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Login Name Post: Green Mountain chamber area        (Topic#308026)
Sparkitoff 
40 Cal.
Posts: 105
08-10-18 10:11 AM - Post#1697048    


Friends,

I have a GM barrel 1:70 .54 cal. on a T/C Renegade stock. I can see all the way down the barrel and it does not appear to constrict very much. If I stick a pipe cleaner in the flash hole I can just barely see it by looking through the barrel. I'm trying to figure out if I have a reduced breach like the original T/C barrel and others so I can address cleaning properly and diagnose any issues correctly. Does anyone know what the breach is on the GM barrels like I have?

 
Davemuzz 
45 Cal.
Posts: 700
Davemuzz
08-10-18 10:39 AM - Post#1697064    

    In response to Sparkitoff

I have two GM barrels and neither one is a reduced diameter at the breech.

 
Rich Pierce 
70 Cal.
Posts: 4228
08-10-18 11:01 AM - Post#1697070    

    In response to Davemuzz

If that is so, the patent breech must have been made with a powder chamber of bore size. That is very unusual because it requires the threaded portion of the patent breech plug to be quite a bit larger than bore diameter (much more than is usual).

And, a cleaning jag would go past the barrel and rifling and into the patent breech, where it might lodge as it engages the rounded end of the cavity or as it re-engages the rifling on withdrawal.

 
Davemuzz 
45 Cal.
Posts: 700
Davemuzz
08-10-18 12:00 PM - Post#1697075    

    In response to Rich Pierce

Rich, I stated I had two GM (Green Mountain) barrels. The one I'm currently working on I purchased the barrel directly from GM, and the breech plug from TOTW. I had the breech threads finished by my local gunsmith. The breech plug has a flat face plate.

 
Grenadier1758 
58 Cal.
Posts: 2344
Grenadier1758
08-10-18 12:42 PM - Post#1697080    

    In response to Sparkitoff

If you have a T/C breech plug installed, then you probably have a reduced chamber.

Is the touch hole installed in front of the ends of the threads of your plug?

Does your ram rod go past the touch hole or does it stop before the rod reaches the touch hole?

 
Zonie 
Moderator
Posts: 26661
Zonie
08-10-18 12:48 PM - Post#1697082    

    In response to Sparkitoff

You didn't say if your Green Mountain barrel was the type used by many of us on custom built rifles or if it is a Green Mountain "drop in", replacement barrel made for Thompson Center guns.

The first type of barrels come from the factory unbreeched and the builder installs the breech plug of his choice.
This is most often a simple, flat faced plug with a tang on it so it will not have any "chamber" or "patent breech" constriction.

The second type, the one made as a TC drop-in uses a breech plug similar to the original TC.
It will have a "chamber" which is smaller than the bore size.

Because the basic Green Mountain barrels come unbreeched, the builder can decide to use either the plain, flat faced breech plug or he can buy one of the snail type of breech plugs that usually have a powder chamber in it.
This allows him to build the typical early longrifle or a later style half stock like one of the original Hawken guns.

If you haven't seen it before, here is a drawing I made showing the various styles of breech plugs used on muzzleloading guns.

PERCUSSION
DRUM2 by James, on Flickr


FLINTLOCK
NOCKS-BREECH by James, on Flickr
Just Jim...



 
SDSmlf 
40 Cal.
Posts: 267
08-10-18 12:59 PM - Post#1697087    

    In response to Sparkitoff

Not flint barrels, but the three Green Mountain barrels (two 45 and one 50) with hooked breeches for TCs that I own all have reduced IDs at the breech. Would think Green Mountain would have used the same breech system for all the ‘drop in’ barrels they made for TCs. Easy enough to check. Run a ramrod with bore size jag on it down the barrel. If it stops before the touch hole you have a diameter reduction. If you want an idea of the diameter try different sized jags (like you find in a set for cleaning c3nt3r fire rifles), the actual diameter will be between the one that fits and the one that doesn’t. Likely around 3/8”.

 
Sparkitoff 
40 Cal.
Posts: 105
08-10-18 03:34 PM - Post#1697106    

    In response to SDSmlf

Using my ramrod with 54 jag to measure it goes past the flash hole.

 
Zonie 
Moderator
Posts: 26661
Zonie
08-10-18 04:26 PM - Post#1697115    

    In response to Sparkitoff

That would indicate your gun has what I called a "Traditional" breech plug.

As for the amount of a pipe cleaner you might see protruding thru the inside of the vent liner, that can be a different issue.

When the vent is located most builders try to locate it so it is close to the front face of the breech plug. For instance, if I am using a vent with a 1/4" external thread on it I will try to locate the center of it 5/32" (.156) to 3/16" (.187) in front of the breech plug face.
Doing that, the threads of the tap will never hit the face of the breech plug.

In this position, the vent hole will always be well clear of the breech plug so a pipe cleaner will easily pass thru it and be seen inside the bore.

If a builder makes the mistake of drilling the vent hole or vent liner threads so that the center of the hole is under or behind the face of the breech plug, something will need to be done to connect the vent with the bore.

One method is to file off a part of the face of the breech plug so that when the plug is installed the resulting chamfer leaves the vent hole exposed.

Pedersoli's Queen Anne pistol ran the vent hole across the breech face as is shown in the left drawing below. (It says "modified" because I enlarged the existing hole).
Queen Anne Breech by James, on Flickr

Both of these methods can work but they usually end up in causing poor reliability because of mis-fires. That's why I modified my Queen Anne to greatly enlarge the groove in the breech face by installing a home made vent liner.
(Note: The Queen Anne does not have a removable breech plug. The bore just stops short of breaking thru the back wall of the barrel.)

If you couldn't get a pipe cleaner thru the vent so it could easily be seen, I suspect your gun's vent hole is located too close to the breech plug face.
Just Jim...



 
SDSmlf 
40 Cal.
Posts: 267
08-10-18 07:03 PM - Post#1697142    

    In response to Sparkitoff

If the full caliber jag goes past the touch hole I would not think the touchhloe liner threads are into the breech face. Reverse how you are looking at this. Run the jag down the barrel. With touchhole liner removed you should able to see the jag through the touchhole and confirm a clear path.

 
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