.30 cal Disco Double

[rsterne]

I was able to get back to machining the breech today, and finished everything except the bolt slot.... The breech accepts a .25 MRod self-indexing magazine, and Lloyd at AirGunLab is tooling up to supply them in a 7-shot .30 cal version.... The breech accepts a 1/2" OD barrel, and the plan is for Sean to supply them in both .25 cal and .30 cal versions.... The boss on the front is 3/4" diameter, to accept a 7/8" OD Carbon Fibre barrel shroud, allowing the barrel to be tensioned by the screw-on Hatsan style air stripper.... Here is what it looks like at the present time....



The breech mounts in three places, at the rear to the RVA (end plug), in the middle to one of the two positions of the 4-48 screw (allowing it to be used on a Disco or 22XX tube), and at the front with a band around the main tube between the trigger and gauge location.... The barrel is solidly mounted with three 8-32 setscrews.... The rigidity of this breech and barrel assembly should be superior to any other breech I have seen on a Disco/22XX, eliminating accuracy robbing flexing and bending....



There are three holes in the middle of this breech because the holes drilled in the Disco tube I have were misaligned, so I had to drill and tap another hole in between them and use that.... Only that hole is counterbored in this breech.... Normally that hole would be absent, and both the other locations would be counterbored.... One difference in this breech is that I left enough material under the magazine to allow a standard 4-48 SHCS to be used instead of the custom low-profile one Crosman uses that accepts that tiny 0.050" allen key.... The thicker head on a standard 4-48 SHCS accepts a 3/32" allen key, so no more stripping out the head of the screw.... The 4-40 screws in the breech band will accept the same size tool....

I have passed along the names of everyone who has asked for a barrel from the initial production run to Sean.... He's pretty busy right now, but will contact all of you with a price as soon as he can.... In the meantime, I'll continue building parts so that when my barrel arrives hopefully it will the the last thing that needs machining....

Bob

[rsterne]

Over the last couple of days I made the two bolts (.25 and .30 cal), the bolt handle, and machined the bolt slot in the breech.... The bolt design is quite unique, thanks to Sean.... at least I have never seen it before.... It is probeless.... The sealing O-ring is in a groove in the end of the barrel and the bolt slides through it....



Rather than have either a hollow probe (which loads the pellet by pushing on the base, and the air flows through it).... or a thin extended probe (which loads the pellet by pushing inside the skirt and the air flows around it).... this bolt simply has a flat end.... The key is in the bolt slot.... The bolt pushes on the base of the pellet (or bullet) to load it, and stops with the base or skirt past the barrel port.... It is then withdrawn to clear the barrel port completely, leaving no obstruction to the airflow once in the firing position....



Sean's original bolt slot design was "T" shaped, the bolt moved past the battery position, was withdrawn and turned down to lock.... We then modified it to a triangular shape, where the bolt handle was pulled down and back on an angle to the locking position.... I then saw a photo of the Crosman Rogue which has a slot shaped like the one in the above photo, and uses the retraction of the bolt to the firing position as a safety interlock (as I understand it).... I adopted that rectangular slot design and Sean's probeless bolt for this rifle.... In addition to having no obstruction to the airflow in the barrel, the bolt handle, once in the firing position, can't pop up on firing.... The bolt in the second photo is shown in the firing position....

Bob

[Doc Sharptail]

How far does the bolt travel under o-ring tension?
Seems a common sense work around, provided there isn't too much space between the base of the projectile, and the bolt face.
There might even be a solution in there somehow, for the loose flopping bolt on cock on open 78 series...

Regards,

Doc Sharptail

[rsterne]

The bolt engages the O-ring, slides in about 3/8", and then retracts 1/4".... The MRod uses a smooth bolt with the O-ring in the barrel, and I have adopted it for my Hayabusa and virtually all my new builds.... I find it seals great, and you never have to worry about the bolt length being just perfect to compress the O-ring, which is the hassle when the bolt carries the O-ring and engages a taper like on the Crosman setup.... Also, if the bolt carries the O-ring it doesn't work well when passing through a magazine,, it's prone to damaging the O-ring....

The distance from the base of the projectile to the bolt face when retracted is just over the diameter of the transfer port, the same as an extended probe bolt.... The difference is, there is no probe in the way of the airflow....

Bob

[rsterne]

I got most of the remaining parts made today.... Here is the front breech mounting band, shown installed on the breech....



It fits over the tube between the trigger and the gauge, and securely clamps the front of the breech to the main tube.... I then made a new hammer and RVA assembly....



The hammer is the same length as a Disco / 22XX hammer, but it is cylindrical and will therefore work with a Disco, MRod, or PRod trigger group.... If an MRod trigger is used, it should be shimmed down 0.030" because it is designed for a thicker main tube.... The hammer is drilled to accept a QB 7X hammer spring, which is longer and larger in diameter, stiffer, and has more travel.... The cocking pin is moved back 0.065", allowing slightly more total hammer travel.... The hammer weighs 58 gr. the same as a Disco hammer, and has a notch to clear the rear breech screw location used on a Disco.... The spring guide is made from a piece of 7/32" drill rod with a nut threaded on, peened, and turned.... It is long enough to protrude through the RVA adjusting screw and serve as a cocking indicator.... It weighs 15 gr., so the total hammer weight with guide is 73 gr., 26% heavier than a Disco hammer....



You will notice that the hammer face is counterbored 0.070" to increase the hammer stroke and limit the valve lift.... In combination with the 0.070" shortened valve stem, the stroke has increased from 0.58" to 0.72" (an increase of 24%), and there is still 0.18" of valve lift available.... I have found that to be more lift than required on all other Disco platforms I have worked on, in fact it is seldom more than 0.12" in a properly tuned gun.... The RVA is made from aluminum, and carries an adjusting screw made from a 3/8"-24 NF bolt.... The spring and guide can be removed through the threaded hole, no need to remove the end plug once installed.... The guide slides easily through the adjusting screw, and protrudes when the gun is cocked, giving a visible cocking indicator.... There are two flats on the adjuster to allow it to be turned with a 5/16" wrench.... and it is locked in place with a thin (jam) nut once adjusted.... I have enough travel on the RVA to go from no preload to coil bind, about 16 turns....

I am now at the point where I can install the breech on the main tube and inlet the stock for the front breech band.... Once that is done, I can get on with sanding and finishing the stock.... Most of the gun should be done by the time the barrels show up....

Bob

[sholo]

...Breech and components look absolutely marvelous, Bob! Best design I think I have ever seen...

I am eagerly awaiting the day I have one of these bad boys in my possession!


Todd

[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]

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]

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