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yourdaddyjoe · Supporting Member

Bob, Thank God you are not a fire arm tinkerer!

You would come up with something like this...

http://youtu.be/xohy9gWz7kk

rsterne · Moderator

25,000+ FPE !?!?!

I'm speechless.... Shocked

Bob

maecolis · New Member

$40 a round.

2500 grain BULLET.
240 grains POWDER.
2100 fps.
25,400 ft pounds muzzle energy.

277ft pounds recoil.

I can't even imagine what you would want to put a hole in that needs this much killing. Maybe an aircraft carrier. The long way.

Shocked

I'm with Bob. I got nothing.

BBgunNoob · Veteran Member

get on bob and modify it to 40000 fpe lol

rsterne · Moderator

I played around with the gun a bit more the last couple of days.... I installed a full length shroud, and although it looks nice it had little to no effect on the noise.... It projects 3" beyond the barrel and is sealed to the barrel 5" behind the muzzle....

I also tried a 2500 psi fill and 4 turns out on the adjuster with excellent results, using the 77.8 gr. EunJins....

758 fps
782
793 - 109 FPE
782
758

It is a completely symmetrical shot string, with a 35 fps spread (4.5%).... The ending pressure was 1500 psi, average energy 104 FPE, and the efficiency was 0.94 FPE/CI.... In addition I sorted some 0.36" buckshot from Ballistics Products into two groups.... About a third of them were under 0.354" and loaded OK, the rest were over 0.357" and were too tight.... I shot a 5 shot string with the smaller size, starting from 2500 psi, and got an average velocity of 812 fps (98.7 FPE).... That string used 1100 psi and the efficiency was only 0.81 FPE/CI.... The pellets are MUCH more efficient than the roundball....

I'm going to have to get a smaller sizing die for my cast slugs, both for this barrel and the .308.... I'm a bit surprised at that, considering the dimensions that the barrels are supposed to be.... This is where my inexperience with big bores is showing.... I don't know how the gun is supposed to be chambered for cast bullets.... Should they load past the transfer port with virtually no effort?.... Should the leade past that point be a shallow taper or a fast one - or no taper at all?....

Bob

rsterne · Moderator

After last nights discussion on another Forum about the strength of the Disco valve securing screws, I decided to make a jig for drilling a Disco valve for 10-32 high-tensile screws.... Here are the photos....

The jig is made from a piece of 1" x 1.5" x 2" aluminum.... A block of steel would be better for production work.... I drilled a 3/4" hole 1.5" deep and then ran a 5/8" mill down along the bottom side and removed 0.015" to form a shallow groove (ie the hole is slightly "pear" shaped).... This forms a "V" block so that when the valve is inserted it only touches at two points.... On the opposite side is a 3/8" SHSS with the end ground flat so that it won't damage the valve when snugged up.... There are two holes drilled in the top beside the setscrew.... The front one is a #21 drill (the correct size for tapping a 10-32 thread) and the rear one is a #11 drill (just clears a 10-32 tap).... There are two brass pins, the smaller one has the end machined to 0.128" which just fits in the 8-32 tapped hole in the Disco valve.... The larger one has the end machined to nicely fit in a #21 drilled hole....

To use the jig, you slide the rear valve half in until one of the screw holes lines up with the front hole and then slide the small pin down until it locates in the 8-32 thread hole.... Then you snug up the 3/8" setscrew to hold the valve in place and pull out the pin.... If you were doing a bunch, you would set the block up in the mill, aligning the hole with the head.... but I just used a hand drill and drilled by eye, using the hole in the block to guide a #21 drill to clear out the threads in the valve.... I used a normal drill, very carefully, to just remove the threads, making sure the point didn't dig in as there isn't much meat between the bottom of the hole and the inside of the valve.... I then changed to a drill ground flat on the end to make the hole full depth with no danger of cutting through.... This operation could also have been done with a 5/32" end mill provided you stopped at the correct depth....

I loosened the setscrew, removed the valve, and the threads were evenly removed, so I turned it around and replaced it in the jig and slid it in until the newly drilled hole lined up with the rear hole in the jig.... I slid in the larger pin to locate the valve and snugged up the setscrew again....Using a 10-32 plug tap, with the hole as a guide, I tapped the hole until the tip of the tap just touched the bottom (about 3 threads).... I then changed to a bottoming tap, and got nearly 6 threads total.... I repeated this for all three holes and then ran a countersink into the top of each hole in the valve just to provide a slight relief because the thread on the screws stops a few thou from the head....

I had to shorten the 1/4" long low-profile 10-32 screws to 3/16", properly finished the ends, checked the thread in a nut (rather than risk galling the aluminum valve), and then installed them to make sure they went all the way down and that the head was seated on the flat on the valve.... Everything worked perfectly, so it was time to install it in my 9mm and give it a try.... The screws I am using are rated at 180,000 psi tensile strength, which means 108,000 psi in shear.... That will give me a 4.4:1 safety margin at 3000 psi, making the screws no longer the weak link in the Disco.... The OD of the head on the screw is a few thou smaller than the holes in the Disco tube, but the head still seats well enough to spread the load and the transfer port still lines up as good as they ever do....

