The reason I used two concentric tubes in the design of my dual tank benchrest is that I could not get a single tube that would do the job.... I would have preferred that.... The stiffness of a barrel is basically proportional to the 4th power of the outside diameter, so if you double the diameter, it will be 16 times as stiff.... Assuming you glue the tubes together, a 12 x 14 inside a 14 x 18 would be stiffer than a 12 x 16 and almost as good as a 12 x 18 (depending on the glue strength).... Gluing it to the barrel will help quite a bit as well, although tensioning the barrel by putting compression on the tube may be just as good (or better, who knows?)....Bob
Mike, my experience with long airgun barrels is that you need a tight barrel band for consistent accuracy. The best group with that setup so far is 12.5mm 5 shots at 100M and I did 3 groups under 15mm in that range session.
Quote from: rkr on March 06, 2023, 03:58:00 AMMike, my experience with long airgun barrels is that you need a tight barrel band for consistent accuracy. The best group with that setup so far is 12.5mm 5 shots at 100M and I did 3 groups under 15mm in that range session.I do appreciate your real world experience. Your. 257 one I often use as a point of reference, a point of measure. On a short tubed gun such as the brod pictured, do you think a barrel band at the tank block would be worth the effort?My .25 Gauntlet is setup that way and can shoot accurately. However one must NOT touch the tank, or POI will move. Applying any vertical or lateral pressure to the tank normally results in a POI shift.The Brod tank/block interface is much more robust than the Gauntlet, perhaps it would not be as sensitive.I may try to fab up a temp/reversible solution and test.
Quote from: mackeral5 on March 06, 2023, 07:19:27 AMQuote from: rkr on March 06, 2023, 03:58:00 AMMike, my experience with long airgun barrels is that you need a tight barrel band for consistent accuracy. The best group with that setup so far is 12.5mm 5 shots at 100M and I did 3 groups under 15mm in that range session.I do appreciate your real world experience. Your. 257 one I often use as a point of reference, a point of measure. On a short tubed gun such as the brod pictured, do you think a barrel band at the tank block would be worth the effort?My .25 Gauntlet is setup that way and can shoot accurately. However one must NOT touch the tank, or POI will move. Applying any vertical or lateral pressure to the tank normally results in a POI shift.The Brod tank/block interface is much more robust than the Gauntlet, perhaps it would not be as sensitive.I may try to fab up a temp/reversible solution and test. I would give it a go, as said copy Bob's design and if it improves your groups you can then try a more permanent solution. I have my barrel band attached to both air tube and the stock making it extra stiff, perhaps you could try making some sort of extension for the stock that extends under the bottle and ties to the barrel band? It would be a good idea for rested shooting as well.
Quote from: rkr on March 06, 2023, 11:10:02 AMQuote from: mackeral5 on March 06, 2023, 07:19:27 AMQuote from: rkr on March 06, 2023, 03:58:00 AMMike, my experience with long airgun barrels is that you need a tight barrel band for consistent accuracy. The best group with that setup so far is 12.5mm 5 shots at 100M and I did 3 groups under 15mm in that range session.I do appreciate your real world experience. Your. 257 one I often use as a point of reference, a point of measure. On a short tubed gun such as the brod pictured, do you think a barrel band at the tank block would be worth the effort?My .25 Gauntlet is setup that way and can shoot accurately. However one must NOT touch the tank, or POI will move. Applying any vertical or lateral pressure to the tank normally results in a POI shift.The Brod tank/block interface is much more robust than the Gauntlet, perhaps it would not be as sensitive.I may try to fab up a temp/reversible solution and test. I would give it a go, as said copy Bob's design and if it improves your groups you can then try a more permanent solution. I have my barrel band attached to both air tube and the stock making it extra stiff, perhaps you could try making some sort of extension for the stock that extends under the bottle and ties to the barrel band? It would be a good idea for rested shooting as well.One last post on the OP's thread regarding my application, because I do believe it is relative and value adding to the topic....I very crudely/temporarily attached the barrel to the tank block and tested. Initial results are very promising. I quickly shot 7 groups. 5 of the 7 had 3 shots in less than an inch. These shots were relatively rapid-fire, without much focus on follow through. Prior to securing the barrel this style of shooting would have resulted in many more fliers. In summary, I plan to add some method of barrel bad securing the barrel to the tank block. The challenge will be finding someone to make it, and of course coming up with a design that allows for some variation/adjustment in assembly
I had a spacer 3D printed for my dual tank benchrest gun....I will be using a worn-drive hose clamp to hold everything rigid and aligned....Bob
I'm not sure the carbon tube approach addresses the barrel getting bumped issue. Maybe a barrel band that is two separate clamps where you can bond the lower section to the tube. Conceptually something like this design.https://www.aliexpress.us/item/2251832761554550.html Of course you could go the airbag route and wrap a pool noodle around the barrel.
Since the bottle is off center on a drop tank, it may be able to move if you bump it and nudge the barrel if it is clamped to it. I love 3D printing but tightening fasteners has some risk. I keep coming back to your approach of indexing the barrel band to the tube, hence the idea of bonding onto the tube. That way, unless you bend the barrel, you should stay on boresight.