.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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…and heres the forward trigger group and scope mount risers I will also be re-using:
Ive had a few emails from guys asking where I got this bullpup stock from, it is one I made a couple of years back for a .25cal Marauder but it is a bit of a cupboard queen at the moment so I have decided to share the stock and the forward scope riser mounts/trigger group with my new .30cal build. So for those of you that missed that build thread here’s a brief look at the making the stock.. (sorry to all those that have seen it before)
The stock started as a large 6 foot long 2.5″ thick slab of 120+ year old black walnut.. I spent about an hour moving the template around the slab trying to envision the finished stock… Im very happy with the chosen piece but wont really see the true grain until I start the shaping. I should have taken a pic of the vast array of router guides and jigging I had set up to get the inletting right, It looked awkward to put it best.. This is my first stock Ive made by hand that is not to be a painted finish so Ive been sooooo careful not to slip with any power tools..
that said there’s a lot of work to do on it yet so if you see a sudden shift from the idea of polished walnut to a nice olive drab paint job, you can assume Ive muffed it up and had to bog something 🙄
I spent about an hour behind the router tonight snorting sawdust and reminiscing my college wood working class…
I made a good start on the inletting. Nice snug fit too.
The scary inletting job is all done, it was a little more complex than I first thought so I’m very relieved to have that stage finished..
Another couple of hours behind the router and its starting to take shape..
I had to shift the lateral router line down about 10mm due to the thin wood in the scope mount recesses. I wanted to be well clear so not to
weaken it or in case they “broke through”. It hasn’t really changed the look to the original design though, they still look good.
I really wanted to stray away from the “chunky” looking bullpup designs commonly seen and endeavour to produce a thinner, slick, light weight
and more ergonomic stock.
I finished cutting the basic outline now and have started roughing out the contours.. Its starting to show some really nice grain now..
I’ve got it all shaped out now which only leaves the sanding part… lots and lots of sanding!
I’m really loving the lines of this stock, the grain follows the curves just as I hoped it would when
I laid out the template. I have tried to keep the Eastern European stile lines which I’m quite fond of.
Probably not every bodies cup of tea.. but then again nor are bullpups
A total of 20 hours work and the stock is finished just applied the final coat of oil. with the recoil pad fitted the stock weighs 980 grams.
The capacity is 255cm3 and I hope to get 8-10 usable shots with the 105gr HP cast bullets maybe a couple more with a 3400psi fill. The decided fill pressure and final tuned velocity to get the best accuracy will determine the air usage though.
As above, I don’t want to discuss the cylinder specs here too much as I think there is too many risks involved for others that may attempt it without the knowledge of other crucial engineering practices .
-how do you anchor the barrel. Solid slugs take a lot of force getting them out of a barrel. Not something you hold with a couple of set screws.
-the tubing for the air cylinder looks quite thin. Did you do any calculations on it?
Cheers
Marc
Thanks Marc,
The barrel is threaded into the receiver, one set screw just locates the rear bush to stop any misalignment with the transfer port and the other is a locking screw on the barrel thread as I didn’t want to use any loctite due to it having several barrel options.
The cylinder tubing is well tested and yes all calculations done. It has a higher safety factor rating than many other PCP cylinders currently on the market so I am more than happy to hold it to my face… It is only 1- 1/4″ OD tube so no need for 1/4″ walls etc. I am a little anxious to post to much detail on the cylinder properties here for fear someone else will try it without using other correct engineering practices so chose to leave the finer details out.
Pretty quick work. Less than 4 weeks and you have an almost fully functional big bore. I too am curious to see know the capacity and specs of the reservoir. How many shots do you think you’ll get out of it at full power at 3,000 psi fill? 6061 and not 7075 tubing right?
skills, you have some mad skills. I really like it looks like a quality build!
Fine looking project. I have just two concernes.
-how do you anchor the barrel. Solid slugs take a lot of force getting them out of a barrel. Not someting you hold with a couple of set screws.
-the tubing for the air cylinder looks quite thin. Did you do any calculations on it?
Cheers
Marc
The build is actually currently well ahead of the documentation this time as Ive been pretty keen to get it to a working state ASAP and thought Id just take a ton of pics and add descriptions later as I get time.
The trigger is fitted and the hammer has been machined to except the drop away sear, it is actually a very nice operating trigger with a crisp let off and no horrible creep.. Do I sound surprised? (I was ;))
I dry fired it down from 3000psi to 0 psi in my workshop to empty the cylinder and it sounded like a thunder clap in a confined space.. its moving plenty of air that’s for sure..
