.30 cal Disco Double

[rsterne]

My new Boattail bullet mold arrived a couple of days ago and I cast some bullets yesterday.... The new .300 cal bullets came out at 70.4 gr. and they function perfectly in the modded MRod magazines....



Here is what they look like....



Since these bullets are a lot heavier than the 50 gr. JSB pellets the gun was built for, I maxed out the hammer spring preload and the velocity adjuster and ran a string.... It peaked at 114 FPE and averaged 837 fps (110 FPE) for 9 shots.... so a little bit more hammer strike is required to get the maximum from one 7-shot clip.... I'm hoping to fix that issue by replacing the valve poppet with one made from PEEK which is a harder material and takes less energy to break loose from the seat.... I shot a couple of other strings with the velocity adjuster run in a bit (which restricts the barrel port with the nose of the bolt), and here are the test results....



The last string was two full clips (14 shots) averaging 86 FPE at an efficiency of 1.11 FPE/CI, which is the same energy as I got with the 50 gr. pellets when the gun was set for 14 shots at fractionally better efficiency.... The way I see it, the best use for these bullets will be with the gun adjusted for a single clip at maximum power.... The bullets were all going into ragged holes between 3/8" and 1/2" OD during Chrony testing, which is pretty good for cast bullets.... Proper accuracy testing will have to wait until the spring, however....

Bob

[OleTomCat]

Great work and excellent information as usual Bob....

[rsterne]

This is a duplicate of the post in the Machine Shop Gate.... All Geeks please respond over there !!!

Well you knew I had to do it !!! .... My .30 cal Disco Double is one of the best documented guns I have, and with the new 70 gr. bullets it's now limited out on performance due to insufficient hammer strike.... or rather it WAS, until today when I made a new poppet out of PEEK for it.... All I can say is WOW !!!

Before I tore the gun apart, I made some measurements of valve lift, using an O-ring on the rear cocking shaft.... This is an easy procedure, you slide the O-ring up against the adjusting screw on the RVA, cock the gun, fire it, and measure the gap.... Voila!, instant measurement of how far the valve opened.... I set the preload to maximum (as near coil bind as will safely cock), opened the bolt stop to maximum (this gun has the ability to choke off the barrel port with the bolt nose), loaded a 50 gr. JSB, and checked the valve lift at 2800 psi and again at 2000.... I did 2 shots to confirm the measurement, and I was pleased that they were the same within 0.001" or so.... I then repeated the test with no pellet loaded (open bore), and then I moved the bolt nose forward with the adjuster to close off the barrel port, loaded a pellet to block the bore even more, and did another set of lift measurements.... These three conditions change the flow around the head of the poppet, from "normal" to as much flow as possible (barrel at atmospheric with no pellet), to virtually no flow where the pressure on both sides of the poppet is nearly the launch pressure (the velocity drops to about 200 fps, you can see the pellet arc through the air to the backstop).... This gives us a way to somewhat quantify the aerodynamic drag trying to close the poppet, relative to the major closing force, which is the air pressure acting on the 5/32" valve stem.... Anyway, I then tore apart the gun and made a new poppet out of PEEK, reassembled it, and repeated the tests.... Here are the lift measurements....



As you can see, the PEEK valve had about a third more lift at 2800 psi, with the same hammer strike, so it is obviously a LOT easier to unstick from the seat.... There can no longer be ANY doubt of that.... The lift of the PEEK valve at 2000 psi was limited by the physical room between the face of the hammer and the back of the valve, which is 0.162".... You can also see that compared to the "normal" case of firing a pellet, with no load (and therefore a bigger pressure difference across the head of the poppet) the lift is slightly less.... while with the barrel nearly blocked (and therefore very little pressure difference across the head of the poppet) the lift is slightly greater.... That difference in lift is due to the aerodynamic drag across the head of the poppet, which shows up as a pressure difference between the valve housing and the exhaust port, which in turn increases the closing force during the valve cycle.... The differences are smaller with the PEEK valve, likely because the head is a smaller OD.... Here are the two poppets....



