.30 cal Disco Double

[rsterne]

Broken Link....

Bob

[tkerrigan]

I just tried it by clicking and copy-pasting in address bar, worked both times.  Regards, Tom

[strever]

Tom
it doesn't work for the rest of us
gives this error message

Unable to read source image dimensions

[tkerrigan]

Try this one      http://imageshack.com/scaled/large/849/fcr0.jpg , I'm so embarrassed, I had deleted all the pics in imageshack, I must have been getting it from browser cashe.   Regards, Tom

[rsterne]

I got a chance to machine the .25 cal barrel and install it yesterday.... I reamed the chamber with a reamer I made myself (thanks Sean for the guidance)....  I cut the chamber in the TJ's .25 ACP liner (14" twist) to fit both pellets and bullets, and it should handle anything that will fit in an MRod magazine.... I then machined the O-ring groove and barrel port, using an oblong port measuring 3/16" x 1/4" which has the same area as the 7/32" transfer port I used for the .30 cal version.... I removed the .30 cal barrel, installed the new .25 cal in the breech and started testing....

I cranked the RVA to just shy of coil bind, left the screw that adjusts the bolt in the wide open position, filled the gun to 3000 psi and tried a few shots with different pellets / bullets to find out the maximum the gun could deliver, with the following results....



Just as the .30 cal exceeded my expectations, so did the .25 cal.... The maximum power is down about 16 FPE compared to shooting simlilar weight pellets in the .30 cal version, as could be expected.... I then proceeded to shoot some strings, and I found out that with the 52.7 gr. RN bullets I have to run the RVA nearly at coil bind (in fact just 1/2 turn out) to get the best string with a 3000 psi fill at maximum power.... Here are the results of testing with those bullets.... The bullets were unsized, and I think the variations in diameter as cast are causing the shot to shot variations in the strings....



When I turned in the velocity adjusting screw (which causes the nose of the bolt to partially block the barrel port) I found that I had to back the RVA out 1 turn to get the best strings.... Here is a summary of the averages for each string shown on the graph....

Maximum: 894 fps (93.5 FPE) for 9 shots at 0.80 FPE/CI
3 turns in: 856 fps (85.8 FPE) for 12 shots at 1.02 FPE/CI
4 turns in: 827 fps (80.1 FPE) for 15 shots at 1.04 FPE/CI
5 turns in: 779 fps (71.1 FPE) for 17 shots at 1.07 FPE/CI
6 turns in: 712 fps (59.3 FPE) for 21 shots at 1.15 FPE/CI

I didn't test the gun with the velocity adjuster in 7 turns.... It wouldn't have been in the 300 fps range like it was with the .30 cal because the oblong barrel port would not have been as completely blocked by the nose of the bolt, and I didn't care about velocities below 700 fps anyway....  In the next post I'll tell you what happened when I switched to the JSB Kings....

Bob

[rsterne]

Tom, I thought understood what you meant, but I wanted to see the photo to make sure.... I don't see how that could work in my application.... You would have to load the pellet whatever length the "streamlined" portion was ahead of the barrel port in order for the full diameter portion to obscure the port.... That extra wasted volume behind the pellet would surely take away from any theoretical advantage to turning the flow smoothly around the corner.... Your setup I'm sure works great when positioned as you would at full power and with a non-retracting bolt.... but with a bolt where you seat the pellet and then retract it to the firing position, you would be loading the pellet further forward than necessary or desireable.... unless I'm missing something?....

Bob

[CarsonRatSniper]

Awesome 25 results Bob!
You're right where I'm at with the Double Tube - I'm getting 10 full power shots and consuming 70 psi per shot.

I'm really eager to see how your TJ barrel shoots. Did you lap it prior to installation?

[rsterne]

Now that the initial testing was done with the cast bullets, it was time to try out the .25 cal Disco Double with my favourite pellet, the 25.4 gr. JSB King.... I left the gun tuned the way it was at the end of the bullet testing, which was about 60 FPE.... I expected I would have to reduce the hammer spring preload with going to half the weight pellet, and that was exactly the case.... Here are the results.... The green line is the way the gun was set for the last test (also a green line) with the bullets.... You can see that it pretty much started on the top of the bell-curve, I had to turn the RVA out 2-3 more turns to move the shot string over....



Here is summary of the averages for the shot strings in the above graph....

