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All Springer/NP/PCP Air Gun Discussion General > Big Bore AirGun Gate

Starting a .308 ELR build

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

Scotchmo:

--- Quote from: JPSAXNC on December 28, 2023, 10:01:48 AM ---You can stiffen the barrel by epoxying a close fitting aluminum tube over it, less weight.

--- End quote ---

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.

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