Disco Double Meets JSB King Heavies

I got 3 tins of the new 34.2 gr. JSB King Heavies a month ago, and finally got around to trying them in a couple of my guns.... They are the earlier version with the larger skirts, and they just hang up when loading in the MRod magazines.... What I mean by that is that some of them fall into the wheel in the magazine, and others hang up of the edge, although they are so close to going in that the wheel still rotates no problem.... However, you can certainly feel resistance when chambering them in my .25 ACP barrel, so I think the Mark II Heavies with the smaller skirts (the same size as the old Kings) might be a better choice.... I can't do any accuracy testing until the snow melts around here anyway, maybe by then the smaller ones will be available in Canada.... Anyway, here is the gun....

http://i378.photobucket.com/albums/o...psc4058736.jpg

It uses a Disco main tube, containing one of my modded Disco valves, ported to the same specs as my Millenium Pumper (but with a PEEK poppet), with 0.219" ports and a 0.266" throat, and mounted with 10-32 screws.... Under that is a Disco Double lower tube (from AirGunLab), and the fill pressure is 3000 psi, with a total of 250 cc (15.2 CI) of air on tap.... The breech is my own, and accepts MRod magazines.... and the gun is a rear cocker.... I have four barrels for it, in .22, .25, .30, and .35 cal.... although the ports are really a bit small for the largest caliber, it tops out at only 170 FPE avg. for 3 shots with 127 gr. BBTs.... It is wayyyyyyyyy overpowered in .22 cal, of course, you have to dial the power back a lot to keep it subsonic, even with heavy pellets.... The gun is optimized for .25 with bullets or .30 cal with pellets, (both at 50 gr.) shooting at about 90-100 FPE either way.... With 25.4 gr. JSB Kings, it has to be detuned a lot, but gets over three mags. per fill at 55 FPE with them.... The stock is a Boyd's Blaster in Forest Camo....

Anyway, the first step with the new 34 .2 gr. King Heavies was to find out what it would shoot maxed out, and I was a bit shocked when it blasted them through the Chrony supersonic at 100 FPE.... Here is a graph showing what this gun does tethered at 2900 psi, with the three types of ammo....

http://i378.photobucket.com/albums/o...ps2zka51dp.jpg

As you can see, the velocity on the plateau with the 34.2 gr. heavies is just above Mach 1.... As you dial the preload back, the knee of the curve occurs where the spring has no preload, and below that the gun is a LOT quieter, and the efficiency increases rapidly.... One feature this gun has is a probeless bolt, that loads the pellet ahead of the barrel port, and then withdraws back into the lower "J" bolt slot to clear the port, leaving no restriction to flow.... However, I added a screw adjuster on the back of the breech that slides a brass pin forward into that slot, preventing the bolt from fully clearing the slot, restricting the port and reducing the velocity.... You can see it in the photo below....

http://i378.photobucket.com/albums/o...ps811e14d1.jpg

The basic method for tuning this gun is to set the hammer spring preload to the knee of the curve at the pressure you are filling to, and if the velocity is too high, then you screw in that pin until the nose of the bolt chokes down the flow and you get what you want.... By playing with the two adjustments, you can get whatever fill pressure you want, and whatever velocity you want.... For the King Heavies, I ended up with the preload out just 1/2 turn from zero, so the hammer spring is just loose when the gun is uncocked.... This preload setting is the same as what I use for the 50.6 gr. Bob's Boattails.... However, where the barrel port is run wide open with the BBT's, to get the velocity into the mid 900s for the Heavies, I needed to close off the barrel port two-thirds of the way.... Here are the shot strings I get with the three types of ammo....

http://i378.photobucket.com/albums/o...psaa4femqu.jpg

Bear in mind that these strings are optimized for a 4% ES and a 3000 psi fill.... With the Kings, the string ends at 1860 psi, with the Heavies and BBTs the ending pressure is 1800 psi.... If I want a narrower ES (with fewer shots), I can either fill to a lower pressure and refill sooner (just using the top of the curves).... OR I can increase the preload slightly, open up the barrel port, and have a few shots at nearly constant velocity with much higher FPE.... If I wanted to shoot off a tether at 2900 psi, I could tune to the knee of the curve and shoot the 25.4 gr. Kings supersonic, the 34.2 gr. Heavies at 1070 fps, or the BBT's at 950 fps (100 FPE).... Of course if I need even more power, all I need to do is throw on the .30 cal barrel....

Bob