Bob

rsterne · Moderator

After a discussion on the strength of the valve securing screws in the Disco, I decided it was time to upgrade the screws from 8-32 of unknown strength to 10-32 high-tensile screws.... The details on how I accomplished that are in the post above.... Now that I don't have to worry about the strength of the valve screws, and given that the strength of the other items in a Disco calculate out with a high enough safety margin to satisfy me.... I now have the ability to try a 3000 psi fill....

NOTE: If you haven't upgraded the valve screws don't even consider doing this, it would be unsafe.... I'm confident that the strength of the rest of the Disco is OK, but I am NOT recommending that you follow suit.... HPA is dangerous, and using more than 2000 psi in a Disco is done at your own risk....

After installing the valve with the new screws and pressurizing the gun and testing for leaks.... I cranked the hammer spring to the maximum preload (just shy of coil bind) and squeezed off the first shot.... 873 fps with the 77.8 gr. EunJins.... 132 FPE !!! The next shot was a bit slower, and then a further drop on the third shot to 838 fps, after which I was down to 2100 psi.... 300 psi per shot, WOW! this thing is using a lot of air!.... I had wondered if the hammer setup I had would be enough to dump 3000 psi, and the answer was yes !!!

I tried backing out the hammer spring two turns, and the shot string started out at only 759 fps, then climbed to 835.... Hmmmmmm.... not enough hammer hit for 3000 psi.... I then tried in between, and at 1 turn out got a nice bell curve of 4 shots.... The results of the testing are in the graph below....

At maximum, 3 shots, falling velocity, averaging 127 FPE, 900 psi used, and the efficiency was 0.77 FPE/CI....
At 1 turn out, 4 shots within 3%, averaging 121 FPE, 1000 psi used, and the efficiency was 0.88 FPE/CI....
At 2 turns out, 5 shots within 5%, averaging 115 FPE, 1100 psi used starting from 2800, and the efficiency was 0.94 FPE/CI.... VERY NICE !!!

I now have the ability to set this gun up for any fill up to and including 3000 psi.... I can set it up for a 2000 psi fill and get about 9 shots at 80 FPE.... all the way up to a 3000 psi fill and about 4 shots at 120 FPE.... or anything in between.... It appears that between 100-120 FPE I will get about 5 shots, with higher fill pressures resulting in more power.... IMO, this represents the "sweet spot" for the gun, filling it between 2400-2800 psi and getting velocities between 760-840 fps.... Throughout that range, the Efficiency plateaus at about 0.94 FPE/CI.... Above that range, the efficiency and shot count drops.... Below that range, the efficiency and shot count increases....

This gun is now shooting at up to 6 times the FPE level of a stock Disco.... I'm a VERY happy camper.... Mr. Green

Bob

maecolis · New Member

Have you considered increasing the thread engagement from the typical 75% to 85%? It would increase the strength of the valve thread a little, but I have not done the calculations to see how large an effect it would have.

I found that some applications need more engagement - especially in soft materials like plastic and aluminum.

Just wanted to ask.

You're doing awesome work. Mr. Green

rsterne · Moderator

The thread engagement is not critical in this application as the screw is not in tension at all.... It is completely in shear.... The head of the screw is tightened against a flat on the valve and the edge of the head bears in an oversize hole in the tube.... It is strictly to keep the valve from moving due to the end force....

Bob

rsterne · Moderator

Here is all the data from my testing on one graph.... It makes it really easy to see how the number of shots decreases as the power increases.... and how the gun uses more air per shot (slope of the pressure lines)....

Interestingly, from 2 to 4 turns out on the adjuster the gun produces 5 shots, with little change in efficiency.... just a higher fill pressure required and a slightly wider range of pressure used.... It looks like with further testing I can fine tune it for 5 shots between 800 and 840 fps (5% variation) using a 3000 psi fill and about 1.5 turns out on the hammer.... Mr. Green

There isn't much more I can do on this gun until I manage to size the cast bullets appropriately for both this barrel and the .308.... I may work on the chamber of the .308 barrel next to see if I can get it to chamber the rounds properly.... I would still like some advice on the proper way to make a chamber for cast bullets if someone can help with that....

Bob

radar · Silver Status Member

Been here?

http://www.lasc.us/Brennan_2-2_BulletDesignFit.htm

It's a start...

rsterne · Moderator

Thanks, the diagram for the throat and leade is useful....

