Regulated Disco Double

[rsterne]

I burned through another tin of 34.1 gr. JSB Beast pellets today (actually more than a tin), playing with the tension adjustment on the barrel.... I found out that a "1 hour" change (about 65 lbs.) to the tension makes a significant difference to the group size.... but that virtually all the groups are better than previously, since I installed the intermediate support on the barrel.... Having a support between the barrel and shroud at 3/7ths of the distance from receiver to muzzle really settled down the variation in group size as you change the tension, bullet, or velocity....

I increased the velocity with the Beasts to 960 fps (70 FPE), and found that there was a sweet spot just past 1 turn of tension.... Using a 24 hr. clock, starting from zero tension, 1400 hrs. (2 hrs. past 1 turn) gave me the best groups.... Here is an example.... Yes, that is a 34 gr. JSB Beast .22 cal pellet sitting in the hole from a 5-shot group....



About half the 5-shot groups at that setting met the "Feinwerkbau Test".... Although I only have 20 ft. available, not the 10M that FWB uses.... I find that when a gun shoots like this inside, if there is a problem at long range, is tends to be the pellet/bullet, not the gun.... At this velocity, I am running right on the knee of the curve at the 2000 psi setpoint with the Beast, it uses 1200 psi for 10 shots, which works out to 1.13 FPE/CI.... I think that is pretty good for a .22 cal at 70 FPE.... With a 3000 psi fill, the last shot (taken below the setpoint) loses about 100 fps, but by filling to 3200 (the gun is safe to at least 3500 psi) the gun is sitting right at the 2000 psi setpoint at the last shot, and the ES is hovering right at 1% (8-10 fps) over the 10 shots....

This is pretty much the kind of tune I was looking for, so I think I'm done with the development of this gun until I can get outside in the Spring to test at longer ranges.... I can chalk this up to another very successful project.... with many thanks to Lloyd Sikes for supplying his new Regulated Disco Double Kit so that I could put it through its paces.... I would highly recommend it for anyone modding a Discovery.... It is capable of a ton of shots at lower pressures, and lots of power when you crank the pressure up.... The external adjustment is easy to use, and covers a wider range than I needed (I used 900-2200 psi during this project).... It can be used with a stock Disco, or one that is highly modified like this one.... I don't see how you can go wrong with it.... Congratulations, Lloyd, on another fine product from the AirGunLab....

Bob

[screwwork]

Nice job on the RDD and get 70fpe at 2000PSI  Bob
You really ran the RDD through its paces
 

[rsterne]

I wanted to find out what affect the Velocity Adjusting Screw on the back of the receiver had, and document it, so I did some testing today.... I tried three different weights at each of the 10 settings (1 turn apart) on the 8-32 adjusting screw by shooting a few shots, taking the average velocity, and plotting the results.... Here is that data....



The nose of the bolt doesn't quite block the barrel port completely, because the front end of the port is rounded to streamline the flow into the barrel.... and when you first start obstructing the port, the velocity doesn't start to drop until you turn the screw in a couple of turns, even though you can see the nose of the bolt through the back of the barrel and transfer port.... I presume this is because there is a bit of wasted volume in the back corner of the chamber, causing a bit of pressure loss when the bolt is fully retracted, which counterbalances the obstruction of the hole when you first start moving the bolt forward.... The black line represents my best estimate of the percentage of barrel port available at each turn of the adjusting screw....

Note that at 2000 psi, the 18 gr. JSB Heavies are Supersonic, hitting nearly 1200 fps.... in fact they don't drop below Mach 1 until the port is about 50% obscured.... I was interested in finding a setting in the mid 900s with each weight, so that I could run a string with each pellet/bullet.... It turns out that with the 18 gr. JSB Heavies I only needed the adjuster open 2 turns, and with the BBTs 4 turns, compared to wide open for the 34 gr. JSB Beasts.... I then used those settings and shot a string starting at 3000 psi, until the velocity dropped off.... Those strings are shown below....



You will note that with the 34 gr. Beasts I get 10 shots, averaging 67 FPE, before the velocity starts to drop off.... With the 30 gr. BBTs, I get 11 at 57 FPE.... and with the  18 gr. JSB Heavies I get 15 shots at 36 FPE.... The efficiency is slightly lower with the BBTs, 1.13 FPE/CI compared to 1.18 with the Beasts.... and even lower with the 18 gr. heavies, down to 1.00 FPE/CI.... This is in sharp contrast to what happens to the efficiency if you back off on the hammer spring preload to reduce the power, instead of choking up the ports.... It makes perfect sense, because choking the port has little effect on how much air the valve releases (other than a bit of increased backpressure), because you aren't really changing the dwell, you are just forcing the air to flow through a smaller port, which is less efficient.... This means that for a quick adjustment in velocity to go with an ammunition change, the Velocity Adjusting Screw is great.... but if you are after increased shot count there are better methods.... Reducing the hammer strike will give you great efficiency, but the velocity will bump up below the setpoint.... A better way is to reduce the regulator setpoint AND the hammer strike, as I showed with the 18 gr. JSBs much earlier in this thread.... I have found previously, however, that a combination of all 3 methods produces the best shot strings with lighter pellets.... In my .25 cal Disco Double, with the basic tune set to shoot 50 gr. bullets at 100 FPE.... the best strings with JSB Kings were achieved with a lower setpoint, less hammer strike, and a slightly restricted barrel port.... It's nice to have the option of that added tuning element when you want to go plinking....

Bob

[rsterne]

The second half of today's testing was penetration and expansion testing of the different pellets and my new Bob's Boattails, both the 29.6 gr. FN and the 27.4 gr. HP.... I used the same settings of the velocity adjuster as for the strings above, 2T out for the 18 gr. JSB Heavies, 4T out for the BBTs, and 9T out (wide open) for the 34 gr JSB Beasts.... The test medium is my standard "Melt-and-Pour" soap, cast into blocks big enough to stop the bullet.... Here is a photo of the block with the four rounds inside it....



