9mm and .308 cal Disco Build

[rsterne]

If the bullet is "lightly engaged" in the rifling, then there would be virtually no load in chambering it.... You would only feel a hint of resistance just as you close the bolt, correct?.... If my math is correct, a 1 degree leade angle over a .004" rifling depth (eg. 300 land, .308 groove) means that the leade would be about 1/4" long.... 1/2 degree would mean the leade would be about 1/2" long....

Bob

[SeanMP]

Yes using TOA I agree with those distances. (Math isn't my thing though I failed calc twice)

That should yield a nice smooth engagement with no shaving and no groove smearing which is often seen when there is a gap to jump.
Throat tolerance key to accuracy

[rsterne]

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....

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....

Bob

[rsterne]

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....

Bob

[SeanMP]

That's a brilliant solution Bob. If you have any bore paste (JB Bore or equivalent) you could whip up a snug fitting mop and work the throat and leade a bit to align the machining scratches and refine the finish.

BTW that string looks excellent!

Post a pic if you can of one of the chambered bullets, I'd love to see it.

[rsterne]

I've never even seen and JB Bore Paste.... can't imagine it is available locally.... Maybe I can get some on the Internet (eBay?).... What do you use for a "mop", a loose fitting lead slug?.... Here is a photo of some pellets/bullets I pushed into my barrels from Aero-Tec....



That .308 bullet was sized and pushed completely into the rifling and then driven back out.... You can see that the sized portion has not scraped in the grooved part of the barrel.... The other two are a 9mm EunJin and a .25 cal JSB King....

Bob

[SeanMP]

For polishing a throat and leade I just use a mop that you would buy at any outdoors store/ctc to screw onto a cleaning rod.

http://www.brownells.com/.aspx/pid=2...TTON-BORE-MOPS

I only use lead slugs for lapping the entire bore. Huge PITA

[rsterne]

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....

Bob

[SeanMP]

143fpe!!!

That's fantastic Bob. Your chamber and leade work is obviously paying huge dividends.

The one really cool thing I have noticed is that whenever you make an improvement your gains are a factor of bore size greater as compared to smallbore.

I think you get another decent gain with the throat and leade lapped and polished

[MSEO1]

I've never even seen and JB Bore Paste.... can't imagine it is available locally.... Maybe I can get some on the Internet (eBay?)....
Bob

You can get the JB Bore paste from Airgun Source on line. Eric might be able to get some in too. I picked some up at my local gun store here in London.