Maximum: 1162 fps (76.2 FPE) for 7 shots at 0.65 FPE/CI (all supersonic)
4 turns out on RVA: 1097 fps (67.8 FPE) for 12 shots at 0.77 FPE/CI (just subsonic)
5 turns in on velocity screw: 991 fps (55.4 FPE) for 18 shots at 1.09 FPE/CI
6 turns in: 922 fps (47.9 FPE) for 19 shots at 1.03 FPE/CI

There are a few useful shots missing on the last string, backing the RVA out to 4 turns out like in the other strings (it was at 3) would have dropped the velocity slighty, added shots, and increased the efficiency.... Tunes in between any of these can be obtained by using adjustments finer than a full turn, of course.... One thing that I was very pleased about was how tight the groups were with the Kings.... I'm used to seeing ragged holes at 20' even when I'm not taking my time and just shooting through the Chrony, but here is what I was getting for 8 shots.... Yep, that's just a 1/4" hole!....



When I was doing the Chrony work with the bullets I was using up some unsized bullets I had that had very uneven bases, bent by the sprue cutter.... The groups were unacceptable, even for Chrony work inside at 20'....



When I was done with the Chrony testing I switched to the last batch of .25 ACP bullets I cast (some 46.4 gr Hollowpoints) and ran an 8-shot magazine through to see if they were any better....



While not as tight as the Kings, this group is consistent with what I usually see with good pellets while doing my Chrony work inside, and usually means I can expect to be able to cover the group with a dime at 25 yards when I move outside.... It certainly proved to me, once again, how important it is to have as perfect and square a base as possible on a bullet.... It just makes sense, it causes the same problems as a bad crown on the muzzle, right?....

One of the things that amazed me during the testing of the JSB Kings was how much I had to restrict the flow to get the velocities in the sub-1000 fps range.... At 5 turns in on the velocity adjusting screw, the nose of the bolt is blocking 0.16" of the 0.25" long barrel port.... leaving just half of a 3/16" hole available for the air to flow through to drive the pellet.... That is basically the same area as a stock 0.134" Crosman barrel port.... AMAZING !!!

Bob

[rsterne]

I lapped it with JB Bore Paste (100 strokes full length; 100 more tapering, more at the breech) and then polished it 50 strokes with Bore Bright prior to installation.... I pretty much do that to every barrel I make.... First time it's off the gun, or if accuracy falls off, I give it 10-20 strokes with Bore Bright again....

Bob

[CarsonRatSniper]

That's what I figured.

If you keep those Kings right around 1020 fps you should see .5" groups at 75 yards.
That is using lubed pellets though..........dry (straight from the tin) my groups opened up.

So the Max useable length on an Mrod magazine is .4"?
Or is it .39?

I can make a Spitzer .390" long and have .150" shank length for bore stability - plus with the base obturating to bore diameter I don't see an issue with yaw down the bore line.
What it weighs we wont know until I've punched one out and put it on the scale.

I love how you've added yet ANOTHER tuneable dimension to your gun with the bolt probe depth adjuster! 

[tkerrigan]

Tom, I thought understood what you meant, but I wanted to see the photo to make sure.... I don't see how that could work in my application.... You would have to load the pellet whatever length the "streamlined" portion was ahead of the barrel port in order for the full diameter portion to obscure the port.... That extra wasted volume behind the pellet would surely take away from any theoretical advantage to turning the flow smoothly around the corner.... Your setup I'm sure works great when positioned as you would at full power and with a non-retracting bolt.... but with a bolt where you seat the pellet and then retract it to the firing position, you would be loading the pellet further forward than necessary or desireable.... unless I'm missing something?....

Bob

Bob, I started coming around to your argument this morning, thinking dead air space all the time might not be good.  I am there now.  Regards, Tom

[rsterne]

A ,25 cal MRod magazine measures 0.48" where the middle of the nose of a pellet would be (eg. on a Predator), but the recess for the nose is only about 5/32" wide and radiused.... If the nose of the pellet is wider than 5/32", it won't go into the recess at all, and then the length available is 0.45".... If the bullet is too tight a fit, and it cocks sideways even a little, it can prevent the magazine from indexing.... My ACP Hollowpoints are 0.436" and the nose doesn't drop into the recess and they work fine.... The RN soild of the same bullet is 0.471" and it fits in the magazine but jams because it doesn't drop quite deep enough into the recess.... I made my new bullets (which have a 0.130" Meplat) 0.455" long and I'm hoping they work OK.... I may have to shave the top of the mold 0.005" if they don't....

Bob

[rsterne]

I threaded the muzzle for the Hatsan air stripper and installed the carbon fibre shroud and stripper.... The accuracy seems unchanged inside at 20', but it's pretty hard to distinguish small changes at such close range.... I tried some Daystate Rangemaster 36 gr. pellets as well, which were pretty dismal in my Hatsan, and they shot nearly as tight as the JSBs, so it looks like the warning on the box about being intended for unchoked barrels may have merit.... I tested them at 950 fps (72 FPE) and they can be tuned both up and down from there.... so definitely worth invetigating....

Bob

[CarsonRatSniper]

.455" long shows a weight of 51 grains. Launched at 895 fps (90 ft lbs) it retains 77 ft lbs and drops 9.8" @ 100 yards.

