Breach Plug

[frogwalking]

I have been trying to decide how to shorten an oct to round 30 ga. pistol bbl.  I removed the factory installed breach plug.  The threaded portion of the plug, 3/4 X 16 thread, is very nearly 3/4 inch long.  The barrel, on the other hand, is drilled and tapped for a distance of .95 inches.  Fortunately, the threads on both barrel and plug look good.  I understand a minimum of .5 inches of threads are needed for adequate strength.   If this is the case, I can cut nearly 1/4 inch off the plug and almost 1/2 inch off the breach of the barrel.  The barrel threads appear to be cut full depth all the way to the end of the drilled area.  Is this done with a lathe in the factory?   Can a 3/4 x 16 bottom tap be bought if the above observation is not correct?  

[northmn]

I have cut down more than one breech plug and barrel to get the fit you want without a lathe.  I use Prussian blue to determine if I have bottomed with the plug.  I have also used thin brass shims between the tang and barrel to save time on fitting.  They kind of look nice anyway.

DP

[rdillon]

Frog, I have a 3/4 16 bottom tap I believe I got it from MSC.

Hope that helps.

Oh and a permanent marker works too if you don't have Prussian Blue

[frogwalking]

Will inletting black work, or is it too thick?  I plan to hacksaw the plug, file it square, champher the edges where it will meet the end of the tapped barrel and do something similar to the barrel.  I have found that extreme care and patience can sometimes allow one to do what a properly skilled craftsman can, only it takes a lot longer.   I have not fitted a breach plug but can certainly do better than the factory did.

Where is the easiest place to get the shims?  Would MSC have them?  I have bought counter bores from them.

[northmn]

The "shims" I make out of any old scrap of sheet brass laying around.  I soft solder them to the tang and then file to fit.  I have a few sheets of brass I have picked up at Ace Hardware for hobbyists which comes in various legths widths and thicknesses. Once you gt the plug to bottom and have a close fit this works.  I also use a machinists square to make sure the breech plug face is square after cutting it off.  I have also made more than one breech plug by welding a tang to a cut down bolt that has been cut off and cut to fit.  Several of us used to do this when you had to index the old Douglas barrels.

DP

[frogwalking]

Breech plug now has 1/2 inch of threads, and the barrel has been cut back to have just over that in tapped hole.  I have also found the barrel threads are not full depth all the way to the end of the drilled hole.  I just ordered a 3/4 x 16 tap from msn as someone suggested.  No  one here in the middle of nowhere seems to have one, although I did not ask James Rogers if he does.  Oh well, I will have one in a week.

Thanks for all the information.