9mm and .308 cal Disco Build

[PACTOO]

Hey sh|tty pants, I mean Hammer Sandwich..... Glad you are back online after 4 fun filled days!

[SeanMP]

I for one am avidly waiting to see the test from your cast slugs when you get them resized. I'm wondering how much work energy is still residual in the full power shots

[rsterne]

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

[chronic]

That looks pretty mean! Man that would be fun!

[rsterne]

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

[SeanMP]

[QUOTE=rsterne;1234

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[/QUOTE]
Bob

The common numbers we used to toss around in BR shooting was the the throat needed to be 0.001-0.002 larger than the bullet and the leade needed to be 1/2 to 1 deg

The tight tolerance on the throat ensure that it aligned the round as it entered the leade. Any sharper than 1deg and you could suffer accuracy loss from shaving

HTH

[rsterne]

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 previous post.... 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....

Bob

[SeanMP]

Bob....I dug this off the web by googling my numbers from previous post



Mountain Molds
What Cast Rifle Bullets Want

Accuracy may have more to do with the chamber throat than any other single factor, more so with cast than with jacketed. If a gun shoots jacketed bullets tolerably well but hates cast bullets, chances are the throat is at fault. Please measure your rifle's throat, with a chamber casting or chamber slug, before you order a mold.
My Credentials as an expert on rifles I'll tell you up front that I am not much of a rifleman, and only occasionally fool around with cast bullets in rifles. No doubt many of you know much, much, more than I do about this subject. Nonetheless, I get asked for advice on rifle bullets nearly every day. I will try to explain the basic principles involved in getting a rifle to feed and shoot cast bullets, and mention some of the potential problems that beginners should watch out for.

[rsterne]

Not quite sure if I follow that.... or how it applies to an airgun.... I understand having the throat 0.001 to 0.002" bigger than the bullet (which I assume is sized to the groove diameter).... I also understand the 1/2 - 1 degree leade angle from the throat to the rifling.... When the round is chambered, however, how much of the bullet engages the rifling?.... Is it barely touching the leade?.... well engaged?.... or does it have to make a short jump before engaging?....

Bob

[SeanMP]

We strove for "just barely touching" in BR

But my gut says lightly engaged would work better in an AR....No reason or rhyme...just feels right

In 50m small bore they are subsonic pure lead projectiles and they are lightly engaged