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Starting a .308 ELR build

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Scotchmo:
Im starting preliminary work on another iteration of my Armada based big bore. For the big_bore/long_range matches that I compete in, the .257 and 7mm versions are probably near optimum for the job, and I havent even fully explored the 7mms potential as it holds together so well at 500m. But Ive got the itch to push it further so Ive been dreaming up what I think would be a more optimum configuration for an extreme long range, subsonic airgun. Longer distances than the current matches require.

To start, I just ordered the NOE 308-154-FN-BF2 5 cavity aluminum mold. The plan is to shoot that slug at 1080fps. It will be a stretch for this platform. The Cothran valve ports are 1/4 and could be opened up slightly, but Ill stick with 1/4 for now. Im somewhat limited in pressure by the Marauder/Armada air tube. Its 3000psi but I might cheat a little and go 3100psi. Also, the tube provides only a 215cc plenum which is a little small for a planned 400fpe airgun. The barrel is going to be 60 long. I have been using DOM steel sleeved 7/8 OD barrels for the 36 long .257 and 48 long 7mm barrels. They are heavy and 60 would be more so. To keep the weight down on the 60 barrel, I would like to use a 7/8 OD, thick wall, UHM carbon fiber sleeve. That would add another $400 to an already pricey barrel. So I might start by slip sleeving the barrel in a crude steel pipe and go to a carbon fiber sleeve later on. The barrel liner is to be a TJ 1:10 twist .308 barrel that is 1/2 in diameter. According to TJ, they can make any length.

Its going to take a little redesign of the breech to accept the longer slugs. Other changes include using 60moa of compensation in the integrated mounts. The 7mm has 50moa.

The original .257 had a floated barrel. Early testing was done with a 3D printed plastic breech. An aluminum breech was necessary to support the heavy 36 barrel. I ended up using a barrel band on the 7mm as the 48 long barrel sagged too much, even with the aluminum breech. I plan to use barrel bands on the .308 as well. Not needing the support for a steel bull barrel means that I might be able to live with a plastic breech for awhile.

Itll likely be a couple of months before any progress reports.

Rob M:
get a second marauder main tube , connect the 2 threaded ends and make a cap for the second tube after chopping it.. this will yeild about 400 cc and the adapter in the middle is pretty simple.. It will also offer support out front for the 60 inch barrel

rkr:
Have you considered tensioning the barrel or doing it FX style or a combination of support discs and tension? That would be much cheaper than CF and save weight over the steel sleeve. In my .257 I used two 2mm thick CF tubes one over the other to sleeve the liner as a mid priced solution.

Scotchmo:
There are Marauder air tube extensions available (70cc extra). I may not need additional plenum.

The barrel is supported by the handguard, and is unsupported forward of that. As long as the sleeve is stiff enough, no additional support is needed. Ive got a couple of options for the barrel sleeve. And I have considered the suggestions. Im going with the cheapest option just to do some testing. A steel water pipe (0.85 OD, 5/8 ID), probably with a few -014 orings to center the 1/2 diameter barrel.

Im not going to do a tensioned barrel. Final barrel sleeve will be either heavy DOM, UHD CF, or two layers of standard CF. The selection of 60 long CF tubing is very limited.

HunterWhite:
I hope you don't mind if I follow along.
Mr. Hull your work is truly fascinating.


Hunter

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