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45 caliber Big Bird Big Bore Build

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I've been doing my 15 - 30 minutes of work here and there on my barrel and will be proceeding with a 45 cal. deer airgun that I will be working on over the course of maybe 6 months to a year.  So updates will be farther between.  This time may be good for planning and getting suggestions.

I'd like to have a 45 caliber deer gun that will operate similar to a Quackenbush (.458 Long action outlaw) in that I am going for a few good shots (declining or top of parabola) with shots 1 and 2 being higher velocity.  Probably 3000 psi since I only have a hand pump.

Barrel (previous to more work)

I have a Bergara 45 cal muzzleloader barrel, (1 in 28" twist) that has about 27" of bore.  I took off the welded on lugs via dremel and it was quite easy.

Lug on

Lug off

It has a 5/8" x 18 x ~1" threaded female breech plug area on the end that I will use for attaching the other half of the breech so breech and barrel are one unit.  The barrel is 1" at breech and 7/8 (was) at the muzzle and I will continue to taper back to the breech.

Working breech end

Ugly Muzzle (I'll clean that up)

So right now I'm looking at a513 DOM steel tubing for the pressure tube and need advice.  I dont see the benefit of 4130 with its lower yield point.  I like 1" x .083 but if it doesn't work for this setup then I can go to 1-1/8" x .095.

Also, everything will be pinned (fill end, valve) as opposed to a real Quackenbush  ;)

I was using the calculator here with 3.5 SF

Here is size, length for ~10 CI,  weight, which is also a concern.

1x.083 = 0.834 ID
25"  x .81 lbs per ft = 1.6875 lbs weight for 10 ci

23" x 1.0452 lbs = 2.0033 lbs for 10 ci

Please bare with me, it may look like muppets from Sesame Street are making it.

Took a couple hours to do the final profile on the front half of the barrel.  Still need to polish out shallow file marks but it has a nice transition from 0.838 to 0.692.
I think it is going to be a light sporter profile.

Still looking for a tube.  Any suggestions?

Madd Hatter:
IMHO, Don't want you your yield strength to close to your ultimate strength.


--- Quote from: Robert Feddeler on August 19, 2019, 11:57:47 PM ---IMHO, Don't want you your yield strength to close to your ultimate strength.

--- End quote ---

Yes.  4130 does have a wider difference.  The real benefit is the UTS in the 90k's psi.  I guess it would be good for engineering a fail in the design so it leaks before it breaks (Something like outboard orings or orings near a tube hole).  That would be a benefit over other tubing.   Or use a burst disk.

I am worried about testing to deformation.  I know Lloyds testing of the Crosman barrels yield happened pretty early (around 7k?  IIRC).  I read Bobs use of 1"x .065 for a tube in 4130 and that would be a great size for my application.

I am always on doing comparisons.  It shows 1020 does have a wider difference than any of the popular lightweight tube alloys (2024. 7075).   Maybe 1020 is a "tweener"?

So, based upon all the feedback and suggestions I ordered a 4130 - Round DOM Tube 1.125" x .083" x 36"
with 24" of 1144 and some other a513 tubes for my 30 caliber and future projects.


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