I have a 15/16 - 36" GM .50 barrel I need to cut off about 2 inches at the breech and reinstall plug. The barrel is already drilled and tapped for a flint liner and I need to convert it to a percussion hooked breech plug. I have the plug (5/8 X 18) and tap. What size drill bit should be used and can I use an electric drill as my small drill press can not be used. After tapping can I grind the tap down to a bottom tap to finish. I have never drilled and tapped anything of that size. This is to rebarrel an old TC Hawken someone gave me and to use a barrel I already had. Thanks Danny
A twist drill would be my second choice here. I would consider buying a 37/64 or 9/16 reamer and ream the existing hole deeper before cutting the barrel. This will be easier than trying to drill or ream with a hand held drill after cutting the barrel off.
Just do not drill/ream too deep. Length of the thread depth plus 1/32 is good.
Tap drill for 5/8 -18 is 37/64s (.578"). But the 9/16 (.562") will work OK for you and give better thread depth. If you know someone with a lathe it would be better to have it drilled in the lathe with the bore centered before drilling.
in any case I would drill it to just past full depth of the plug.
You want the plug to run right to the end of the bore to avoid fouling traps. This is mostly an inconvenience with BP though it can increase corrosion. If its shot with some "replica" powders, however, it can be very serious and can greatly reduce the service life of the barrel and breech.
You need to tap the hole with a taper tap then grind it to remove the taper somewhat then tap again then grind even shorter and tap again to get the threads full depth as near as possible to the depth of the plug. It needs to be tapped right to the end of the bore, within 1 thread anyway. BUT the tap can damage the end of the drilled hole and create a fouling trap. So try to keep this area as drilled.
When fitting the end of the plug should seat against this cone or flat to seal. It might be necessary to file the front thread back 1/2 to 3/4 turn to get the plug to the end of the hole.
If you drill slightly deeper than the plugs threads you can then remove material from the breech of the barrel and the front thread of the plug to get the threaded portion seated properly as the body of the breech seats against the breech end. Further filling of all contact points will allow the plug to match the flats. I "paint" the contact areas with blue magic marker to find contact points to be removed.
On gaps, unprotected threads etc.
Fouling/oil traps are mostly just a PITA with blackpowder. However, with powders made with potassium perchlorate "fouling traps" are far more serious. This stuff is aggressively corrosive. While BP fouling is largely inert when cutoff from moisture potassium perchlorate is always bad and will continue to eat barrels and breeches under an oil film.
It will produce "crawdad" holes as it burrows into the steel. A friend found this years ago in MLs that had the breeches pulled after long use with this stuff. One was close to "exiting".
This was re-enforced just a few days ago when a European shooter on another ML site reported that the German proof house had told him that these powders were "bad" and this (the deep individual pits) was one result of using perchlorate powders.
I have been taken to task by people on this site who should know better for insisting on fouling tight threads whenever possible. Usually because breeching in this manner costs too much/takes too long and they don't want to bother. But there is a VALID reason for keeping powder fouling out of the crevices and gaps produced by quick, loose tolerance breeching. It is doubly important when the firearm may be used with propellants that produce highly corrosive residue that, all claims to the contrary not withstanding, is difficult to adequately remove. Leaving fouling traps in the form of poorly fitted breeches simply makes it impossible.
The comments on the corrosive propertys of the fouling are based on MY personal experiences, experiments and observations of effects the use of perchlorate powders while working for a major manufacturer of 19th century single shots as gunsmith, gunstocker and all around "tech guy". Add this to the research conducted by a friend who posts here now and again and I can assure you this not "blackpowder elitism" at work. It based on sound, irrefutable, science. The results can be and are, repeated all the time and are readily observable by anyone paying attention.
"But lots of old guns have breeches simply threaded into the bore".
Yes, but:
1. The powder they were using was not a potassium perchlorate based powder.
2. In many cases the tap that made the threads was the same one that made the screw plate or the screw plate made the tap.
Tighter thread fits and relatively non-corrosive fouling make this an apples and oranges comparison when compared to the modern "thread tolerance" found in today's barrels/breeches/taps/dies and powders containing potassium perchlorate.
Dan
