Dual Tank Benchrest Rifle

[rsterne]

But still high enough your not looking at the muzzle, lol.

sb327.... I checked the scope I have coming, and at the lowest magnification (8X) the field of view just touches the top of the muzzle when the scope is at maximum elevation (160 MOA).... so I think I should be OK.... The bell of the scope will clear the barrel by just over 1/8" as well.... I am waiting for the scope to arrive to double check everything, and I have some more aluminum bar stock coming as well.... Getting closer to making chips.... 

Bob

[rsterne]

Well, after months of collecting parts (I still haven't received the barrels, but LW has them on order), I finally got back into my shop to do some work on this project.... The shop needed a thorough cleaning (3 years of dust and debris) but it was sure nice to be working on an airgun again.... I decided my first project would be the adjustable scope mount.... I haven't machined the radius into the bottom to fit the barrel yet, but the rest of it is finished.... The base will sit on a 20 MOA slant, to put the scope centered at 100 yards, and the adjustment gives me a total of 140 MOA up from there in seven 20 MOA steps, for a total of 160 MOA from the boreline.... The secret is in the asymmetric octagonal cam.... Here is a photo of the top view of the parts....



and here is the bottom view of them....



On the top is a 6" long Picatinny rail, with the cam follower bolted to it.... The groove on the bottom of it is to locate the upper arm of the spring, which is from a clothespin.... The middle portion (bottom in those photos) has a large groove machined into the top to hold the spring, two threaded holes to mount the Picatinny rail, and on the bottom it is machined to clear the mounting screws and the bump in the baseplate.... The shouldered pivot bolt, and the cam mounting screw, are threaded into the side of it.... The baseplate is an "L" shape, machined from a 3/4" x 1" bar of 6061-T6 aluminum, the same grade material is used for the other parts.... There are two mounting holes on 4" centers, and the vertical part has holes for the shouldered pivot bolt and the cam support, which have very close tolerances to minimize play.... The "bump" has a transverse hole to accept the bottom leg of the spring.... Here is a photo showing the spring and cam in place....



The next photos show the mount assembled.... The first two are the right side, showing the mount at minimum and maximum height adjustments....





The next two show the left side, again at the lowest and highest settings....





To adjust the height, you loosen the two screws on the right side (cam and pivot) a half turn, and turn the knob to the height desired.... It has a nice snap from one position to the next because of the generous flats and the spring tension.... When you reach the height you want, you snug the two screws up again, to draw the middle section against the vertical part of the baseplate, while pinching the cam against it as well.... The cam screw moves up and down with the mount, which is why there is an asymmetric hole through the cam.... When the screws are tight, the mount is completely rigid....Here is a photo of my Vector 8-32 x 50 Sentinel scope adjusted to the maximum tilt....



I will be mounting this setup on my .183 cal, on the bull barrel, just ahead of the loading port.... I have already determined that I can get the required eye relief with the scope, and as you can see, it will clear the barrel at maximum tilt.... First part of many, basically finished.... 

Bob

[subscriber]

Very nifty, Bob!

[sb327]

Glad to see and hear you are making chips!

Appears the mount is going to be pretty solid. Nice work 👍

Dave

[rsterne]

I got my lathe repaired, so today I roughed out the two tank blocks.... My milling attachment for the lathe has a vice that is only 1" x 2.5", and opens just 2.3", so I had to mill the middle of the 1.25" thick block to fit into it....    .... I had to turn the block end for end, and make one block on each end.... A lot of it will get cut away to shape it.... Here is the one for the 18 x 1.5mm Regulator (for the .183 cal)....



and here is the one for the 5/8" x 18TPI Regulator (for the .22 cal)....



All four of the threaded holes for the tanks are 18 x 1.5mm.... I have some all-thread studs made from Metric bolts to attach the tanks.... The smaller through holes, which will be on the front of the tank blocks, have yet to be threaded 1/8" NPT, one for a gauge (tank pressure) and one for a male Foster fill fitting.... I also need to drill the air passages to connect the tanks to the regulator, and plug them at the bottom with threaded plugs with O-rings.... Once all that is done, then I can cut and mill away all the extra material....

