.30cal big bore PCP build
My mission is to build a light weight short .30cal big bore PCP from scratch suitable for taking deer and pigs with cast projectiles.
I have 3 different barrels to test with a range of twist rates and bore diameters for the range of .308 cast projectiles which are in themselves tunable in weight and balance with an adjustable depth hollow point.
Ive got my money on the 1-14″ twist .307.5 match barrel for the 105gr cast projectiles but also have a 1-18″ twist for 80gr -120gr projectiles and a 1-10.5″ twist shallow cut microgroove for up to 160gr projectiles.
This is the bullet I have chosen, it is a 115gr Lyman bullet
The mould has been modified into a hollow point that now casts a 95-110gr HP pure lead bullet depending on hollow point depths.
My goal is to get about 170-180ftlbs from this rifle so although making a bullet for a barrel before the rifle is even built may seem a little backwards but it was a big part of the R&D process for this project and I needed to be very sure there was a bullet that would suit my needs and will be stable at a lower than intended velocity range in one or more of my barrels. I pushed a sized one through the microgroove bore to get a feel of the bore friction and check out the rifling groove cuts in the bullet.. the weight and balance can be fine tuned by altering the hollow point depth and diameter at a later date if needed.
For the action I cut a piece of 1-1/4″ 6061 T6-51 alloy to length and skimmed one side down on the mill to 1″ to form the start of the receiver.
Then I marked the barrel/bolt centre, fitted up my 4 jaw chuck and drilled the pilot hole with a long series 8mm drill bit..
Followed by the 1/2″ barrel clearance size drill and a 10mm bore for the bolt.
I then cut the loading port for the feed tray and the 45 degree chamfers to pretty it up..
A little more milling and the basic shaping is done.. It will get a raius to mate up with the cylinder, transfer port and then the bolt and cocking pin cut outs on the underside..
A few more components to the list.. the rear plug is yet to be bored out and threaded for an external power adjuster/spring guide and the bolt still needs the probe, O ring groove and handle to be made.
The 1-14″ twist match barrel has been pre fitted and is ready for the transfer port to be drilled.
I have decided to use a modified German made Gecardo trigger because Im too lazy to make one from scratch.. 🙄
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Next up the high flow valve..
I cut a length of 6061 T6 bar a little longer than I need for the valve but Ill use the other half on the front plug/fill valve.
After turning to the inside diameter of the air cylinder I drilled a 5mm pilot hole right through and then parted it off a little over the length it will end up and bored the various port, valve shank and seat internal diameters.
Once all the internal bores were cut I measured up and drilled the transfer port and then milled a counter sink for a transfer port tube.
The valve was then slipped into the air cylinder and located it with the transfer tube. Once locked in place I drilled the holes three valve retaining cap screws and countersunk them with a 3/8″mill cutter so the heads of the cap screws are in shear load not the threaded shanks. This “pinning” system is far stronger and safer that threaded end plugs that can strip out and thin the cylinder walls in the threaded sections.
Back in the lathe for a polish and to cut the O ring grooves
Then a hole and 1/8″ BSP tapered thread to fit a downward facing manometer.
With a final polish and clean it is ready for assembly.
The gauge will get a brass extension to drop it down flush with the walnut stock so I haven’t wound it tight into the tapered threads
Progress has been a little slow this week as I have been a little pre-occupied but I spent a little time on the lathe tonight and made the bolt handle.
Because of the heavier hammer springs needed on big bore airguns, there is a trend to have a separate cocking handle directly into the hammer and protruding from the cylinder but I’m not a big fan of that “double action” system.
This is normally done to lessen the cocking pressure but I have decided to have the hammer cocking of the bolt so it had to be built tough and of a big enough diameter to take extra load
After shaping and trimming to length I heated it red hot and put a nice bend in it. It is drilled into the bolts shaft and held tight in place by a recessed 4mm cap screw.
What a nice project!
Make a bolt action mini bullpup 177. There will be a long waiting list than your cannon that has Too much power for backyard 😀
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Solid progress right there. What’s your background since you have some good machinist skills. Pics of your workshop?