Starting a .308 ELR build

[Scotchmo]

I’m 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 haven’t even fully explored the 7mm’s potential as it holds together so well at 500m. But I’ve got the itch to push it further so I’ve 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 I’ll stick with 1/4” for now. I’m somewhat limited in pressure by the Marauder/Armada air tube. It’s 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.

It’s 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.

It’ll 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. I’ve got a couple of options for the barrel sleeve. And I have considered the suggestions. I’m 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.

I’m 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

[JPSAXNC]

You can stiffen the barrel by epoxying a close fitting aluminum tube over it, less weight.

[Scotchmo]

You can stiffen the barrel by epoxying a close fitting aluminum tube over it, less weight.

Aluminum is about 1/3 the density of steel, which gives a lower weight, but it has a correspondingly lower modulus. CF tubing is a lot better.

Modulus:
aluminum = 10msi
steel = 30msi
CF = 33msi
HM CF = 57msi
UHM CF = 110msi

[rsterne]

Hey, Scott, always good to see what a fellow Mad Scientist comes up with.... It will be interesting to see what you can do with the 154 gr. BBT.... I have a newer design with a smaller (40%) Meplat and slightly shorter and larger BT base, if you want an alternative (3.0 cal long instead of the 2.5 cal BBT in your 7mm) maybe we can rattle Al's cage to make it?....



155 gr. as a FN in .308 cal (SD = 0.233).... just scale up all the dimensions from the drawing.... 

I don't know if you have checked out Clearwater Composites, they sell UHM tubing in 72" lengths.... You could sleeve it in 2 stages, use regular CF for the 1/2"ID > 3/4" OD ($164), and then UHM for the outside (by far the most important for stiffness).... They go up to 72" long and up to 1" OD in UHM (7/8" OD is $189)....

https://www.clearwatercomposites.com/products/carbon-fiber-tubes/round/uhm-round/?filter_nominal-id-range=0-500-0-999

For a 60" barrel, I would go for broke (literally) with the 1" OD UHM (72" for $408).... over the SM for the .5-.75" inner layer.... That 1" OD UHM should be like a (very light) crowbar!....

Bob

[Scotchmo]

Both my .257 and 7mm barrels are 1:14 twist, which is a good match for 2.5 caliber long boat tail slugs. So if I wanted to push it, I was going to need a new barrel anyway, with a faster twist. I also wanted to stick with slugs currently in stock. I decided to redesign the breech to accept longer projectiles. Prior slugs were limited to 0.750” long and I was already at 0.710” long with the 7mm. I wanted a caliber that this platform could handle, with 30 caliber being a practical limit. I considered the 4 caliber long .257, but I think 3 calibers long is a better limit for subsonic/transonic projectiles. The 3 caliber .308 with the TJ 1:10 twist looks like a good combo to push as close as possible to sonic velocity.

Ok. Today I started machining the 0.84” pipe for the sleeve. But now I plan on skipping that idea. And this is why I like feedback on this forum. I’m going to be bold and go with a 1” sleeve. I’ve got some thinking and redesign to do.

[Scotchmo]

I made some progress on the design tonight. Since this is an Armada based design, there are some common dimensions that I retain. Because if this, the 1” sleeve will interfere with the air tube, unless I relieve it along the bottom edge.  0.946” is the maximum sleeve diameter before it touches the air tube.

[bear air]

Following along. Good luck with the new rifle Scott.

[Scotchmo]

I guess I won’t be calling this latest build an “Armada”. In order to get to my design goals, I felt the need to break away from the Armada constraints. The air tube is the only Benjamin part remaining in the design layout. And it’s been modified as well as most everything else in the tube. Cothran valve, modified end cap and gauge block, and electronic hammer.

[Scotchmo]

I’ve made some progress on the design for the .308 ELR airgun. I received the slug mold and the 1” x 60” long DOM barrel sleeve. I’ve got some other materials on order. No parts remain in common with the Marauder. Though it will be fitted with a Cothran Powerhouse valve. Anyone running the Powerhouse valve over 3000psi?

Below are three images of the AutoCAD solid model.

2nd image shows just two parts. White part will be the 3D printed ABS receiver. It will be inclosed in a square steel tube that also acts as a handguard (shown in green).

3rd image shows a side view of the mostly complete assembly (minus bipod and buttstock). Finished length will be 6’-4”.

[Back_Roads]

 Following, I have a similar cal. and twist barrel liner, I have been debating on a build with for some time, didn't cut it on a Daisy 880, so PCP is my next move, I had nay sayers on the twist rate, for PCP when I first got the liner

[rkr]

Really interesting design. Is that a single shot loader that has a transfer port in it? Then you push the bullet in to barrel with that red lever and pull it back? That square steel tube and ABS receiver is certainly new. Following your build.

[Rob M]

now i can see why you wont need the tube extension , didnt know it was gonna get another 2l tank. ALSO yes , ive run cothran valves at 3600 , no issues aside from the common stiction on occasion.

[Scotchmo]

Following, I have a similar cal. and twist barrel liner, I have been debating on a build with for some time, didn't cut it on a Daisy 880, so PCP is my next move, I had nay sayers on the twist rate, for PCP when I first got the liner

I think that a 1:14 twist is about max for a practical big bore airgun. The reason one would want this faster 1:10 twist is to launch a longer/heavier/higher_BC projectile. In order to realize much benefit over the 1:14 twist, you’ll need to launch it at least nearly the same speed and preferably even faster. That takes a proportionally higher pressure or proportionally longer barrel. With this build (in it’s initial form), it will not be possible to do anything other than bench or prone.

[Scotchmo]

Really interesting design. Is that a single shot loader that has a transfer port in it? Then you push the bullet in to barrel with that red lever and pull it back? That square steel tube and ABS receiver is certainly new. Following your build.

Yes. Just as you described

[Scotchmo]

now i can see why you wont need the tube extension , didnt know it was gonna get another 2l tank. ALSO yes , ive run cothran valves at 3600 , no issues aside from the common stiction on occasion.

The Cothran valve is designed for a Marauder which has a factory recommended limit of 3000psi. I’m not sure what the limiting criteria was. Possibly for an adequate margin of safety? Did you do anything to account for that when running 3600psi? I guess 20% overfill is not outrageous, but I get leery without some justification as to safety.

[Rob M]

Mine was on a disco size tube where the longitudinal force was the main concern ( not hoop stress ) So upgraded hardware was needed ( I think I was using grade8 10-32 as opposed to 8-32 retention screws ) it would  be different for the larger i.d marauder tube , I havent done the math as both the hoop and longitudinal forces are far greater in the mrod size tube \ Bob would be able to clarify whats needed for the mrod tube