Bob

[rsterne]

I have the tank blocks pretty much finished....



The 18 x 1.5mm studs are done to mount the bottles, the air passages are drilled to connect the bottles to the regulator, and the outer end of them is drilled and tapped 10-32 for plugs, which are a simple low-profile SHCS with a 5/16" OD O-ring under the head, fitting flush in a milled hole.... I have used these before and they work perfectly.... You can see one of the air passages in the right hand hole in the left hand block, and you can see the two 1/8" NPT threaded holes in the bottom of those, which allow a male Foster and gauge to be mounted on the front of the block.... The two tapped holes in the bottom of the tank blocks are to allow me to mount a Bipod, borrowed from my .257 Monocoque PCP, if I want to.... Here is a photo of it, mounted to the front tank band from that rifle....



It is adjustable for height and tilt (the pulleys act as handwheels), and has a level built in.... I probably won't need it on these rifles, because they will sit level on the dual tanks, but adding a couple of tapped holes to allow it to be attached just made sense.... The hole in the right hand tank block is to lighten it a bit, it is for the .22 cal pellet shooter, which will have a carbon sleeved barrel and aluminum plenum.... There was no point trying to lighten the one for the .183 cal slug shooter, as the barrel alone weighs about 7 lbs., and it has two 700cc bottles and a steel main tube.... Probably a 20 lb. gun?.... 

Bob

[subscriber]

Did you cut the threads on your lathe, Bob?

[rsterne]

The male threads on the Bipod are threaded rod.... and the studs that will hold the bottles are just shortened 18 x 1.5mm bolts and drilled through for the air passage.... All the female threads were done in the lathe, holding the workpiece in the milling attachment (it replaces the compound feed on the crossfeed carriage)....



For large taps, I hold it in the 3-jaw chuck, turned manually.... for smaller taps, just using a piloted tap holder, turned by hand, guided by the 3-jaw.... Here is what I use for tapping large threads, or for doing single-point threads up to a shoulder....





Unplug the lathe, remove the drive belt, and crank by hand.... 

Bob

[subscriber]

Thanks, Bob

[sb327]

Nice update Bob.

It’s always a relief when the drill bit pops out in the right spot when connecting these internal passages. We all have seen a bit wander off course.

Dave

[rsterne]

Using a milling attachment really helps.... The block is held square in 2 planes, and the third can be angled (by rotating the vice) so that the drill bit (when laid across the top of the part while in the chuck) intersects the center of both holes.... Then you set the height in the vertical, and it drill along the course you set.... I have never had a problem.... Of course you have to properly sharpen the drill to have the point on center, or use a new drill bit....

Bob

[rsterne]

I finished up the tank block for the .183 cal today.... The tank mounting studs are fitted and loctited into place, and the regulator is also in place.... I removed the gauge and fill fitting, and moved them to the front of the tank block, and plugged the holes in the regulator with 1/8" NPT brass plugs.... I could then screw the bottles into place, pop the tank spacer in, and slide a tube into place to see how everything fits.... 



It all worked out exactly as planned....  .... I have to make the adapter to hold the regulator in the front of the plenum tube, but it looks like the whole idea is going to come together.... I will do the other tank block to the same stage before continuing....

Bob

[rsterne]

The bottles are the primary HPA reservoirs, two x 700 cc = 1.4 litres at 300 bar for the .183 cal, and two x 450 cc = 900 cc at 250 bar for the .22 cal pellet shooter.... The regulator is mounted between the "inverted V" shaped tank block and the tube, which is the plenum.... The valve and hammer will be inside the plenum, with the stock on the back of it, and the barrel above it.... The plenum in the .183 cal will be longer (because the bottles are longer) and be made from 4013 CrMoly, running at about 3600 psi.... while on the .22 I will be using 2024-T3 aluminum above the shorter bottles, running at about 2200 psi....