You wll notice that the Disco poppet, which has seen maybe 500-1000 pellets during testing, is pretty badly pounded.... While it is not extruded like the one in my Hayabusa was (which had a larger throat), it would appear that 3000 psi on a 1/4" throat is really more than it should see.... The ID of the Disco valve, BTW, was bored out to 5/8" ID so there is lots of room for flow around the head.... When I made the new PEEK poppet, I made the OD the same size as the valve spring.... The stem is made from 5/32" drill rod with the end threaded 8-32 for 1/4" of length and then the stem is slimmed behind that to 7/64", the same as I did on the Disco stem.... The PEEK valve head is drilled and tapped 8-32 and glued onto the stem with medium viscosity CA glue before final finishing.... Thanks, Lloyd, for the idea of threading it on for extra strength !!! ....

Of course my testing didn't finish with just measuring the lift, I had to see if the new PEEK valve increased the performance.... Well, the first thing I noticed was that the gun was louder and a lot thirstier for air.... My SCUBA tank was down to 2800 psi, but that was enough to drive a 50 gr. JSB at 1033 fps (119 FPE) compared to a previous best of 1011.... The 70 gr. Boattail, which I could only get to 852 fps yesterday screamed across the Chrony at 918 fpr (131 FPE).... I'm sure those numbers will increase further at 3000 psi.... I then started backing off the preload, with the pressure in the SCUBA tank gradually dropping, and it was obvious that even with the heavy, 70 gr. bullets, the gun, rather than being limited by hammer strike, was operating on the velocity plateau for the pressure available.... I had to back the preload down 5 turns before the velocity started to drop significantly.... At 6 turns out, the velocity pretty much matched the way the gun shot with the Disco poppet at maximum preload, starting at 830 fps and climbing gradually.... There is one problem, however.... The gun is suffering from a LOT of air robbing hammer bounce.... It appears that the PEEK valve is so easy to open that the returning hammer is burping it a couple of times, even at velocities where I was getting a decent string yesterday....

My ShoeBox is currently chugging away topping up my SCUBA tank, so further testing will have to wait.... I'm guessing that I'll have to use a shorter, stiffer hammer spring to get rid of virtually all of the preload to eliminate the hammer bounce.... I know that something will work, because my Hayabusa doesn't have this issue, nor does Sean's .30 cal QB.... It looks like a whole new learning curve is required to find the best combinations to work with the new PEEK valve.... There can be no question, however, that by using this material for the poppet (or seat) you can immediately reduce the hammer strike required for a given pressure and valve throat size.... Can lighter hammers be in my future?.... certainly the gun just got a whole lot easier to cock....

Bob

[rsterne]

I had a chance tonight to find out how the .30 cal Disco Double responds to hammer spring preload with the new PEEK valve.... Previously there wasn't any point as the gun couldn't reach its maximum potential as the hammer strike was the limiting factor, not the porting.... With the new valve, and tethering the gun at 3000 psi, I could now explore the guns power curve.... Here it is, plotted for both 50 gr. pellets and 70 gr. bullets....



You can see the same velocity plateau as the gun maxes out as you get on a regulated PCP, where additional hammer strike just wastes air.... The "knee" of the curve is where the plateau starts, and there is no point in running more hammer spring preload than that.... This occurs at about 4 turns out from coil bind with the pellets and 3 turns out with the bullets.... Going past that point gains only about 20 fps and doubles the amount of air used.... Even at that setting, the first shot will basically be the fastest, normal unregulated bell-curves start just below the knee of the curve.... at maybe about 5 turns out with the bullets and 6 turns out with the pellets....

The absolute maximum values obtained with this gun were 117 FPE with the 50 gr. pellets and 135 FPE with the 70 gr. bullets....

Bob

[rsterne]

I was a bit worried about the excessive air consumption with the PEEK valve due to air-wasting hammer bounce.... The only string I recorded yesterday was 6 shots, 855-864-833 with the 70 gr. bullets, and that used 1100 psi for a horrible efficiency of 0.59 FPE/CI.... Something obviously had to be done !!!

