.30cal big bore pistol build
This build started as I had a short off cut of air vessel tubing from a length used on a rifle build..
However it is VERY short on big bore standards as they require a large section of tube in the rear to accommodate a large valve, hammer stroke and hammer spring, in this case if I was to build the same action as the .30 cal bullpup it would account for 3/4 of the length of the tubing which would be kind of pointless as there would be no volume left for air.
My goal is to keep this pistols OAL to 300mm which means no just adding a larger rear cylinder end cap to extend the hammer springs working stroke rearward which would have been easy.
This is where my challenge started and I am building a bit of a prototype to test a few new dimensions and components.
The first thing I wanted to achieve was the longest barrel and the shortest receiver length I could possibly get away with which meant shifting the valve body as far rearward as I could without loosing so much
hammer travel that power would suffer.
I have used a different hammer spring which is shorter and larger in diameter than the one I used in the .30 Pup which gave me 12mm less length on the springs total bind length when cocked.
Also being a short barrelled pistol a lighter hammer can be used to shorten dwell time on larger valve ports to get similar velocities form a larger but shorter burst of air, this gained me another 6mm.
The last and most drastic design change was the valve body its self, it is 8mm shorter rear of the transfer port than the rifle length valve and I have manage to claim back another 13mm by recessing the valve return spring into the rear of the valve.
Another change Ive made is reduced the poppet valve shank from 1/4″ to 5/32″ and made a stepped collar with a retaining grub screw that locks down on a filed flat..
A recessed valve spring seat means a flat or slightly recessed faced hammer can be used meaning more weight and a shorter hammer.
The new valve is much shorter in total length and still has loads of meat for countersunk retaining cap screws and still within the minimum safety measurement to the edge of the material. The smaller 5/32″ valve shank means I can use off the shelf M6 X 6mm cap screws to retain the valve in the cylinder and they still wont breech the bearing surface of the valve shanks bore like it did in the rifle valve with the 1/4″ shank which required the cap screws to be cut very short.
The over all valve travel remains the same as the rifle sized valve.
The valve spring I have used a stiff little bugger too which should help with efficiency not that that will matter much with this little pistola as it will have a low shot count with maximum power tune.
The best part is the new dimensions make the back end super short which will give me a 7″ barrel and a 130mm receiver compared to the 150mm of the pups, which will be low profile and made from a smaller material than the pups, the greatest advantage is I now only have a 140mm section of tubing behind the air reservoir for the firing mechanism compared to the 200mm of the rifles design but will have the same air delivery only with the dwell tuned to a pistol length barrel.
If all goes to plan this will lead to future pistol builds being shorter lighter and have more air capacity for a higher shot count.
I will be using a rotary multiple barrel transfer port design in this pistol which uses 3 smaller rotary ports within a cut groove rather than one large transfer port cut into the lower section of the barrel. This will allow me to use smaller cast bullets, round ball and JSB pellets without the problem of them dropping into the transfer port and jamming things up. more on that later.
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Today I made a start on the receiver, mostly slow going where the machine set up takes an hour then the job itself is done in a couple of minutes.. ::)
So after carefully marking it all out the off centre bores were drilled and threaded in the 4 jaw chuck then it was onto the milling.
I first drilled the 4 holes that will secure it to the cylinder with cap screws and then milled the top down for a 3/8″ dove tail to mount optics.
Once roughed out with a end mill cutter I used a 60 degree dovetail cutter to cut the dovetail and get the final dimensions
followed buy the loading tray, rear bolt handle cut out, all the various cosmetic angles and the radius to mate up with the cylinder.
This time round I am determined to set this pistol up as a “cock off bolt” system.. the bullpups intention was there but due to design changes and the power output it was just not pleasant to use in this manner. With this short stroke pistol however I have corrected a few of the design features that worked against me on the pup such as bore hight, slot dimensions and side load/friction points which have now given me a super slick cocking stroke that I can cock by hand even without a receiver and bolt to assist.
The design change started with a much narrower slot cut into the cylinder for the cocking cap screw and a free rolling brass bush made to run in the slot.. both the bush and cap screw will be cut down and polished once the receiver is mated up
Note: the hammer in the fist pic is not actually the .30 pistols hammer it was just one I had on the bench from another .44 build.. (my hands were too dirty to go get it for the pic from inside) the .30 hammer is the one in the bottom pics with 3 contact bands and a more central cocking pin.
I have decided to use a Crosman trigger and grip frame on this pistola as it will do the job nicely..
They are not the toughest built frame but with a bit of sear work and some shim washers it has made a very acceptable trigger. With a quick pre-fit of all the parts and a 3000psi fill I have tested the firing mechanism and valve operation on max power and there is a very pleasing and loud discharge of air moving through the open transfer port.. It looks/sounds promising to push the 85gr HP bullets to the fun side of 100ftlbs ;D
So with the engine room and fuel tank completed I have moved onto trying to finalise what I want this little hand cannon to look like..
I have done a quick concept on Photoshop of the receiver and front shroud design I have had floating around in my head since I started this build.. it may look a little more like an “auto pistol” than the traditional exposed barrel look but Im still undecided. This target stile grip design however I have decided on and will be sculptured from the heart walnut.
As most of you know wood is by far not my favourite media to work with however Im quite looking forward to making the grip for this one.. it seams a little less daunting than a full sized rifle stock and some of the past bullpup stocks I’ve tackled leave me saying “never again”..
I will be making an ambidextrous target style grip for this one.
Part of my attraction to this dusty project is that I have been saving some very special pieces of timber for just such a project.. a really nice piece of heart walnut with some great colour and grain and also a piece of rare Wenge timber.
Wenge is the product of the Millettia laurentii tree. Native of the Republic of Congo, Cameroon, Gabon and equatorial Guinea. The tree was put on the endangered list due to the destruction of its habitat and over exploitation for timber. This piece was brought to NZ by a south African Knife maker and after many years of sitting on a wood rack came to my possession.
The colour of the wenge is a nice dark misty brown with almost black striped grain.. a very beautiful colour when oiled. It is VERY hard withj a tight grain and was often used for knife handles and inlaying etc.
Well Ill try to cover a little of what I missed on the last bullpup build on this one so it doesn’t all just get repetitive and boring..
I got the cylinder all marked out, centre punched and milled all the counter sink cap screw holes and transfer port into it ready to seal up and pressure test.
Then it was onto the fill probe so I could get air into this new high pressure vessel..
I went with a different idea on this build as I want to keep smooth lines and minimal length I didnt want extra material out the front off the front plug to drill in the probe port like I did on the pup so I drilled these front plug retaining cap screws 180 degrees to the valve retaining screws and drilled an 8mm probe port between and forward of the two lower cap screws.. this puts it out of sight and no need for unsightly protruding Foster fittings.
Due to the new location of the port I couldnt use the probe I made for the Pup because it was longer and would have come too close to the top cap screw so I made a special “pistol” probe and included a male foster into the design to save the need for a screw on adaptor. This way it plugs straight on to my whip line.
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With only a 7.2″ barrel this little hand cannon Im going to need some light weight lead so I have gathered a small selection of lighter bullets to run through it to keep the velocity up..