Got some bar stock......

[CaptTHighbiter]

 I just came into 10 feet of 4-1/2" seamless tubing, with 2-1/2 ID".  Steel. 

I am in NYC, any one want to swap machining services for say 2/3 of this material?

[b. snipes]

I wish I was closer (Mount Vernon, WA.) I had a 10 foot piece of that too. Exact same size. I made several "jumpin billys" out of it. I have a 28" piece left which I am going to make a Lyle gun replica. I make flanged breach plugs, 2 5/8"x8 threaded plugs. Works well. Add some trunions and go for it. You will find the bore (unless machined) is not smooth and true. I am a blank load kind of guy so it is of no matter to me. beech

[Double D]

You are going to have to either band the breech or put in a chambered breech plug to fall with in recommend safety guidelines

[Don Krag]

I'd go with a little of both.....a 1.75" ID chambered plug with some 3/8" banding.

[CaptTHighbiter]

I just found out this is 4140 alloy steel, left over from a Navy job this company didnt get. Already been NDT'ed too.  Nice stuff!  Heat treatable, though I am not sure I want to do that.  Any one with an opinion chime in here, please.

[GGaskill]

I just found out this is 4140 alloy steel ...

Good strong stuff BUT it is essentially not weldable.  So you would have to use a screw in breech closure.  Not all bad.  And a Parrott-like band would be a good idea.  And a band for the trunnions.

[Victor3]

 It is indeed "essentially not weldable" for the average guy in his garage, but there are procedures involving pre/post heating to produce a reliable weld with 4130.

 For critical applications though, NDT is generally advised after welding because it's prone to cracking.