Short breech plug?

[PIKELAKE]

Many years ago before I got religion, in my attempt to breech a barrel, the breech plug ended a little short. I realize that it should be at least 1/2" long and it isn't. The plug now mates perfectly with the shoulder of the barrel but the threaded portion is only about 7/16" long or 5 full threads.There is enough room in the barrel to cut a couple of threads in front of the touch hole liner and then replace the plug. What does everyone think? The plug is a 3/4" by 16 thread plug and about 5 full threads,( 54 cal ) and if it is suggested that more threads should be cut and a longer plug be installed, I'll have to send it out. I would only load about 65 grains of powder at the most as it stands now. I have been reviewing my work from 30 plus years ago looking for things that might not be quite right. Thanks to all.....JZ
 Out of curiosity .....do other folks review their earlier works ?

[rich pierce]

I’ve pulled the breechplugs on old builds from the 70s and 80s to see how things are down there.

I can see how maybe we might differ from standards on guns that are ours, figuring we know risk and how to mitigate that risk. Like your modest loads. I guess the fact is, someday, somebody else is likely to be shooting our guns. I try to keep that in mind.

[Mike Brooks]

Many original plugs I have seen were 3/8" long and very course threaded.

[rich pierce]

I’ve got one original barrel that can be unbreeched using fingers only. Probably won’t shoot it.

[jerrywh]

 Have friends that are engineers. I have consulted all of them on this. The answer is this. Three full threads is as strong as it can be. Besides, because of the difference in area, the barrel is about three times as likely to fail before the breach plug will. If your threads are good I would not change it.

[Bob Roller]

The last time I devoted more than 10 seconds of thought to this subject,
it occurred to me that the patches ball moves forward easier than the threaded plug can move backward..
I did unbreech a new made "Harpers Ferry"import years ago that had stubby threads and poor fit as well.
Bob Roller

[D. Taylor Sapergia]

I have a pair of Staudenhauer 15 gauge barrels whose plugs are exactly 3/8" long, and no shoulder for them to seal the bore.  They still have case colours on them yet the barrels have seen use during their life both in flint and percussion eras.  Don't touch your plug.  It's fine.

[Dphariss]

If you want to get really anal find the actual thread length of a 3/4-16 nut. Then figure the tensile of the 3/4-16 mild steel bolt. Then calculate the breech thrust of a 54. This would be the groove dimension of the bore in square inches x the pressure generated by a proof load. Say 25000 psi.
Here is a hint, the area effected by the gas pressure of the properly installed plug in a .54 is less than .264 square inches. So IF we can make 25000 (unlikely) then the pressure on the breech is 25000 X .264 is 6,600.  This is about 1/3 the proof load for a 3/4-16 grade 2 bolt and nut since the NUT has maybe 2 more threads (remembering that the nuts are rebated at each end) than the breech mentioned the breech at 7/16" . So if we loose a significant strength with the shorter thread length, say 1/3, we still have DOUBLE the breech thrust of a very heavy loaded 54.  This is why its impossible to blow a properly  threaded breech from barrel unless VERY, VERY short in the threads. OR the barrel bulges.
HOWEVER, there is no reason to make a breech with less than 1/2" of threads IMO. Though I have done it a time or two and one was REALLY tested and held fine.

Dan

[PIKELAKE]

Thanks to everyone for their opinion and insight.  This was one of the first rifles that I ever built, some 40 plus years ago. At the time, there just wasn't the info available  like there is now. I think that I'll just load it up and shoot it. It has never been fired; don't ask me why. Thanks again........JZ

[Darkhorse]

Have friends that are engineers. I have consulted all of them on this. The answer is this. Three full threads is as strong as it can be. Besides, because of the difference in area, the barrel is about three times as likely to fail before the breach plug will. If your threads are good I would not change it.

I agree with Jerry. During my career with the DOD I was a toolmaker for some 20 years and built many fixtures to manufacture structural components on NC Machines. Due to space constraints threaded holes could not always be threaded as deep as we would prefer. It was accepted that 3 full threads gave full strength to that threaded hole. Most of these holes were used for clamps to hold down the part being machined.
I personally would prefer my breech plugs to be a little longer. But the fit of the threads must be considered also. A good fit is stronger. A sloppy fit is weaker.