Dotted lines are for the .183 cal version.... where I will be using the adjustable scope mount to give up to 160 MOA elevation (dotted line) for long ranges....

Bob

[subscriber]

Awesome, Bob!

[rsterne]

I got the other tank block finished today, and both assemblies fitted together for the first time....



You can clearly see the difference in the size of the bottles.... One of the 3D spacer blocks my son printed for me is in between the long tanks near the back.... They are going to work great, with a simple clamp (or even duck tape) around the outside to hold the tanks aligned with the plenum tube and rigid....

Bob

[Spacebus]

I'm looking forward to seeing how the .183 turns out. I've been fascinated with tiny bore rifles since stumbling upon yours and other .172 builds.

[rsterne]

I finished the Regulator Adapters today, and drilled and tapped them for a (low-side gauge), and for the three 10-32 SHCSs to mount the plenums....



The front end of the adapter (made from 1.25" OD 2024-T3) is tapered to match the back of the regulator, and of course bored for the O-rings and tapped 1/4" NPS to accept the regulator output.... I lubed and tightened each regulator to its adapter, and then used the tapered sides of the tank block (they are at 60 degrees to the base) as a reference to orient the gauge and the upper plenum screws.... The lower screw is on the bottom, referenced off the base, and you can just access it with an allen driver between the tanks.... The drilling of the holes to mount the tube is so accurate I can orient the tube any of the 3 possible ways, which actually surprised me!.... 

Other than making the O-ring grooves (I will do them along with the ones for the valve, all at one time), the tank and regulator assemblies are now finished, and fitted to the plenums.... Here is the one for the .22 cal pellet shooter, on the 2024-T3 aluminum plenum....



and here is the one for the  .183 cal slug shooter, on the (longer) 4130 CrMoly plenum....



I should get about 50 shots at 80 FPE with the 39 gr. BBT slug, and about 100 at 50 FPE with a 25.4 gr. JSB Monster pellet....

Bob

[Rob M]

looking great Bob , I went back and looked , whats the length gonna be on the .183 barrel ? i did read that its 7 lbs

[rsterne]

The .183 cal LW barrel is 26", and stock it is 32mm OD (1.26").... The .22 pellet barrel from Mike Niksch (a very slow twist LW) is 28" long, and "only" 16mm OD, but I will be sleeving it up to 30mm with carbon tubes....

Bob

[rsterne]

I started on the valves today by making the poppets.... These valves will be a semi-balanced design, similar to others I have made, but with the balance chamber reversed, with the "cylinder" being part of the poppet and the "piston" mounted on the front of the valve body.... Here is the basic design....



The position of the piston is adjustable to tune the volume of the balance chamber.... Smaller volume acts more like a conventional valve, larger volume "blows open" more, which requires less hammer strike but resists tuning via hammer strike.... Motorhead (Scott) uses this style, and the idea of adjusting it came from sb327 (David).... I decided to make the poppet from 2 pieces, a PEEK sealing portion and an aluminum sleeve/cylinder for the balance chamber.... I was concerned the PEEK may not be strong enough to resist the 3600 psi, and might "creep" over time to pinch the "piston".... Here are the poppets....



The one of the right is complete, the other one has the rough machining done, but it shows the internal O-ring to seal the two part together.... The 1/8" stem is threaded 5-40, and when assembled everything is glued up using Loctite 638 (green).... Final machining of the chamber is done after assembly, with the stem in the lathe chuck, to assure alignment.... The inside is done with a 1/4" mill, and the OD is 0.354", which reduces the force holding the valve against the seat by 50%.... At 3600 psi, it drops from 354 lbf. to 177 lbf.... The minimum clearance from piston the chamber will be 0.075", which is 1/4 of the valve throat diameter.... This means the valve can reach full flow without the poppet touching the piston.... That volume can be tripled if desired, to make the valve open easier.... The vent through the stem is 3/64" (0.047"), as is the hole through the side of the stem....

Bob