Author Topic: Any Interest in 6mm ? - Build Thread  (Read 7328 times)

Monkeydad1969

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Re: Any Interest in 6mm ? - Build Thread
« Reply #105 on: March 18, 2018, 02:16:46 PM »
I just tried a couple of Airsoft BBs in the 6mm at 2800 psi.... 1944 & 1963 fps.... So much for the 1650 maximum.... *LOL*.... Next time I have the 6mm upper on the unregulated gun I will try a couple of shots at 3800 psi.... maybe even 4200....  ;)

Bob

Lol.
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Alan

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Re: Any Interest in 6mm ? - Build Thread
« Reply #106 on: March 18, 2018, 03:58:29 PM »
Bob, do you remember those old Sierra thin-skinned varmint bullets made for a .22 Hornet or .218 Bee? They used to unglue at about 2,500 FPS, and I suspect the airlift pellets will too. So let us know what happens, and take photos if you can. Thanks.
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #107 on: March 18, 2018, 04:11:37 PM »
I didn't have a fresh piece of paper for a target this time.... but when I did the Disco using airsoft at 1750 fps there were nice round holes at 20 ft., one per shot.... but about a 3" group at that distance....  ::)

If I test again, I'll put up a fresh target....

Bob
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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #108 on: March 19, 2018, 03:50:56 PM »
I tried to glue up the 0.047" vent through the poppet and redrill it with no success, so today I made a new poppet for my SS Valve.... I am still using the 3/8" big end and 1/4" small end, but I added a thin (1 mm) O-ring inside against the shoulder of the poppet as a bump stop.... It should never touch, but if the valve was driven fully open (or blew fully open) the O-ring bumper would take the shock instead of either the shoulder on the poppet hitting the inside shoulder of the thimble, or the spring going coil-bound.... It was just a safety measure, but also a way to find out if the valve was opening that far.... As I expected, the O-ring made NO difference, which is proof that the valve is not trying to open more than the 0.19" travel before the O-ring hits.... This was a useful piece of information (and expected), and I plan on incorporating this feature into any future SS valves I make to insure they don't break from being overdriven, or from machine-gunning.... I used PEEK for the poppet, not because it was required, but just because I had some and wanted to try it, and it machines nicer than Delrin....

When I reassembled the 6 mm regulated gun I finally nailed it.... I used the 32 gr. MDS hammer, and the 11 lb/in spring with 1 lb. of preload on the SSG guide.... I was able to easily get to the plateau with all three bullets, and have to run 5 turns of SSG gap with the heaviest and 7 turns with the lightest to get about 3% below the plateau, where I want to operate to get reasonable efficiency.... Here is a graph of the velocity vs. SSG gap for all three bullets at 2800 psi....



I am getting nearly 200 fps of adjustment before the valve falls off the cliff, which is plenty, I will never run it that low.... You will note that there is one additional line on the graph, the dotted black line.... I set the SSG gap at 6 turns, and using the 63.8 gr. bullets I tested the velocity adjuster on the back of the receiver that restricts how far the bolt retracts by moving a pin into the J-slot for the handle.... The reason for the slight increase in velocity when the adjuster is turned in 2 turns is that I had to shorten the nose of the bolt 1/16" because it was pushing the bullets too far forward, which made the 73.4 gr too hard to chamber.... This meant that with the bolt handle pulled all the way back in the J-slot it was creating more wasted volume in the chamber, behind the barrel port, which was dropping the velocity slightly.... It shows the importance of minimum wasted volume between the valve seat and the base of the bullet.... From the maximum velocity setting I have 8 turns of adjustment on the 8-32 screw, which moves the bolt nose from the back of the port to nearly obstructing it.... at which setting the velocity is below 500 fps.... The cool thing about this adjuster is that you can easily try different velocities, to find out if there is an optimum for accuracy.... Of course you should adjust the Harmonic Tuner at each velocity, just to make sure what you are seeing is a true velocity effect, not a change of accuracy due to barrel harmonics....

Once I am sure that there are no leaks, I will remove the "tiny tank" and replace it with my 500 cc CF bottle.... At that point I will be able to test the efficiency at various velocity settings.... These two guns are nearing the end of their development, which is a relief because it won't be long before the Motel gets busy and my shop time disappears once more.... BTW, Alan, I shot three more 6mm Airsoft BBs today, all in the mid 1900s, at a new target.... At 20 ft. I was getting about a 1" group, and I was not taking my time and using a red-dot.... The holes in the paper were perfectly round, and there was only 1 per shot, so the BBs are not coming apart....

Bob

« Last Edit: March 19, 2018, 03:53:20 PM by rsterne »
  • Coalmont, BC

rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #109 on: March 20, 2018, 02:25:31 PM »
The gun only lost about 100 psi overnight, so I figure that is "good enough" and I installed the 500 cc CF tank today, and attached the AR style stock.... Here is what it looks like....