Of course that length will be lighter due to the base punch.

Can anybody figure out the cubic volume of a cone?
Its .190" in diameter at the base and travels at a 35* angle until point intersection.
If I know the cubic volume of the base punch I will know how much weight to deduct from the Border Barrels design model.

Where are you getting this melt & pour soap? Sounds like I might want to have some on hand.

[Bill G]

V=Pi(r^2)h/3

Surface area= Pi(r^2)+Pi*rsqrt(r^2+h^2)

Hope that helps

Bill

[CarsonRatSniper]

It does, except offhand I don't know the value of 'h'.

[rsterne]

For 35* per side, "h" is 0.135", ie if the cone is 0.190" at the base and 0.135" tall, the angle at the top will be 70*.... I get a volume of 0.00127 cu.in.... Lead weighs 0.409 lb..cu.in. adn there is 7000 gr. per lb., so that works out to 3.65 grains....

Bob

[CarsonRatSniper]

Thanks Bob!

[rsterne]

Today I tried several different tunes for the JSB Kings around the 55 FPE level.... The plan was to try adjusting the hammer preload weaker (and drop the fill and refill pressures) while opening up the barrel port by backing out the velocity adjusting screw.... I wasn't fussy about getting exactly the same velocities with each tune, I recorded all the shots within a 4% ES (~40 fps), and all the shots were between 965 fps and 1018, so the strings were within about 15 fps for the peak velocity.... I backed off the hammer spring preload a turn, dropped the fill pressure 200 psi, and opened up the barrel port to keep roughly the same velocity for each string.... Here are the results....



The numbers on the legend are the number of turns in from maximum power on the bolt travel (barrel port restriction) and the hammer spring preload, measured from the lock nut to the end of the adjuster (bigger distances mean less preload).... The -5T and 0.358" curve started at 3000 psi, and the last string shot, (-2T and 0.568") started at only 2000 psi....  I know it's a bit hard to see, with all the lines on top of each other, and the curves look silly with the drastically expanded vertical scale.... but here are my conslusions....

1. There was little to choose from for the efficiency.... All the strings were between 1.02 - 1.10 FPE/CI, with no real trend apparent.... The differences may have been more due to some strings being a few fps faster, or missing a shot or two on the front end of the string because the starting pressure I used wasn't quite high enough....

2. As the pressure range dropped and I compensated by opening up the porting to keep the same power, the shot strings got shorter and more curved.... The range of pressure between the ends decreased.... Once again, it is apparent that wide open ports can develop good power at low pressures, but the price you pay is a low shot count.... You can equate big ports to a hot cam in a racing engine.... More power for a given pressure, but a narrow power band....

3. All these tunes used about 50 psi per shot from the 250 cc Disco Double reservoir.... in fact the range was narrow, between 48-52 psi per shot.... Each vertical line on the graph therefore represents about 50 psi.... With the ports wide open, I was able to get 1000 fps (55 FPE) from only 1700 psi, which is pretty amazing, IMO....

4. The longest shot string was at about 2800 psi fill pressure, and ran 18 shots, but it was missing 1-2 shots off the front end of the string, and peaked at over 57 FPE.... I feel that with some fine tuning, I can achieve 20 shots at 55 FPE average from a fill, within a 4% ES.... I can already get 13 shots within a 2% ES, and 17 shots within a 3% ES, averaging closer to 56 FPE with the Kings....

While the purpose of this gun was to achieve a higher power level using cast bullets, I'm very pleased with the flexibility demonstrated to shoot JSB Kings at 1000 fps with a good shot count.... and once again I was provided with the opportunity to learn more about PCP tuning....

Bob

[CarsonRatSniper]

Fantastic information (as usual!).
I've learned more about tuning a PCP from reading your information and test procedures than just about all other sources combined.

My Double Tube for my Challenger is 22" long vs the standard 18" which gives me roughly 285 cc's.

With my hammer preload screwed all the way out (off of the hammer spring but still attached to the adjuster body) I get an average of 1023 fps from a 3k psi fill.

Starting with 2900 psi, here's a 10 shot string:
998
1010
996
1023
1022
1030
1033
1038
1040
1041
Ending at 2550 psi

My guns powerband using cast bullets is from 2600 psi down to 1900 psi.

For pellets I try to keep my pressure between 3000 - 2500 or it fires the JSB'S too fast and I lose accuracy.

If I trade my 10.6# hammer spring for a lighter one it will be more pellet friendly but I will lose my horsepower for cast bullets.

I still think the Discovery valve design is the best platform for building on! Especially with a Double Tube kit.

Sorry Mrods.................

I have to give it to you Bob - you've built the most tuneable non regulated powerhouse I've seen with that breech of yours on that Disco Double!!!