Best aluminum for tensioned shroud?

[mackeral5]

2024T3 aluminum comes to mind as it is the preferred aluminum for pressure tubes...   But do those same qualities make it the preferred aluminum for a TENSIONED shroud? 

I have 2 applications.  There are space constraints so material dimensions must closely match what is described below.  Both applications are expected to tension the barrel as such that it vibrates at a higher frequency, at less amplitude when tensioned.

One is a 7/8 x .065 wall, 24" long this one is simply a tensioning sleeve, is not configured to also affect muzzle report.  ----I am likely to use 4130 here, unless someone convinces me otherwise.  weight is not so much of a concern here.  The application generates 380fpe and only weighs a little over 8lbs, it will not hurt to add a pound. 

The second application is just under 17" using 1 x .049 wall, this one has a vented muzzle nut, thus the shroud is both a barrel tensioner and makes things a bit more quiet.  Weight is not a deal breaker here, but it would be preferable to use aluminum if it will perform as needed.  This is an 80-100fpe application. 

Carbon fiber tubing----I am over my love affair with CF for now, I have no interest in any CF related input at this time....

Bob---I know you have written on this topic previously and I have searched but have come up empty.....

Thank you in advance.

Mike

[rsterne]

I assume the barrel is in tension, so the shroud will be in compression?.... If you use the same material for the shroud as the barrel, then thermal expansion will not change the barrel tension with temperature.... Any other combination of materials I would suggest you use a short stack of Bellevilles at the muzzle to reduce the change in tension with temperature.... Generally speaking, an aluminum that has a higher tensile strength would also have a higher compression strength, so 2024-T3 would be better than 6061-T6, but it has been a while since I looked at the Thermal Coefficients of the two.... Here is a thread you may find has the answers you are seeking....

https://www.gatewaytoairguns.org/GTA/index.php/topic,81226.0.html

Bob

[mackeral5]

I assume the barrel is in tension, so the shroud will be in compression?.... If you use the same material for the shroud as the barrel, then thermal expansion will not change the barrel tension with temperature.... Any other combination of materials I would suggest you use a short stack of Bellevilles at the muzzle to reduce the change in tension with temperature.... Generally speaking, an aluminum that has a higher tensile strength would also have a higher compression strength, so 2024-T3 would be better than 6061-T6, but it has been a while since I looked at the Thermal Coefficients of the two.... Here is a thread you may find has the answers you are seeking....

https://www.gatewaytoairguns.org/GTA/index.php/topic,81226.0.html

Bob

thanks Bob.  The toughest decision to make for me will to back away from wanting my cake and wanting to eat it too.  Tensioning the barrel is one thing...  A desire to use said tensioner as a sound reducing shroud creates a set of compromises that may or may not conflict with my goal of having a stable barrel system.  Stable both in the context of rough handling as well as being thermally stable.

I recently did some testing with a 2024t3 shrouded gun.  it is a rather complex system of a 700mm FX liner system with call it 16" of unsupported barrel.  I used an old WAR tensioning adapter as the barrel nut.  The WAR adapter has vents so the shroud provides a bit of sound supppression. 



I shot a group immediately after going outdoors when the gun was around 72F or so.  Then I let the gun sit in the sun until the gun measured 97-100F using an infrared temp gun, and shot another group.  These are 100 yards groups of Altaros ATP Smooth 49.5gr at 850fps.  These are randomly shot, not cherry picked nor was great care taken in shooting.  If anyone were to pick up this gun, they should be able to shoot these or smaller groups.

Sunbathing between groups:






The lower left group was with the gun at 72F, it was standing in a corner, butt down, leaning with the moderator against the wall.  I carried the gun outside by the shroud.

The lower right group was after 30 minutes or so of sunbathing.  Notice the one low shot, I am unsure if that was regulator/hammer strike related or if that was just the gun shooting where it shot.   Perhaps the upper group of 4, was that higher velocity?  or, after the expansion event, and the one shot, did that one shot shock things into their new home after expanding? 

I plan to test more, including velocities etc.  This was just a quick look to see where I was with this setup.  it seems to be resilient to rough handling, at least.

[rsterne]

Which target did you shoot at on the second group, the lower right?.... Is so, after settling? it looks like the POI moved up (but the 4-shot group is at least as good)?.... A quick look at my info in the previous thread would indicate the tension increased?....

Bob

[mackeral5]

Which target did you shoot at on the second group, the lower right?.... Is so, after settling? it looks like the POI moved up (but the 4-shot group is at least as good)?.... A quick look at my info in the previous thread would indicate the tension increased?....

Bob

Agreed.  I was aiming at the lower right bull when shooting the hot gun.  But, I neglected to take velocity measurements before and after each group.  Next time I do that test I will record velocity to ensure that isn't a factor.....

[Conecrusher]

Tom Costan, of AAA in San Diego, has posted some ideas on this topic on the AGN

[JPSAXNC]

It looks like your using a moderator even with your vented shroud. I would stay with the moderator, and fill the shroud with expanding foam to insulate the barrel from changes in temp. And the foam would  increase overall stiffness.

[mackeral5]

It looks like your using a moderator even with your vented shroud. I would stay with the moderator, and fill the shroud with expanding foam to insulate the barrel from changes in temp. And the foam would  increase overall stiffness.

For my purposes I prefer a shroud that is functional for sound moderation.  Otherwise no need for a large diameter shroud.....  Ultimately I abandoned this configuration----an FX liner system with a tensioned shroud resulted too many variables for repeatability.  Once setup it was very accurate and very stable.  However, returning to zero after disassembly/reassembly was just to much of a PITA.  At some point I will revisit the tensioned shroud but with a solid barrel.