As a cure, I decided to try a shorter, stiffer hammer spring to get rid of the significant preload with the QB spring.... Lloyd was the first to notice an efficiency increase from eliminating the preload, and both Sean and I have observed it too, so it seemed like a logical place to start.... I had to shorten a 2.5" x 0.059" wire spring that I had to 2.0" by cutting off 4 coils.... The spring was a snug fit in the hammer (so it would bind on compression), so I had to drill it out one size as well, to 3/8" ID.... I was still able to use the 7/32" spring guide / rear cocking rod.... I had the preload set for 5 turns out with the QB spring, and to my surprise, with the new spring, that put me right at zero preload but no slack as well, exactly where I wanted to start.... The first shot across the Chrony was at 1000 fps with the Daystate 50 gr. pellets, so I knew I was close.... 

The new stiffer spring is a LOT more sensitive to preload adjustment, of course, at 1 and 2 turns in the velocity maxed out at 1021 fps, which showed me that zero preload was right where the plateau started.... Since I wanted to be on the knee of the power curve, that meant I would have a slight amount of free play in the spring, which I hoped would eliminate, or at least severely curtail, hammer bounce.... At 1/2 turn out the velocity was 972 fps, at 1 turn out it was 956, 1.5 out it was 935, and at 2 turns out is was 904 fps, so I chose 1 turn out for my first string.... My SCUBA tank was down to 2980 psi, so the first string started there, and I got 7 shots (1 clip) within just a 7 fps range (0.7% ES), averaging 100 FPE, and with an efficiency of 0.96 FPE/CI.... WOW ! I've never seen anything that flat before unless regulated.... Maybe this PEEK valve DOES produce tighter strings?....



I backed the preload down another turn, and cranked the velocity adjuster in 2 turns (which does very little) and shot another string.... This time I got 13 shots, starting from 2920 psi, 904-940-906, averaging 94 FPE at an efficiency of 1.09 FPE/CI.... I cranked the velocity restrictor in one more turn and repeated the string, starting at 2900.... I got 14 shots, 878-897-864, averaging 86 FPE at 1.10 FPE/CI, and the string should have started 1 shot earlier if I had a full 3000 psi in the tank.... Even so, this 2-clip string spanned less than a 4% ES, and over 14 shots will be a bit tighter with a 3000 psi fill.... The string stayed within a 4% ES down to 1860 psi, which is 100 psi lower than with a similar tune using the Disco poppet.... The PEEK poppet was truly showing its stuff !!!

I'm very pleased with the new PEEK valve, now that I have the proper hammer spring to compliment it.... The valve is easier to knock off the seat, reducing the hammer energy required to get this gun to full power from over 1.12 FPE (over, because I never quite got there with the Disco poppet) to 0.90 FPE.... or putting it another way, the valve gets a third more lift from a 1.12 FPE hammer strike.... The PEEK valve opens so easily, however, that it is prone to being reopened by hammer bounce.... I cured that by using a shorter, stiffer spring, that instead of preload, had a bit of free space for the spring to rattle around when uncocked.... This DRASTICALLY reduced the report of the gun from reduced air wastage.... I had thought previously that this was the case, but today's experiment proved that it can and does work.... The only downside is that the new spring is harder to cock.... The ultimate solution would be more hammer travel and going back to a QB spring without preload, but adding that much hammer travel is not an easy thing to do when I've already increased the travel from 0.58" to 0.75".... One thing I know for sure, there will be more PEEK poppets in my future, particularly on high pressure, large throat valves, where driving them open is difficult anyway.... 

Bob

[rsterne]

I finally got around to fitting my new .357 cal barrel this week.... Sean Pero and I designed a new airgun specific barrel to run anything from roundball to about 160 gr. bullets.... It is 0.357" groove, 0.350" land, with a 26" twist, with 6 narrow lands, and is hammer forged in CrMoly with a 1/2" OD.... The one I got is 28" long.... I had a chambering reamer I had made for my LW 9mm barrel, and it worked just fine.... I made the chamber long enough to work with the 132 gr. Lee RF bullets I had, but in fact even the 154 gr. will chamber without too much effort.... When you chamber a JSB .35 cal Exact, the head is most of the way engaged into the lead and the skirt stops just short of the beginning of the leade, which starts about 1/10" past the barrel port.... The bolt is sealed with a 9mm x 11mm x 1mm O-ring in a groove in the barrel.... It's a very skinny O-ring but seals well and so far seems to be standing up just fine.... The only thing that is a no-no is pushing the pellet out backwards into the breech, the skirt cuts the O-ring.... I found that out when I was checking what the rifling engagement was like....