The BBT Hollowpoint is entering from the left (at 57 FPE).... From the right, top to bottom, are the 18 gr. JSB Heavy (at 36 FPE), the 34 gr. JSB Beast (at 67 FPE) and the 30 gr. BBT FN Solid (at 57 FPE).... Those FPE levels are the result of wanting all the bullets to be in the mid 900s fps range when they hit the block of soap.... The 34 gr. Beast had by far the greatest penetration, at 6.5".... The 30 gr. BBT and the 18 gr. Heavy were nearly identical, at 4.2" and 4.1" respectively.... while the 27 gr. BBT HP exploded violently and dumped its energy by creating a large cavity before stopping only 2.4" into the soap.... The volume of the wound channel in the block is roughly proportional to the FPE.... I then cut the block apart to photograph the wound cavities with the bullets still in place.... Here is the photo of the 18 gr. JSB and the 30 gr. BBT FN....



Now you can see why the BBT stopped in about the same distance as the 18 gr. JSB.... even though it had way more FPE.... The pellet hardly changed at all, which the large Meplat of the BBT expanded to well over bore-size, while leaving a much larger diameter wound channel than the pellet.... kind of the difference between an ice-pick and a sledge-hammer.... Next is the photo of the 34 gr. Beast and the BBT HP....



The block cracked behind the JSB Beast, so I didn't get to photograph it, but the wound channel was long and thin.... and you can see the pellet was hardly changed at all by the 67 FPE impact.... Not so the 27 gr. BBT Hollowpoint.... It literally blew apart after entering only about 3/4" into the soap, leaving three large fragments of the nose (and 1 small one) behind as the base (and 1 more fragment) continued penetrating.... Below is a photo of the bullets and pellets after recovery.... BBTs on the left, pellets on the right....



As you can see, round nosed JSB Pellets punch through and penetrate with very little change in shape.... Both pellets showed a very slight flattening in the center of the nose, and you can see how the skirts have obturated (blown out) from the pressure inside the skirt.... The Meplat on the BBT FN solid nearly doubled in diameter, showing how effective the large (70%) Meplat is in providing energy transfer to the target while limiting penetration.... The Hollowpoint BBT, however, is in another class entirely.... The nose section blew apart, creating a wound cavity in the soap nearly 3 calibers wide, while the rear portion of the bullet mushroomed completely and continued to penetrate, although less than 60% of the distance penetrated by the solid.... I would suspect that bullet to be devastating on Varmints.... and look forward to trying it out in the Spring....

Bob

[Buldawg76]

Those are some very interesting results from the pellets and BBT and I would agree the BBT HPs will be devastating on varmints and drop them in their tracks. The pellets should go right thru most game unless they hit a bone directly.

Mike

[Machinist]

Lots of interesting data here to read thru.  I'm surprised at the velocity you got from only 1150 psi with 18.1 pellets. The effect barrel tension had on accuracy was cool too.

The gun has lots of cool features, methods of adjustments, details. Looks like a real winner.

Steve

[Machinist]

How long does it take to change the reg setpoint?  Are there 2 gauges on the gun, or just one?  I was thinking of how easy it is to make adjustments on this gun. Went back and looked at some pictures too.  When the data is overwhelming, I look at the pictures.

Steve

[rsterne]

Just a few minutes.... You shoot the air pressure down to a pressure below the lowest setpoint pressure you want or have.... pop the plastic cap off the upper tube, and using a slotted screwdriver adjust the pressure setting.... Refill the reservoir and check the new setpoint pressure on the onboard gauge.... There is only one, and it is in the original position for a Disco, on the bottom, just in front of the trigger, and reads the setpoint pressure....

If you want to increase the pressure, you turn the screw CCW, and you only have to bleed the gun below the existing setpoint.... If you want to decrease the setpoint pressure, you turn the screw CW.... Since you are screwing the seat towards the regulator piston, you must first bleed the pressure to a point BELOW what your new setpoint will be.... Otherwise, when you screw the seat inwards, you could damage the seal in the end of the piston.... For most of the pressure range, it takes about 1 "hour" on the clock face for 150 psi of pressure change.... ie 1/4 turn is about 450 psi....

Bob

[rsterne]

I made one additional test today, I reinstalled the SSG.... I had not tried it since I increased the hammer travel to 0.75" and adjusted the setpoint to 2000 psi.... There was no problem getting either weight of BBT into the mid 900s with a decent gap, and I could just barely push the 34 gr. Beasts at 950 fps (68 FPE) with the gap adjusted to zero (but no preload).... There is still room to increase the hammer travel slightly, which would give me room to create a gap and still shoot the Beasts at 70 FPE.... but with the stunning performance of the BBT Hollowpoints in my expansion tests, and assuming I can get them to shoot accurately, I think I will set the gun up for those.... With the gap increased to 0.10" (2.5 turns out), the velocity with the 27.4 gr. HPs was 960 fps, and I shot a 10 shots string with an ES of just 10 fps (1%) using just 740 psi of air.... and a LOT quieter.... That works out to 1.45 FPE/CI, a 28% increase in efficiency compared to the same spring with preload at the same FPE.... The gun is a bit harder to cock, which could be fixed easily by using a longer, softer spring, but I don't have one on hand at the moment.... It sure shows the benefit to an SSG over a conventional setup, though.... With the ability of this gun to be filled safely to 3500 psi, I should now be able to get two 10-shot mags on a single fill, which at 56 FPE is pretty impressive....

Bob