessary rake on the cutting edge....

Bob
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Christopher

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Re: .257 Monocoque PCP
« Reply #3 on: February 28, 2017, 09:04:31 AM »
Glad you are able to get back on this project.  Anxious to see how this behemoth performs with those massive slugs. Entering some uncharted territory here.....exciting times.

Chris
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rsterne

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Re: .257 Monocoque PCP
« Reply #4 on: March 02, 2017, 04:38:46 PM »
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....

Bob
« Last Edit: March 02, 2017, 04:50:37 PM by rsterne »
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Christopher

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Re: .257 Monocoque PCP
« Reply #5 on: March 02, 2017, 05:38:24 PM »
CONGRATULATIONS!!!

Have been anticipating you shooting this gun ever since you started this build many months ago. Achievement of your goals no doubt leaves you with a sense of fulfillment and satisfaction. Nice efficiency, great speed, and incredible energy. Keep forging ahead to further develop the hobby that we all so much enjoy.

Thank you,
Chris
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rsterne

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Re: .257 Monocoque PCP
« Reply #6 on: March 02, 2017, 08:48:54 PM »
Just for giggles, I tried a 34 gr. JSB Heavy pellet.... 1362 fps (140 FPE) at 4000 psi....  ::)

Bob
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sixshootertexan

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Re: .257 Monocoque PCP
« Reply #7 on: March 02, 2017, 09:05:18 PM »
Well done Bob. Glad to see it getting finished and can't wait to see a target from it.
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Christopher

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Re: .257 Monocoque PCP
« Reply #8 on: March 03, 2017, 02:03:31 AM »
Quote
can't wait to see a target from it.

+1

Chris
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rsterne

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Re: .257 Monocoque PCP
« Reply #9 on: March 03, 2017, 10:21:10 PM »
Today, I took apart a Ninja 4500 psi "high-pressure" 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.... Other regulator brands (or clones) may not be acceptable/safe when doing these modifications.... although if rated at 4500 psi input, and properly engineered to withstand a failure to that pressure, they should be....

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 "high pressure" 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.... nor do I know if "clones" are engineered to the same standards, even if rated at 4500 psi.... as many do not have high enough output pressures to require a 5K output burst disc.... and only come with a 3K.... Substituting a higher rated output side burst disc may be pushing the safety of any regulator not so designed....

Bob
« Last Edit: March 04, 2017, 12:13:45 PM by rsterne »
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rsterne

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Re: .257 Monocoque PCP
« Reply #10 on: May 10, 2017, 01:24:58 PM »
Well, a year after this project came to a grinding halt, I finally got a day to take her out and let her stretch her legs a bit.... I built a new shooting bench, and the Monocoque was the first gun to try it out at 100 yards.... Here is a pic of the setup....



and here is a photo of the range, currently set up for 100 yards....



The field I am set up in has the ability for me to move the bench back to 300 yards, or pretty much anything in between.... The rancher I shoot Varmints for allows me to leave the bench and target stand in his field.... I have wind flags on stakes, currently at 25, 50, and 75 yards.... and recently purchased an anemometer that I keep on the bench with me....

The Monocoque was tethered to my Great White by a 3800 psi regulator, and I shot enough to use 1000 psi from the tank today.... I had 12 different bullets to try, or rather 4 different bullets, and three diameters of each, 0.256", 0.257", and 0.258".... The bullets were the HP and FN versions of my two heaviest Bob's Boattails from NOE.... The bullets tested were a 98 gr. HP, a 100 gr. FN, a 109 gr. HP, and an 113 gr. FN.... All bullets were cast in 2% tin....

The Millet scope is mounted on a Picatinny rail that I milled to angle the scope downwards, to hopefully center the POI at 100 yards with the scope centered, and it worked perfectly.... I was within 4" at 100 yards, and only need to tweak the turrets a bit to roughly center me with the 109 gr. bullets.... The 113 gr. were a bit low, and the 98 and 100 gr. were a few inches high, because there is an 80 fps difference between the lightest bullet, at 960 fps, and the heaviest, at 880 fps.... The first targets were 3 shots groups, intended just to prove that the POI was intended, and one of those was the best group of the day....



I saw a pretty clear trend, with the heavier bullets begin more accurate, and the smallest diameter the least accurate.... I'm not thrilled with the average 5-shot groups I shot today, but that 3-shot above, at 1.12", show that the gun has promise.... One thing is clear, the holes in the target are perfectly round, showing no sign of yaw, so the 7" twist is sufficient even for the longest bullets....


Bob
« Last Edit: May 10, 2017, 03:46:42 PM by rsterne »
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Christopher

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Re: .257 Monocoque PCP
« Reply #11 on: May 10, 2017, 03:18:27 PM »
Can't remember all the details of this build, but looks like this barrel might be a good candidate to experiment with your barrel tension and maybe some sort of harmonics dampener to slide up/down the barrel if possible....Still though, glad you finally were able to get out to your permission and stretch her out a little. Nice group too.

Chris
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rsterne

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Re: .257 Monocoque PCP
« Reply #12 on: May 10, 2017, 03:35:14 PM »
The barrel is tensioned within the upper tube, using my Belleville system.... I have a lot more to learn about this design, and shooting cast bullets in particular....

Bob
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Aceflier

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Re: .257 Monocoque PCP
« Reply #13 on: May 10, 2017, 05:42:50 PM »
Looks good on the initial testing! That's some mega power in .257! Nice lookin home build. Wish I had the know how and skills to do such things. Looking forward to future updates. Let the naysayers wallow. Said the same thing about the .45 boat tail and resized it shoots great in my texan. Thanks for the design on that btw!
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sixshootertexan

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Re: .257 Monocoque PCP
« Reply #14 on: May 10, 2017, 09:24:34 PM »
Great first time out with the rifle Bob. Just keep working at it it will get better. Took me awhile to get all the kinks out of my .308. I shot a lot of bullets thru my .308 until I got the mold for the 135gr BBT. It's now the only bullets I shoot in it. Thanks Bob.
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