Barrel Boring

[sparkie]

I have an old shot-out .45 cal that I was considering boring out and makimg a smoothbore/small cal shotgun out of! It has a 7/8" barrel with a 5/8" x 18 breech plug, the dovetail for the rear sight is .070" deep with a soldered on front sight. How far can I bore this barrel out and still be safe?

 

[Bookie]

From a professional business standpoint, I wouldn't take it past 47 caliber. 50 makes them pretty thin walled and you never know when your best bud loads her too heavy at the range....Bookie

 

[Zonie]

IMO, a 20 guage (.615 dia) is out of the question because it would remove all but .010 of the breech plug threads. :grin:

If you want to stay with a common shotgun bore size that takes you down to a 28 guage (.550 dia).

Using Green Mountains barrels as a starting point, they offer a .54 cal rifled barrel with a 5/8-18 thread in a 15/16 barrel.
Your 7/8 barrel is only .032 thinner in wall thickness than the 15/16 barrel and with a .54 cal bore you will have .168 thick wall.
Taking the sight dovetail into account does bring the wall down to about .100 which is getting a bit thin in the breech area.

If you want to be totally safe based on the barrels offered by GM, you could use a .50 cal bore.