GM drop-in barrel safety question

[Ken Rummer]

I have a GM 58 cal barrel that has been abused. The hook on the breech plug has had metal removed.; Looks like a rough tool was used like a chisel or a cutoff wheel, definately not a file. The underlug was also beat up. Since it won't fit well in any of my 1" TC stocks I thought if I had the breech plug and underlug replaced it may be usable.

I will not touch the breech plug but I am qualified to remove the underlug and dress up the dings with a file. This underlug is attached with two screws which I removed.

I measured the depth of both screw holes with the depth mic feature of my Frankford Arsenal vernier caliper. The measured from .150 to .160 depending on the rotation of the depth mic shaft.

The screw thread dia. is .133. Holes are 6.75 and 7.62 from the breech plug/ barrel joint.

The barrel flats are 1"
The groove diameter is .602 measured at the muzzle.
Subtracting and dividing by 2 the barrel wall is .199 thick.
For a .160 deep screw hole the wall thickness at the screw location is .039.

Thread engagement is .072 so the holes are twice as deep as they need to be.

This seems very wrong.

I would appreciate your comments.

 

[ebiggs1]

Man there is no way I would give you a thumbs up, or down for than matter, from just a web site post.
I doubt anybody else will either. You need to have someone actually hands on check it out, if you have concerns.

Better safe that sorry if you truly have doubts. :thumbsup:

Good luck.

 

[Ken Rummer]

I am looking for general guidelines from people who actually drill holes in barrels. How much wall thickness should be left intact? Does it vary with distance from the breech?

Green mountain sells barrels that are like a piece of smooth pipe, no cuts or holes. The builder then cuts dovetails and drills & tap holes as needed. I would think there are some standards for how deep to go. I would not think each builder would have to learn this on his own.
Where could I go to learn this stuff?

I apperciate your caution. I will never shoot this barrel. This .039 wall would have me flinching every time I pulled the trigger. Too much stress.

 

[Zonie]

My standard for drilled holes in the wall of a barrel is that they should not break thru.

As long as some barrel wall exists between the bottom of the drilled hole and the bore/rifling the barrel will be safe for use as a muzzleloading barrel.

When one stops and thinks about it for a moment they will remember that the vent or touch hole passes completely thru the wall and it is located right in the area where the greatest pressures occur.

With a vent liner installed rather than a plain drilled touch hole, an even larger amount of the barrels wall material is removed and still, these are safe to shoot.

In the case of a side mounted drum for a percussion fired gun usually the threaded hole is even larger than most vent liners and it is still safe to shoot.

The only reason my (self made) guideline says the hole must not break thru is because if it does it will create a feature that will rip patches and will also be impossible to clean.

From a stress standpoint the corners at the bottom of a drilled hole which could be considered as stress risers are uniform and continuous forming a closed loop. They are also usually rather small so a build up of high stress that could cause a failure of the adjacent material is highly unlikely.

A dovetail on the other hand is unsupported at the ends and usually has a sharp corner which is a stress riser.

When I ran a stress analysis on the area around and behind the muzzle on the barrel in the picture below I found that a dovetails stressed corners could create stresses that could exceed the barrels yield strength.
For that reason, I soldered the front sight in place.

 

[TennesseeJed]

Were the underlug holes any deeper than the front sight holes? I have a GM drop in .58 and have never had any problems at all out of it after initial fitting it to the stock.

I suppose you could have the "questionable" area of the barrel cut off and the undrilled portion rebreeched with an underlug replaced to a depth of your choosing. The .58 is a really fun gun to shoot.

 

[roundball]

I owned several GM drop-in barrels for the T/C Hawken... .40cal, some .54cal smooth bores, a .58cal, some .62cal smooth bores...and they all had the wedge pin under lug attached with screws.
Unless someone has tampered with your under lug screw holes (and I can't imagine why anyone would), I can't imagine they would be a problem.

PS:
A couple of the under lugs on some GM barrels I had would hang down into the ramrod channel so far they'd interfere with the ramrod sliding in.
The good news was that the thickness of the under lug metal was so great there was plenty of material there so filing off .020"-.030" to provide the needed clearance for the ramrods to pass was a simple fix.

 

[Ken Rummer]

I pulled the underlug off a GM 62 SB (.610 bore)and measured the screw depth at .125 so it may be the case that GM adjusts the screw hole depth depending on caliber. Min wall on the .610 is .070 with .125 deep holes

I bought this barrel to rework to a .620 smoothie.
At .620 with the current screw hole depth the min wall would then be .030. Zonie may be right that it may be OK but I have decided to cancel the project. I would have to spend $65+ to have GM install a new breech plug and $60+ to have Bob Hoyt do the reboring.

I think others have done this conversion without incident but I would still have that lingering feeling.

Thanks to all for the feedback. It has been very helpful.

 

[roundball]

I bought this barrel to rework to a .620 smoothie.
At .620 with the current screw hole depth the min wall would then be .030. Zonie may be right that it may be OK but I have decided to cancel the project. I would have to spend $65+ to have GM install a new breech plug and $60+ to have Bob Hoyt do the reboring.
I think others have done this conversion without incident but I would still have that lingering feeling.

Not trying to belabor this but I had one of my GM .62cal drop-ins 'rifled' which cut .012" grooves on both sides of their .610" bore so to speak and there were never any safety issues raised by the barrel / machine shop.

 

[BrianL]

If you want to see something downright scary, google 'minimum shotgun barrel wall'. You will find that .018-.025" is considered 'Proof' at 9 inches from the breech. That is the thickness of your patch material, uncompressed.

Keep in mind that you have the screws in the holes of the tenon and this is belt and suspenders more safety factor than the clean out screw that is directly connected to the combustion chamber.