.30 cal Disco Double

[rsterne]

I installed the cocking pins in the bolts today and machined out the breech to clear them....



When the bolt is in the "down" position (shown), the head of the 4-40 SHCS rotates out of the way of the hammer pin into the recess milled in the side of the breech.... When the bolt rotates to the "up" position to open and load a pellet, it engages the hammer pin, drawing it back to cock the hammer.... Nothing magic, but complicated to measure umpteen ways to make sure it all works properly and allows the bolt to retract far enough to remove the magazine when the gun is cocked....

I inletted the stock for the breech band, finish sanded it, and put on the first coat of Watco Danish Oil yesterday.... The stock will be done in about a week.... I wet sand each of the next six coats, two with 220 grit, two with 400, and tha last two with 600.... and then give it a final coat of Minwax.... I think the "Forest Camo" combination of black, brown, and green in the Boyd's Laminated Blaster stock should be quite attractive when done.... It is much darker than the first photo with it unfinished....

I think all the machining is done except for the barrel.... and a shoulder on the Hatsan air stripper to locate the carbon fibre shroud.... and then the last step will be to make a transfer port from a piece of 5/16" Teflon rod.... Basically, once the barrel arrives, the gun is only days (hours?) away from initial testing.... I've already pulled the trigger and heard a satisfying BANG when the hammer opened the valve for the first time.... I won't be bluing the Disco Double tube and any small parts until after testing....

Bob

[Doc Sharptail]

Kind of curious about the mechanics of that transfer port. I don't "see" a shoulder for it in the breech (could be the photo), or will it butt up against bbl? Or are you going to mill a shoulder onto the teflon?

Regards,

Doc Sharptail

[rsterne]

I changed the gun over to a rear cocker last night, and also enlarged the transfer port to 7/32".... Here is the hammer, which now weighs 82 gr. with the cocking knob....



The gun is now very easy to cock.... I put a small O-ring on the shaft which can be used to directly measure the valve lift.... You simply slide the O-ring up against the back of the RVA screw, cock the gun, fire it, and the O-ring slides back exactly the amount the valve opens and you can measure the space with calipers.... I did that for all RVA settings from coil bound to 12T out, for the first shot, and the last shot before the velocity fell more than 4% below the peak.... The smallest lift measured was just under 0.06", and the largest was 0.13".... In virtually every case where there was a proper bell-curve, the lift at the first shot was about 1/16".... and at the last shot was less than 1/8".... This is a very valuable piece of information, and while it won't be the same for every gun, for this gun it indicates that limiting the valve lift to 0.15" (eg. by having the hammer hit the back of the valve at that point) would not have any affect.... That means I can lengthen the hammer stroke on this gun to a full 3/4" by grinding a bit more off the valve stem or counterboring the front of the hammer a bit more.... The fact that the valve lift was virtually the same regardless of pressure and preload.... once the gun was adjusted for a proper bell-curve.... was a bit of a revelation to me.... though in retrospect it makes perfect sense....

The larger transfer port increased the power a bit, made the shot strings more curved (and hence shortened them), and required about 3 turns less preload on the RVA for the same FPE level.... Possibly 1 turn of that is likely due to the slight increase in hammer weight with the longer guide and the cocking knob.... Here are the results of the testing, from coil bind to 12 turns out, by which time the fill pressure required was less than 2000 psi....



You can see that with a 3000 psi fill, anything within 6 turns of coil bound, while producing a bit more power, drastically shortens the shot string, and of course wastes a lot of air.... I saw 1000 fps (102 FPE) with everything maxed out at 3000 psi, however.... but the gun ws using nearly 200 psi per shot, and was down 4% in just 3 shots.... The best setting I could find was 7 turns out, starting from 2900 psi and ending at 1900.... It peaked at 929 fps (88 FPE), and averaged 913 fps (85 FPE) over 11 shots.... but the efficiency was only 0.88 FPE/CI.... This is the first gun I have tried where the area of the transfer and barrel ports (0.219"D) were larger than the area of the throat, which is the equvalent of a 0.203" hole.... I'm pretty sure that is why I couldn't get over 1.0 FPE/CI like I did with the 3/16" transfer port in the initial trials yesterday.... This leaves me with two choices, either drill the throat out to 1/4" (it is currently 15/64").... or refit the 3/16" transfer port.... The former should increase the power even further, and may increase the efficiency slightly.... The latter will drop the power a bit, but increase the shot count and efficiency.... I will wait until I get the new longer barrel to make the final decision.... but since I'm already pushing these pellets into the mid 900s more power may not be the answer.... so I'm leaning towards going back to the 3/16" transfer port with the 25" barrel....

