Fitting plug to new GM barrel

[Dave Young]

I am getting ready to fit the plug to my new Green Mountain barrel for a Vincent rifle. Does it matter, now days, which barrel flat goes up? I read somewhere that with barrel run out, they used to mark the top flat with the caliber and barrel maker. Is that still true? The barrel has one flat that has, 45 caliber with the GM logo on it.

 

[jtmattison]

Might still be true. If you don't like the stamping you can always draw file it out.

HD

 

[Otter]

IIRC, GM mills the flats after the barrel is bored and rifled, indexing and rotating 1/8 turn to mill each flat. The bore should be centered so close, I doubt you could find any run-out. I know I tried on mine and found none. It will be more important to get the breech plug fitted properly than to have the factory stamping show (or not show). YMMV

 

[fitter]

Don't depend on it not to have any run out. Most have some. Generally the mark goes on the bottom,you want the runout to be vertical rather than horizontal so you don't have to move your sights for windage so much. You have to check it and see.

 

[Dave Young]

How do I go about checking for run out?

 

[Otter]

One way to do it is to mark the same flat on each end of the un-breeched barrel and carefully measure (with a dial caliper or micrometer) from bore to center of each flat on each end of the barrel and writing down the measured dimensions. Make sure you have the same flat measured to the corresponding flat on each end (measure clockwise on one end and counter-clocwise on the other end of the barrel). If the dimensions are substantially different, you have some runout. If the dimensions directly opposite from each other vary much, you want to make sure that flat is either the top or the bottom when the barrel is fitted to the breech plug and the stock - with the larger dimension on top, the rifle will probably shoot low, with the smaller dimension on top, it will probably shoot high. What is a lot of varience? You have to decide that for yourself, or somebody else here will chime in and give a number.

I still bet it will make no difference with your GM barrel . . .

 

[ZUG]

Put the logo on the top if you can't remember the caliber or put in on the bottom if you can remember the caliber :thumbsup:. And- yes their is some slight amount of runout sometimes not enough to make a difference :v .

 

[Goldhunter]

I've got a Green Mtn in my .54. I did the look through test etc and didn't appear to have any run out (that my eyes could tell). I put the stamp down, so the barrel is still stamped but not visible. It shoots straighter than I do.

 

[Zonie]

When folks talk about "barrel runout" and it being an important issue many of them are thinking of some of the old barrels made by Numrich, Douglas and a few others that were very bad.
I'm talking 1/8 inch or more.
There was a post a while back that showed one of these old barrels and try as I might, I can't find it. Anyway, the bore was off center by over 3/16 of an inch as near as I could eyeball the picture.
Now, THAT'S runout!
That's also enough to make it important to install the barrel with the error to the top or the bottom.
That barrel by the way was so far off that I suggested that the it be installed with the thick side at the breech "up" simply because cutting the rear sight dovetail could have seriously weakened the barrel.

Anyway, all of the currently made barrels that I've seen (Green Mountain, Coleran, Rice) are well made and the runout error cannot be seen with the naked eye. Barrels that are that good can be installed which ever way the builder wants to install them without affecting the shooting of the gun.

 

[fitter]

Bought a GM barrel 2 yrs ago. Cut off 10" from the breech to make it 32" and rethreaded it for the breech plug. Any way it had over an 1/8" of run out. I've seen several like this.So check it.15 years ago I used sme GM barrels and they were almost dead nuts centered so who knows.Maybe their QC is not what it once was.