The valve in the gun is the same PEEK valve I recently made for it, and it has a 1/4" throat and 0.219" ports, which are on the small side for this caliber, so I really didn't know what to expect for power.... The gun was still set up for the .300 cal I recently tested, so the first couple of shots were at that setting, and went across the Chrony at about 920 fps, which was a pleasant surprise, at nearly 150 FPE.... I topped up my SCUBA tank during dinner and this evening got down to some serious testing.... I filled the gun to 3000 psi, cranked the hammer spring preload to just shy of coil bind, and tested all the various 9mm/.357 ammo I have.... Here are the results.....

68 gr. Roundball: 999 fps (149 FPE)
78 gr. JSB Exact: 961 fps (159 FPE
90 gr. Air Venturi HP: 925 fps (171 FPE)
117 gr. Air Venturi RN: 848 fps (187 FPE)
126 gr. Air Venturi RN: 817 fps (187 FPE)
132 gr. Lee RF: 808 fps (192 FPE)
154 gr. Lee RN: 742 fps (188 FPE)

In every case, the second shot was a lot slower, and 2 shots dropped the pressure to 2260 psi, which works out to an ear-splitting 0.40 FPE/CI for (non)efficiency.... Well, the gun was a bear to cock like that, but at least I had enough hammer strike to get everything out of the restricted porting that was possible.... It was pretty apparent that with the porting the gun has, the 78 gr. JSB pellets, which were the original design intention for this gun, were just about perfect.... so the rest of the testing was done with them.... I shot strings at gradually reducing hammer spring preload, recording all shots with 4% of the peak velocity for that string.... Here are the results....



At Max. preload the second shot was down more than 4%, and at 1 turn out the velocity was only down 2 fps, but the gun did manage a 2nd shot within 4% ES.... The velocity was obvious plateaued at the 3000 psi fill pressure, confirming that I had enough hammer spring to get everything out of the gun.... It got 3 shots at 2 turns out, and 4 shots at 3 turns out, with the first two nearly identical, and at that point the efficiency had improved to a still dismal 0.65 FPE/CI.... At 4 turns out the spring has no preload but also no slack when the gun is not cocked, and the gun was now showing signs of a normal shot string, delivering 6 shots with 4% ES at 0.78 FPE/CI.... At 5 turns out, the velocity had dropped quite a bit, and the first two shots were more than 4% below the peak, but the string had 9 shots with a 4% ES, starting from 2800 psi, at an efficiency of 0.96 FPE/CI, not bad for a big-bore.... I turned the preload back in 1/2 turn for the final test, and got 7 shots with 4% ES, the first 6 were within just 17 fps (2% ES) at an efficiency of 0.85 FPE/CI, with the string ending at 1940 psi.... The 7 shots averaged 884 fps (135 FPE), which I'm pretty darned happy with....

This barrel was kind of an afterthought for this gun, the porting was optimized for .25 cal bullets and .30 cal pellets, weighing about 50 gr.... I'm pretty sure that with larger porting the efficiency would increase significantly, and it would probably be possible to tune for a string in the mid 900s with the JSB pellets (ie 150-160 FPE) with decent efficiency.... Anyway, when you consider the gun started out as a humble Disco, and it's still using a stock valve with the ports just opened up to 0.219" and a PEEK poppet.... I think it's pretty impressive to get 6 times the original FPE with a usuable 7 shot string....

Bob

[Big Bore Bart]

  Bob, that thing is just nasty.     When are you going to chase down a moose with it?

[wwonka]

Great information and practical demonstration as usual Bob. 

The 7-shot string seems a nice compromise between power, ES and count. I'm reading someone who is working on tuning up the Rainstorm and reducing the ES at the same time as well - and his early results (with minimal changes) are v. promising. Wish you'd turn your eye to that gun.  I'd almost donate one to you to see your results 

That 28" barrel must be quite a sight on the Disco. Seems that you swapped it in pretty handily onto the Disco. What do you think it would take to put one of those barrels (maybe a 25) on an Evanix Rainstorm?