Bob

[Doc Sharptail]

I like your thinking on the transfer ports.

It's becoming more apparent to me that efficiency, low e.s.'s and shot count are what matters in a hunting PCP. Velocity becomes sort of a redundant issue, after a certain point...

I'm so close to equalling the 29 gr CB cap (which is an efficient grouse harvester this side of 25 yds), that I'll leave the AR alone, until I can evaluate how it behaves on game. I had been hoping for low to mid 8's with it and the predator. After seeing how the predator groups, and expands at the mid 7's, I may not have that much more work to do...

Regards,

Doc Sharptail

[rsterne]

Since this gun is going to have a 7 shot magazine, I was curious what I could get for 7 shots starting at 3000 psi.... Here is the string....

984
1000
1007
1003
997
993
981

Average fps = 995
Average FPE = 101.1
Total FPE = 708
Press. Used = 750
psi / shot = 107
Total CI Used = 786
FPE/CI = 0.90
% ES = 2.6%

I'm thinking this is a heck of a Coyote gun....

Bob

[Doc Sharptail]

I get a headache thinking about thread indexing- one of the things I've had little success with. I know it can be done, but at the very least it takes an accomplished machinist- and a boffo cnc set up for production, to boot. I've always been reduced to finding/scrounging a threaded part that "comes close"- which ain't very practical.

That's gonna be a doozy of a gun when it's done. Your grouse and chucks had better be checking over the fine print on their life insurance policies...

Regards,

Doc Sharptail

[rsterne]

Here is a photo of the roughed out breech.... all the longitudinal machining is done and it's bored 1/2" end to end.... This basically took me all day....



Lots of work yet to finish it; loading port, bolt slot, cocking slot, transfer port, and all the mounting holes including the slot for the front band....

Bob

[rsterne]

The hole is the breech is straight through, there will be a 5/16" OD shoulder on the barrel and the transfer port will seal directly against it....

Bob

[rsterne]

Update on these barrels.... The mandrel is made, and the barrels will be produced to the following specs:

.30 cal Airgun barrel intended for the new JSB 46 & 50 gr. pellets
Made from CrMoly steel, hammer forged
Groove 0.300"
Land 0.294"
Twist 1 turn in 26"
6 Lands
OD 0.500" (0.590" also available)
Standard length 25" (12.5" for pistols)

What they are NOT: They are not intended to use 0.308" cast bullets, nor are they intended to use any bullet over 0.48" in length, as those will not fit into a modified .25 cal MRod magazine, and may not stabilize in the 26" twist....

Sean will be placing an initial order as soon as we can collect pre-orders and determine a price.... For THIS ORDER ONLY, and only for BARREL BLANKS with no machining, we will accept orders for lengths other than the above, but be advised there may be a small surcharge from the manufacturer for barrels over 29" long....

I will be contacting those who have expressed an interest in pre-ordering barrel blanks.... Anyone else who wants to get in on this initial order, either for blanks or for machined barrels.... please contact me ASAP.... Sean will be finalizing the list within a week, getting prices, and then contacting you to arrange payment.... Sean will be offering all machining services (chambering, porting, and crowning, and can even add a choke) at extra cost if you desire, please ask him for details directly....

Bob

[rsterne]

While waiting for our new barrels to arrive, I thought I'd do a practice run using a .308 cal PB barrel I had.... I cut off the .300 Win. Mag. chamber, ending up with a 21" usable section, and then I turned down the breech end to 1/2" OD to fit into the breech, and machined in a chamber and leade and a transfer port and O-ring groove.... Here's how it turned out....



I assembled it into the breech to check the functioning of the bolt, and the nose pushes a pellet just past the transfer port and then withdraws into the "J" slot so that it is just clear of the port and completely out of the airflow, just as designed.... I also machined out the groove in a .25 cal MRod single shot tray to take the .30 cal pellet.... It clips onto the end of the barrel which protrudes 0.060" into the magazine bay, the same way the magazine mounts....



By adjusting the diameter of the O-ring groove inside the barrel, you can customize how tight the O-ring grips the nose of the bolt.... I have set it up for a noticable drag, but the pellet slides through fine.... I'm sure it will seal perfectly as I've done this type of breech seal in the past with great results.... While outside the original design parameters of the gun, this barrel and loading tray will allow me to test cast .308" bullets up to 120 gr. in single shot mode (providing I cut the chamber deeper), just to satisfy my curiousity....

Bob