As shown it weighs 8.5 lbs., and without the Harmonic Tuner, exactly 8 lbs.... It balances right in the middle of the tank.... I'm very pleased with the way it came out....

Bob
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Monkeydad1969

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Re: Any Interest in 6mm ? - Build Thread
« Reply #110 on: March 22, 2018, 08:21:12 AM »
Looking good, Bob.

Joe
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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #111 on: March 23, 2018, 01:58:48 PM »
Well the 6 mm regulated gun has been sitting with the 500 cc bottle on it for a few days, and shows no visible drop in pressure after the initial post-fill cooling.... I topped it up from my SCBA tank, with the digital gauge in between, shut off and bled the tank valve, leaving the digital gauge connected to the gun with a microbore hose (there is a check valve on the "inlet" side of the gauge for this purpose).... I took a couple of shots until I noticed the digits dropping on the gauge, so I knew it was accurately reading the pressure in the 500 cc CF bottle, let it stabilize a couple of minutes, recorded the pressure, and shot 10 shots through the Chrony with the 63.8 gr. bullets, with the SSG gap set at 7 turns.... I recorded the average velocity (939 fps), the ES (11 fps) and the SD (2.8 ), and after a couple of minutes for the pressure to stabilize again (it rises slightly after the string as the tank warms back up to room temp) I recorded the pressure again.... The tank dropped 465 psi for the 10 shots (which averaged 124.9 FPE), which means I used 978 std. CI of air to produce 1249 FPE, which works out to 1.28 FPE/CI.... That should give me about 30 shots per fill (with the VanDerWaals correction), and I am very pleased with that....  I can dial the velocity up or down from there, which will lose or gain shots of course.... but I certainly have a good starting point for testing.... It shoots 982 fps with the 58.3 gr. Bowman HPs at this setting.... should be deadly on Varmints....

Bob
« Last Edit: March 23, 2018, 02:35:27 PM by rsterne »
  • Coalmont, BC

rsterne

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6mm Hits 2097 fps on Air
« Reply #112 on: March 23, 2018, 06:19:38 PM »
I swapped the 6 mm upper onto the unregulated gun tonight and filled it to its MSWP of 4200 psi.... I set the SSG to just a whisker of gap and fired one shot to settle things, topped it up and fired two shots with the 73.4 gr. bullets.... 1028 & 1029 fps (173 FPE).... Then I did something I have just been dying to try ever since I came up with the idea of building a 6mm.... I loaded up a 1.8 gr. Airsoft BB, topped it up to 4200 psi and took a bead through the Chrony.... 2093 fps.... repeated it.... 2097 fps.... once more.... 2087 fps.... That gives a three shot average of 2092 fps (17.5 FPE) with a basic PCP, just shooting very light ammo.... No special equipment, just a standard SS style valve, porting that turns through 180 deg., and a rifled 29" barrel that is a poor fit on the BB.... it will actually roll out the muzzle if you hold it pointing downwards.... Running air at room temperature (the pressure in the 300 cc reservoir dropped ~ 250 psi each shot), not Helium.... Lloyd still holds the record at 2162 fps.... but he better be looking over his shoulder....  ;D

Bob
« Last Edit: March 23, 2018, 06:23:28 PM by rsterne »
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rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #113 on: March 24, 2018, 12:17:38 PM »
Although I plan on running the .257 barrel on the unregulated gun, while I had the upper removed to try the 6 mm on it, I thought I would try the .257 upper on the regulated lower.... It had a slight leak in the transfer port area, but I did my testing anyways.... Here are the results for the 73.5 gr. Lyman 257420 at 2800 psi....



At 5% below the maximum velocity I was at 894 fps (130 FPE) at 8 turns of gap on the SSG, and the efficiency was 1.17 FPE/CI.... While that is similar to what I was getting on my .257 Hayabusa, it was a little below the 1.28 FPE/CI I got on the previous testing, which I attribute to the leak.... I was either losing a bit of velocity (likely), using more air (unlikely), or both.... but no question it would affect the FPE/CI either way.... If I was going to set this gun up as a regulated .257 I would likely bump up the setpoint pressure to 2900-3000 psi, although that may not be necessary with the 66 gr. HP version I have of this bullet, it might well shoot better at this pressure....

This pretty much completes the basic testing of these two guns.... I am extremely pleased with the results for both power and efficiency.... The plan is to put the 6 mm upper back on the 2800 psi regulated gun and run the .257 barrel on the unregulated version, and tethered it at 3800, but tune it for a few shots off tether.... All I hope now is that the snow disappears before the Motel gets too busy so that I have a chance to see how they shoot....

Bob
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