I just need to shoot into town and get some O rings for the bolt probe etc so I can test some lead through it and run a few strings over the chrony. I will fine tune it with a selection of hammer weights once I get close to the desired velocity. [/color][/size]
On a positive note: the proportions look pretty sweet in its walnut bullpup stock. The .308 match barrel is currently cut to 23 inches, this cast bullet barrel will be a deer, pig and goat slayer so noise is no issue and will probably stay unsuppressed.. I may make a nice break for the muzzle though.
With the air cylinder sealed up and the valve working I moved onto fitting the receiver to the cylinder.
The transfer port was used to centralise the action on the cylinder which consists of a steel transfer tube compressed between the two parts in blind counter-bores sealed with external O rings.
Once the transfer port was mated up I drilled the action for four flush fitting 4mm cap screws to lock it all down in place
After some careful measuring of the hammer travel and total spring compression range I then cut a slot for a cocking pin and mated it up with a second slot in the receiver for the opposing cocking pin/bolt lock up. I have used a much bigger cap screw in the bolt and recessed the head a little as it is subjected to a lot of rearward pressure on discharge, I will also be adding a second smaller “safety screw” on the bolt in case the main locking screw ever failed and tried to exit the action rearward. Its not as much as a safety hazard in a bullpup as it would exit over top of your shoulder and probably leave you with a ringing ear but in a conventional rifle set up the bolt is right in front of your nose 😯 😮
The action works smoothly but it does takes quite a bit of effort to compress the hammer spring with the added side loadings of the cocking pins etc so regretfully I think I may have to cut a second slot in the side for a stand alone direct cocking handle. :sadn:
Knife
i can believe it. in my shop area i have a lathe, belt/disc sander combo, 4 rollaway tool boxes. a verticle mill, a cylinder hone, lateral mill, 3 welders and a few benches. all in a 10″x10″ area. lol.
its not junky, just cramped and cluttered because i have no room to expand to the rest of the shop which is crammed. i will have to take pictures next time im in the shop in town. we moved a few months ago and still havent sorted stuff out yet the rest of the way
Man, Your shop has the one thing I don’t have. ROOM!!! I can hardly walk around it to work. Not junky, just packed with equipment. Would you believe I don’t have the room for a lathe! Grrrr!!!
Knife
A little more progress in the last few days…
I decided to use a fill probe rather than a foster fitting to keep the cylinder shorter and tidy so started by turning a fill probe out of a piece of 12mm brass hex rod and milling a blind 8mm hole into another chunk of alloy bar. I wanted to leave it “blind” so the fill port was only visible from the underside of the cylinder and lessen the risk of introducing debris in to the air cylinder and valve etc.
Once the plug was turned to a nice slip fit into the cylinder and fitted with an O ring I centre drilled the 3/32″ air inlet port with a long series
drill bit into the 8mm fill probe port and cut a 3mm thread for a countersink head cap screw to create the one way fill valve.
I assembled the simple one way valve design consisting of a 3mm countersunk cap screw with the thread filed flat on one side snugged down on a 005 O ring and tested/adjusted its flow rate on the dive bottle.. I adjusted it to a restriction pressure of 500psi on a fully open dive bottle valve (3200psi) which will be enough to let air in slowly but completely seal and not let air back out when the probe is removed. Works a charm.. ;D
The front plug is held in place by three countersunk 12.9 grade cap screws like the valve body again with the heads in shear load not the threads.
A small mod was needed after initial testing as it was hydraulicing the air in front of the probe which prevented me from pushing it all the way home.. a 1mm vent port drilled into the top side of the fill probe hole sorted that out and allowed the probe to seat all the way home.
So with both shiny new parts fitted up to the air cylinder I filled it with air and submerged it in a bath of water to check for any leaks then left it over night to check for any leak down..
Sunrise came and the cylinder hasn’t dropped any pressure at all.
Thanks Mr Fudd,
My background is in mechanics and engineering in the aircraft industry but I have always tinkered as a home machinist and some light gunsmith work.
My workshop is nothing special, it contains a cheap Chinese made lathe/mill combo machine, a belt and face sanding machine an plenty of quality hand tools. The small lathe and mill has its limitations and is often slow going but I generally get along just fine with it.. most of what I do by hand could be done ten times faster with the correct machinery but hey.. it keeps me occupied. :whistle:
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If your hammer spring is that hard to cock you might consider using thicker trigger linkage or gudes. In my .357 Evanix pup I started with M3 rod but it flexed badly and I had to upgrade to M4 and even that flexes a bit. I know the Evanix sear geometry adds to the pressure but I have fixed that to a degree by reshaping the sear engagement geometry.