[rsterne]

Backing the preload out just a whisker from the 4.5T setting might squeek out an 8 shot string starting from 3000 psi within a 4% ES in the 850-890 fps range.... My Disco Double has a repeater breech (single shot loading in .35 cal) that I made myself, and it has a 1/2" ID the same as the breech on a .25 cal MRod.... I'm not at all familiar with the Rainstorm, but if the barrel OD is 1/2" it would slide right in, if it's larger, the barrel may be available with a larger OD, or it could be sleeved to fit.... I know Sean is pretty busy right now, but I'm sure he could do pretty much anything you want....

I've thought about working on a Rainstorm at some point, but funds and time are always the problem.... It certainly can't happen this winter, and during the summer I don't have the time because of the Motel....

Bob

[rsterne]

I put the .300 cal barrel back on today because I had more testing to complete.... I never worked out a useable tune with the PEEK valve, new hammer spring, and the 70 gr. Bob's Boattails.... So, here is the data, along with that for the JSB 50 gr. (Daystate) pellets....



In a similar state of tune, the heavier, cast bullets produce about 20 FPE more energy than the pellets, but at about 80 fps lower velocity.... To do that, they require one more turn of preload on the hammer spring to compensate for the greater mass.... The shot strings are shorter because you are extracting more energy per shot from almost exactly the same pressure drop.... In both cases, for the "black" strings, the end pressure was just over 1800 psi, starting from 3000, which is a fairly wide range to stay within a 4% ES.... That seems to be a characteristic of the PEEK valve, extending the useful pressure range, and hence shot count.... Here are the tabulated results:

50 gr. JSB pellets:
7 shots @ 100 FPE @ 0.93 FPE/CI
13 shots @ 94 FPE @ 1.02 FPE/CI
14 shots @ 86 FPE @ 1.01 FPE/CI

70 gr. Bob's Boattails:
4 shots @ 131 FPE @ 0.51 FPE/CI
6 shots @ 119 FPE @ 0.76 FPE/CI
9 shots @ 110 FPE @ 0.85 FPE/CI
11 shots @ 104 FPE @ 0.93 FPE/CI

The valve in this gun, which has a 1/4" throat, is plenty for the 90-100 FPE range for the JSB pellets, but it's struggling a bit with the 70 gr. boattails, just as it is with the .357 cal barrel.... It simply can't flow enough air to develop over 100 FPE with good efficiency.... I understand that Sean is planning to fix that by sending me a new valve to play with.... My problem will be to figure out how to make the transfer port large enough to handle the extra flow....

Bob

[lloyd-ss]

Bob,
Looks like it took some work to get the peek valve figured out. I know the peek valve is easier to open, but does it make the valve poppet any lighter?
So the short, heavy spring ended up being the most efficient?
How many additional foot pounds do you think the peek valve was able to extract from the DD? You are getting incredible power from a valve with only a .25" throat. Excellent.
Lloyd

[rsterne]

The PEEK valve is not really allowing me to get more power, technically.... It's allowing the hammer to open the valve without having to use so heavy a spring I can't cock the gun.... *LOL*.... I never tried the .357 barrel with the Disco poppet, but on the .300 cal, the best I got was 113 FPE, but with the PEEK poppet I got 135 FPE, nearly 20% more, and that was using 70 gr. bullets.... I have no doubt the difference would have been larger with heavier ammo....

Yes, getting 192 FPE from a 1/4" throat is pretty impressive, but the efficiency is dismal at that power (0.4 FPE/CI).... Unfortunately, even set up for a proper shot string, the efficiency isn't that great (about 0.9 FPE/CI).... Not only are the ports being sonically choked, they are just plain too small for that bore, or even for the .30 cal at anything over 100 FPE.... The short heavy spring helped the effiiency quite a bit, I was getting a lot of hammer bounce with the QB spring and the PEEK valve.... just too easy to crack, I guess.... Maybe be careful what you wish for?.... *LOL*....

