run-out on Green Mountain barrel?

[dlocke]

Has anyone experienced run-out on a Green Mountain barrel? I have one and am about to fit a breechplug. I noticed that one flat is stamped .50 (for 50 cal, of course). Is it possible that this stamp was put on at the factory on that specific flat to indicate that it should be the top (or bottom) flat?
DJL

 

[Blackfoot]

Those barrels are turned on center so there is no run out on them.

 

[fitter]

That's not true. I got a GM barrel not long ago,
13/16 .40. 42'' long.It has a good 1/8th runout in it,so I would pay attention to what flat you seat the breech plug on.

 

[k.jacobfrost]

Would you mind explaining what run-out is? I don't think I have ever heard the term before.

Thanks
Frost

 

[talon]

About 3 out of every 5 Green Mountain barrels I have seen have run out. Thats why I don't buy them any more.

 

[Joel Boyles]

In simple terms, runout is the amount a hole varies from center over the length of the part. That means that the bore in question deviates from center by X amount over the length of the barrel. It also means that at one end or the other, the bore is not concentric to the outside diameter. Does that make sense? (If so, I may have explained it wrong.)

Enjoy today,

Joel

 

[Birddog6]

Generally if you have runout on a barrel at the breech, they stamp & mark that flat to be the bottom. Runout at the breech is not nearly as criticle as having it in the middle (making the bore arc) or having it at the muzzle. (looks like H)

I had a barrel a couple of years ago in a small bore .36 cal. with runout in the middle, it shot a foot low & a 9" to the left at 25 yards. No way of compensating for that. Manufacturer replaced the barrel at no charge & when I cut the old barrel in two in the middle, it was amazing how bad off center it actually was. Looking thru the bore I could not tell it though. New barrel shot 2" low & 1" left on first outing & was easily adjusted for. Now that same .36 cal rifle is shooting 1 hole 3 shot groups at 50 yards. :thumbsup:

 

[Blackfoot]

That's not true. I got a GM barrel not long ago,
13/16 .40. 42'' long.It has a good 1/8th runout in it,so I would pay attention to what flat you seat the breech plug on.

Just goes to show you that you can't believe every thing that you read...I just read that about the green mountain barrels on a web sight that sells them. Sorry if I was wrong.

 

[mckutzy]

can u bend the barrel to compinsate for the runout?

 

[Birddog6]

Yes it can be bent. How much will it take ? anyones guess. I know a Green Mtn. barrel is pretty tough steel & it's not gonna be easy to do.

Personally I would send it back to them rather than bend the barrel & build a rifle Knowing I started with a major component defective.

 

[CrackStock]

Gee, I am quite lucky since I have 3 of them and none have run out.

Okay, who got the lemons?

CS

 

[Blackfoot]

Ok, I found where I read that the Green Mountain barrels are "Turned on center" and it was on the Track of the Wolf.

 

[Zonie]

The "gun drills" I'm familiar with all work off of centers. The centers are hollow so that the drill passes thru them as the barrel is rotated.

This does not keep the bore from having "runout" relative to the outside of the barrel.
If the centers were not centered relative to the outside of the barrel, the bore will not be parallel with the outside of the barrel, thus, you will see "runout" relative to the outside.

In other words, if someone messed up when they machined the centers, you will have problems.

If the barrel does have "runout" relative to the outside of the barrel, position the barrel so that the runout at the muzzle is pointed "up" (best) or "down" (not as good).
With the barrel positioned this way, the gun will not shoot right or left, just high or low. This is easily corrected by setting the sights to correct for the error.

zonie

 

[CrackStock]

Zonie,

What if you bored the bore into a steel blank; rifled it and then milled the flats while set in centers? This would give you zero runout. I understood that this was how Green Mountain made their barrels. Am I confused?

CS

 

[don getz]

You only get runout when you drill a preshaped octagon blank. This is how the old Douglas barrels were made, and I have heard that Green Mountain does "some" this way. I am almost sure
that none of their swamped barrels have runout
because they must be shaped after the hole is drilled, maybe even reamed and rifled. And Zonie,
you're not quite right on how a deep hole drill works...it does not run on centers. You are correct in that the barrel spins and the drill is
merely fed into it, but it is basically a self piloting drill. The smaller the calibre, the more
tendency it has to wander, or drill off center when it exits the far end of the barrel blank....
Don

 

[Zonie]

The only gun drilling machines I'm familure with were the ones at the company I worked for.
They were used during WW II and my company bought/leased them.
We used them to produce very long hollow shafts out of materials like Inconel 718 so the actual cutting area was made out of carbide.
High pressure cutting oil was forced down thru the center of the hollow tube which the carbide cutter was silver brazed to. This provided cooling, lubrication and washed the chips back out of the hole.

Yes, the drills are self piloting once they are started however they were fed into the shaft thru a hollow center. This started them exactly on the centerline of the two machined centers.
The solid center and the hollow center were engaged with the parts machine centers during the entire drilling process.

zonie

 

[don getz]

Zonie....yes, we're talking about the same thing.
I think it would amaze a lot of people if they could see the drills....wippy, like a fly rod, but
with that carbide tip on it. They are started into the barrel thru a hardened drill bushing, which gives them a straight start into the barrel.
Amazing how they work.....Don

 

[CoyoteJoe]

I believe GM barrels are bored from round stock, the bore is then "straightened" and set up on bore centers to mill the outside, thus there should be no runout. But that depends on how "straight" the bore is after straightening. I've worked with quite a few GM barrels, cut them to various lengths and have never found a bore off center, not to say it can't happen. I don't believe the location of the caliber stamp means anything on a GM barrel.

 

[Gordon Horn]

Coyote Joe, thats exactly what a brochure that I got with my GM barrel several years ago explains. The bore is drilled into a round blank and then placed onto a CNC machine where the flats are milled to a tolerance of one ten thousandth of an inch. Any barrel that is'nt within that tolerance should be sent back MHO. Of course I don't know whats happened since Knight Rifle bought them out.

 

[dlocke]

Well, I took the Starret's to the breech & muzzle today & measured the inner to the outer on each flat, taking care to do it several times at the same distance in (breech) and dealing with the rifling (muzzle). The measurements are within .005" at the breech and .003" at the muzzle. Looks like run-out is not going to be a prob with my barrel. I also test fit the breechplug. Looks like I'll need to file about 1/8" off the breechface before it snugs in. I decided to order me a Pedersoli kentucky flint pistol (.45 cal) so I would have something to work on along with the rifle, as the rifle will hopefully be ready sometime this decade. I figure I can practice my inlaying and carving more on the pistol, and it will be fun to shoot.
DJL