Dual Tank Benchrest Rifle

[rsterne]

Thanks for those thoughts.... much appreciated....

Bob

[subscriber]

Bob,

That sort of thinking comes easily to me.  I had pause before posting, because I am not qualified, being that I do not own, nor have I made a bench gun of any configuration. 

I find that thinking in analogies often helps with perspective.  I was granted an honorary blackbelt in analogies by a Swede once

[sb327]

I have built a couple bench rest powder burners. Not used in competition but both turned out extremely accurate.

A couple of things I made sure in both builds

1. Free float barrel. No bands.

2. Rigidity from chamber area to BOTH bags.

One I made a long clamp top that secured barrel to chassis (stock) with eight machine screws. The bolt action floats within the chassis.

The objective is to have the effect recoil has on the shot be as consistent as possible.

Bipods are not the best in this case. A front bag that the stock can easily slide on and the stock can be placed the same before each shot.

If the buttstock is to be on a bag, the lower portion of stock needs to be parallel to barrel/forestock.

Just some suggestions Bob. You have a sketch of your general layout you have in mind? I like the idea of an easy to machine receiver. I think it can definitely be done and still be somewhat modular.

Dave

[rsterne]

I have decided to use bags instead of a BiPod, and the bottom of the stock will be about 6" long and parallel with the barrel.... It will likely be a 2x4 on edge, screwed to a tube that slides over the extended back end of the plenum tube, so adjustable in length and clamped to it for rigidity.... At the front, the twin bottles will supply a foot long, wide base, parallel to the barrel.... The front bag will have to sit on a wooden block to raise it up to the correct elevation.... No meaningful drawings yet, but the basis for the general arrangement is my 6mm regulated bottle gun....



Two 700 cc bottles will replace the single 500 cc, same PRod trigger group, flat bottom stock as described above....

Bob

[subscriber]

Bob,

It looks light an minimalistic.  Thus attractive or scary, depending on perspective.  I like it; but you should look at the weight of the parts and how bending moments are created and resisted.    That should provide peace of mind or warn based on your expectations for rigidity in normal use.

[rsterne]

Well, the .183 cal barrel weighs about 7 lbs, and the plenum/valve/hammer tube is CrMoly, so also heavy.... Benchrest only, probably 15 lbs. total?....

The .22 pellet version will be MUCH lighter, carbon sleeved LW barrel, aluminum tube, two 450 cc bottles instead of two 700cc.... Similar to that drawing, except the breech / bolt assembly will be inside the 30mm OD barrel sleeve....

In both cases, the scope will be mounted on the barrel.... 

Bob

[rsterne]

Well, I'm still collecting bits and pieces for the new guns.... The barrels are on order, I'm getting a couple of Crosman PRod pistol trigger assemblies from Lloyd Sikes along with some tubing, and I gave my wife a big hint for a new 8-32 x 50 Scope for the .183 cal. for Christmas.... I have a Hawke 6-18 x 44 I took off my Hatsan for the .22 cal pellet barrel.... I have some more 6" long Picatinny rails coming, and my son has done up some tank spacer blocks with his 3D printer.... I got a new Ninja/Umarex regulator from Hajimoto, set to 3600 psi for the .183 cal, and I have a 2200 psi Ninja regulator for the .22 cal.... These will mount between the "Inverted Y" block that holds the two tanks and the front of the plenum/main tube.... The benchrest rear stocks will be a simple 2x4 attached to the bottom of piece of PVC pipe, slid over the main tube.... The bottom will be parallel to the barrel so that it will slide straight back on the rear bag during recoil....

Anyways, I haven't made any chips yet, but I'm getting closer.... 

Bob

[rsterne]

I was looking at different designs for an adjustable rail to allow setting the scope on a downslope to the barrel, to allow shooting out to at least 500 yards.... I found one called the Eratac (patent pending) that uses an asymmetric octagon to change the height of the back of the scope rail....



It only has a total of 70 MOA of adjustment, and I needed about twice that, so I came up with a linkage to do that....



As you turn the adjusting wheel (black) clockwise, each time it engages a new flat on the blue cam, it raises the back end of the red bar by 0.020".... Since the distance between the center of the cam and the pivot is (3600 / 1.047 / 1000) = 3.439", each thou changes the angle by 1 MOA, so each step changes it by 20 MOA.... There are 7 steps (140 MOA), and I intend to machine 20 MOA of angle into the bottom of the base that bolts to the barrel (which sights the gun at about 100 yards), so that gives me a total of 160 MOA available, which should be plenty to allow shooting at a target at over 500 yards away with pretty much any decent slug, starting at a MV of 900 fps or higher....

Bob

[sb327]

Interesting concept, looks like it can be solidly locked once the adjustment is made.

Dave

[rsterne]

Yes, there are two screws, one on each pivot, that I assume pull the movable rail over against the fixed one when tightened.... The trick is to make to whole thing low enough that the scope isnt waayyy higher than required to clear the barrel when the objective tips downward....  .... I'm guessing that is why the Eratac has the rings machined as part of the movable section of the mount.... 

Bob

[sb327]

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

That is pretty nice the rings are built in. Just makes things that more solid.

Can’t wait to see some chips made!

Dave

[subscriber]

Simple, clever scope jack.

[rsterne]

I got three tank spacer wedges for my Dual Tank Benchrest guns today from my son.... He used his 3D printer....





They are 100% infill, so solid as a rock, easily capable of taking the compression from a hose clamp, which is what will hold the tanks rigid at the back.... This was a perfect solution, I'm sold on 3D printing for certain jobs such as this....

Bob

[Motorhead]

That should work !!

[subscriber]

Simple and effective.

[rkr]

I've sometimes wondered why we use barrel bands instead of hose clamps and filler wedges/pieces, probably because barrel bands look cooler.

[rsterne]

 Yep, that would be my guess.... However, in this case it would be huge and hard to make, so a worm-drive hose clamp will be just fine!.... also much thinner!.... 

Bob

[rkr]

Yep, that would be my guess.... However, in this case it would be huge and hard to make, so a worm-drive hose clamp will be just fine!.... also much thinner!.... 

Bob

Yes, I have used metal band with screw and nut to tighten it and spacer in between barrel and airtube without issues. Easy to make and shims can be added if needed. I eventually made a one piece band for that gun but IMO it's not really needed in most cases.

[rsterne]

Just to make you jealous, here is a Google Earth view of one of the locations I have permission to shoot ground squirrels and marmots on, just 10 km. away....



The two red lines are 500 and 1000 yards long, so running out of range for testing airguns is not an issue.... 

Bob

[customcutter]

Wow!  Almost looks like a golf course!  You'll have to start a NUAT club.