Bob

rsterne · Moderator

I spent the afternoon chambering the .308 cal. barrel for this gun.... I drilled 5/16" to a depth of 0.6" past the front of the transfer port which gave me a parallel chamber that the bullet would slide nicely into.... that was the easy part!.... I didn't have a chambering reamer, nor any reamer with a 1 degree taper, and I needed to taper the end of the rifling from the top of the lands (0.300") to the bottom of the grooves (0.308") over about 1/4" of length.... I machined up a piece of steel bar to the appropriate dimensions, coated it with a very fine abrasive paste, and with the barrel in the headstock and the tool in the tailstock, I turned the chuck by hand while applying pressure with the tailstock handwheel.... This was a VERY laborious process, it kept getting stuck, and at one point I succeeded in welding the pieces together and had to drive the tool out with a 1/4" brass rod.... It took me a couple of hours of fiddle-pharting around, but I finally got a decent leade on the rifling.... When you chamber a bullet and then push it back out, you can see a smooth, tapered imprint of the rifling for about 2/3rds of the front driving band on the bullet, which was sized to 0.308"....

After dinner, I just had to put a few rounds through it, of course.... I had two different bullets, both Lee that I cast myself, one a 120 gr. Round Nose and the other a 113 gr. Flat Nose (which actually weighs 118 gr.).... The two bullets are virtually identical except for the flat.... I had some that were as cast (0.309"-0.311") and some that had been sized to 0.308" with my Lee press.... Cast bullets always seem such a tight fit to me.... I tapped a sized bullet into the barrel and then drove it back out (it was too tight to push through) and the rifling was deeply engraved, right down to scraping slightly on the grooves of the bullet in some spots.... There was no sign of the driving bands being shaved by the bore between the rifling however, so I decided to try them out.... They chambered easily, especially the sized bullets.... My SCUBA tank was down to 2900 psi, but I shot a few groups, enough to at least find out where I'm at.... Here are the results....

The first string I shot with the hammer spring preload at maximum, just shy of coil bind.... and was rewarded with four shots, the third of which was the fastest at 685 fps (123 FPE).... but the 5th shot was only 598 fps, and when I checked the pressure remaining, it was only 1400 psi.... 300 psi per shot, an efficiency of just 0.73 FPE/CI !!!.... I backed the preload out 1 turn for the rest of the testing, and to my delight, just that small change dropped the air usuage to 250 psi per shot and gave me 5 shot strings starting at 2800-2850 psi and ending at 1550-1600.... The 118 gr. and 120 gr. bullets shot nearly the same velocity, so the heavier one had a tiny edge in FPE.... The sized 120 gr. averaged 112 FPE and the 118 gr. were 110 FPE.... The efficiency was about 0.80-0.81 FPE/CI.... The unsized bullets were a LOT slower, losing 40-48 fps, so they were obviously way too tight in the bore.... The 120 gr. was only 99 FPE and the 118 gr. just 96 FPE.... and the efficiency was down to 0.70-0.72 FPE/CI.... The dotted lines in the graph show the unsized bullets....

I'm actually quite pleased with the results using the sized bullets.... I expected the FPE and the efficiency to drop because of the smaller bore.... but the heavier bullets, with their much higher SD, gained back much of the loss.... At the same hammer spring setting, the 9mm only had about 9 FPE more energy and about 14% more efficiency.... I do have a bit of a leak at the nose of the bolt to deal with, so when I fix that, I may find that the .308 gains a bit more.... In addition, I hope to get a smaller sizing die and try the bullets at 0.306"-0.307" and see if the velocity increases.... In any case, the gun is now interchangeable between .308 and 9mm.... Mr. Green

Bob

rsterne · Moderator

After yesterdays success in chambering the .308 barrel I decided to rechamber the .357 barrel today for cast bullets.... The process went better than yesterday, and when I put the bigger barrel back on I got the following results....

As I expected, this combination gave the highest FPE levels I have achieved so far.... the peak level with the 132 gr. Lee Round Nose was 698 fps which is 143 FPE.... With the hammer spring at maximum preload I got four shots using 300 psi per shot.... The average was 687 fps (138 FPE) at an efficiency of 0.84 FPE/CI.... I backed the preload out 1.5 turns and got 5 shots starting at 2800 psi and ending at 1550 (250 psi per shot).... They averaged 650 fps (124 FPE) at an efficiency of 0.90 FPE/CI....

The new longer chamber cost me about 10 fps with the EunJin pellets at the same spring preload.... However, by winding in the spring 1/2 turn and increasing the fill pressure to 3000 psi I gained that back and still kept 5 shots at 250 psi per shot.... So the current results are 5 shots at an efficiency of 0.90 FPE/CI with either pellet/bullet at the same 1.5 turns out on the hammer spring.... The 77.8 gr. EunJin shoots 812 fps (114 FPE) using a 3000 psi fill, and the 132 gr. Lee shoots 650 fps (124 FPE) filled to 2800 psi....

I'm pretty darn pleased with the results, and I certainly learned a lot building this Big Bore Disco.... Who would have thought you could take a gun all the way from 22 FPE to 143 FPE.... Mr. Green

Bob

radar · Silver Status Member