Bob

[lloyd-ss]

Bob,
Sorry to keep pestering you with questions, but this is very intriguing.  So if I am understanding this correctly, the peek valve, being much easier to open, allows lighter (compared to the usual Delrin poppets) spring forces to open the valve.  And this allows you to max out the flow in the system without it seeming like you are hammering the valve to death. And now the new maximum is higher than the old because you can drive the valve harder.  But then you have to change the spring methodology to avoid hammer bounce. Wow! Quite a journey but that is really good as it does open up some very new possibilities, as you and Sean have already show with the other peek applications.

The dismal efficiency you refer to when driving the valve really hard (with the barrel area being.... gulp.... over twice the throat area!!!!).... I assume the calculator shows that the valve is staying open well past half way down the barrel and the low efficiency isn't being caused by something else?

Thanks a bunch,
Lloyd

[rsterne]

You summary of how the PEEK valve changes things is correct.... As to the dismal efficiency (when cranked to the max.), it's no worse than my DAQ .308 Exile was when I first got it, at 0.38 FPE/CI for only 3 shots with a 90-95 fps (rapidly dropping) spread and using 400 psi per shot....

As you know, Lloyd, your spreadsheet has no way to differentiate between dwell and efficiency, they go hand in hand.... The further the pellet is down the barrel, the lower the FPE/CI, and vice-versa.... Just using the ratio of HPA released to barrel volume, my calculations show that the valve is open for 76% of the barrel length.... The port to barrel area ratio in this gun (38%) is only slightly worse than on a stock .22 cal Disco (42%), so we're not in uncharted territory.... However, getting 192 FPE from it would be like trying to get (over) 71 FPE from a .22 cal Disco through the stock 0.140" transfer port.... admittedly with 3000 psi and a 50 gr. bullet....

Bob

[cambriajohn]

I've been watching this thread with great interest. I've ordered a 30 cal barrel from Sean. Now I NEED a 357 and some bullet molds  From an engineering perspective, Do you think there is any efficiency to be gained by going the smoothtwist way and only rifling the last 4 inches? Would doing this minimize damage during loading as I assume that the pellet would be well centered by the time it hit the rifling?

[rsterne]

I don't understand how the smooth twist idea works, as the rifling is about a 16-18" twist, but only results in a 100" or slower twist rate, so the pellet must be skidding across the rifling.... When they start winning all the benchrest contests instead of conventionally rifled barrels, I'll likely pay more attention....

Bob

[cambriajohn]

I shouldn't have mentioned smoothtwist as it implies other concepts and baggage that I know nothing about....

My real questions are:  Do you think there is any efficiency to be gained by only rifling the last 4 inches? Would doing this minimize damage during loading as I assume that the pellet would be well centered by the time it hit the rifling? It seems to me that this might be a good way to go when using a liner as the edges of the rifling are not as sharp.

[rsterne]

For the same reasons I stated, I can't imagine why you would want to do that.... Instead of the pellet being engaged into the rifling before it starts it's journey, and being accelerated in rotation at the same time as longitudinally, it will be travelling at high speed with no rotation before hitting the rifling, and while it will pick up "some" spin at that point, if will also be stripped of it's outer diameter, sizing it essentially to the land diameter.... If you look up photos of recovered pellets from smooth twist barrels (essentially what you are describing) you will see that they show virtually no residual rifling marks.... While that smoother surface may increase the Ballistics Coefficient by decreasing the drag, the resulting very slow spin rate (it has been measured as low as 1 turn in 156" - 13 feet).... would never work with a cast bullet.... Without the inherent drag stabiizaton of a diabolo (waisted, or shuttlecock) pellet, the bullet would be prone to tumbling with such a low spin rate....

IMO, the proper chamber length is one where the nose of the projectile is engaged in the tapered leade between the smooth chamber and the full depth rifling....

Bob

[cambriajohn]

Thanks Bob,

Now I get it. They made a pellet only smooth bore barrel with a choke that imparts a small amount of spin.

John

[rsterne]

Correct, good description of it.... When they choke it, they do so over a mandrel with the rifling on it....

Bob