Author Topic: GM Breech Plug Question Please?  (Read 1029 times)

Offline Marcruger

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GM Breech Plug Question Please?
« on: September 26, 2021, 08:19:55 PM »
Hello folks,

I bought from Keith Lisle an older, neat little swamped Green Mountain A-weight .40 caliber 38" barrel.  It measures .946 across the flats at the breech.  I view swamped GM barrels like gold.  Accurate barrels for sure, and sad that they no longer offer swamped barrels (though Kibler is now using GM for .50 and above and swamping). 

Does anyone know what breech plug Green Mountain would have used on this one? 

I looked on their site, and the .40s offered are listed as a 5/8 x 18 currently.  I am not sure if that holds true for the older swamped barrels. 

Also, is GM the best source for a matching plug if the 5/8 x 18 is the correct thread?    I am thinking thread depth comes into play here too, right? 

No real hurry, as I just bought this barrel for a future rifle some day. 

As an aside Keith Lisle does a fantastic job of packaging for shipping, and he is super easy to deal with. 

God Bless,   Marc

Offline snrub47

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Re: GM Breech Plug Question Please?
« Reply #1 on: September 26, 2021, 08:26:23 PM »
5/8x18

Offline flinchrocket

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Re: GM Breech Plug Question Please?
« Reply #2 on: September 26, 2021, 08:56:37 PM »
I would also measure the depth of the threads down to the shoulder and trim it down to 1/2”, if it’s more than that. GM barrels normally have 5/8” deep threads, which will put the touchhole at least 3/4” from the end of the barrel.
« Last Edit: September 26, 2021, 09:02:01 PM by flinchrocket »

Offline smylee grouch

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Re: GM Breech Plug Question Please?
« Reply #3 on: September 26, 2021, 11:19:05 PM »
I think flinchrocket is right about the plug depth. All those I have worked on had a little extra length in the plug. Something you might want to take into account when choosing/laying out your lock position.

Offline Not English

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Re: GM Breech Plug Question Please?
« Reply #4 on: September 27, 2021, 01:49:34 AM »
Marc, We used a lot of GM barrels in the Kentucky rifle class I taught Any 5/8 -18 breech pug will do with a little work. The barrel threads were 5/8" deep as mentioned in earlier posts. Measure thread depth and then mill the breach end of the barrel down to 1/2". You will need a 5/8-18 bottoming tap to finish tapping the threads all the way down to the seat. Most breach plugs are also long. If you mill 3/32" of the end of the breach plug it will mean less work when fitting the breach plug.

Dave

Offline Marcruger

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Re: GM Breech Plug Question Please?
« Reply #5 on: September 27, 2021, 02:24:15 AM »
Many thanks to all for the good advice.  God bless, Marc

Offline heinz

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Re: GM Breech Plug Question Please?
« Reply #6 on: September 27, 2021, 10:50:46 PM »
 Just bear in mind that if you decide to whack a 1/8 inch of the breach of the barrel you also have to shorten the breach plug to 1/2 inch and then re-index the breech plug to align with a barrel flat.  You would, of coure=se be doing the indexing on a non-fitted plug anyway.  An understanding of how the face of the breech plug is going to mate with the shoulder of the breech thread hole is needed to get a fit that does not trap powder fowling.
kind regards, heinz

Offline kutter

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Re: GM Breech Plug Question Please?
« Reply #7 on: September 28, 2021, 07:13:43 PM »
You can leave the breech plug alone especially if it's already fitted nicely in the bbl. If you don't want to have to go thru the process of  not only cutting the bbl, but also cutting the plug to length and then refitting the whole thing so it indexes as well..
you can just mill or file a radius coming off the side of the tang and onto the bbl.
Bring the entire radius cut out to the side flat of the bbl.

That leaves you with a radius or curved transition on the oblique flat on either side of the bbl to the tang and gives the look of moveing the bbl back (or the wood forward depending on how you look at it!)

The flash hole is then closer to the breech end of the side flat of the bbl.,,1/2" +/- .
The lock  sits in a position that looks better having some more wood just behind the back edge of the flash pan and not seeing the bbl flat there.

Here's a pic of one I did on a GM bbl. The breech threads were 5/8 deep. So I milled a radius approx 1/8+ forward with the plug installed in the bbl.
Since the bbl had already been inletted, I then filled the voids in the wood on either side of the radiused bbl with wood plugs and finished them off.
My glue line is starting to show a little. But not bad for 3 yrs of use. Need a new flint!