Barrel wall thickness runout

[ar15barrels]

We have talked about this before, mostly when talking about why some barrels will have a zero that changes as the barrel heats up.
I just happenned to notice the runout on this barrel while I was shortening and threading it yesterday so I stopped for a couple minutes to take a video.

[pklin1297]

Crazy...

I'm sure there'll be some barrels like this from almost every manufacturer, but whose blank (I'm assuming you're machining a blank) is this from?

[Hateca]

Crazy...

I'm sure there'll be some barrels like this from almost every manufacturer, but whose blank (I'm assuming you're machining a blank) is this from?

Every barrel has run-out. This is why you index a barrel in the lathe off the bore not the outside. The outside could be square for all I care, one needs to pay attention to the bore. Run-out will change depending on where the barrel is cut and where the measurement is taken in the bore.

In 25 years I have never seen a perfectly straight bore.

[ar15barrels]

whose blank (I'm assuming you're machining a blank) is this from?

This was a factory Remington 5R barrel being shortened and threaded and a brake fitted.

[kcstott]

This was a factory Remington 5R barrel being shortened and threaded and a brake fitted.

It's not saying much nor is it unexpected but I chopped a AK or RPK barrel once.. you didn't need an indicator to see the run out in the barrel and bore. you could have used a tape measure. It was about .05" off center

[CRTguns]

that runnout is likely much worse the closer you get to the middle of the barrel.

[ar15barrels]

that runnout is likely much worse the closer you get to the middle of the barrel.

It usually is.

[grymster]

Even turning the OD while centered on each end of the bore does not always guaranty uniform wall thickness. I've seen some serious "walking" in gundrilled holes, leaving a bore that's not so straight.

[ar15barrels]

Even turning the OD while centered on each end of the bore does not always guaranty uniform wall thickness. I've seen some serious "walking" in gundrilled holes, leaving a bore that's not so straight.

Exactly.
The way I explain it is that you can think of the bore like a jump rope.
If the barrel is contoured straight to both ends, but the middle is curved and you spin the barrel, the curved hole is moving around within the steel cylinder like a jump rope...
When you CUT the barrel in the middle of that curve, you get the results that this thread is showing.

[sonofeugene]

I can certainly imagine that having more mass on one side of the bore than the other would result in the barrel curving as it heated up. The problem is complicated, though. Assuming you could center the bore in a lathe, indexing at either end, you could then turn/grind down any eccentricities on the OD along the whole length. However, unless the bore is perfectly straight, this wouldn't accomplish much. If the bore wandered off axis along the length of the barrel, you'd still have areas of uneven mass around the barrel.

Another option might be to turn/grind the OD of the barrel so that the wall thickness is perfectly even all around the bore at all points along the length of the barrel. But to do this, you'd need some fancy capability where you could know where the bore is going off axis and in what direction and by how much. You'd end up with the OD looking somewhat strange, but with even wall thickness, perhaps it might actually shoot better.

Things to think about.

[grymster]

Yes, jump rope is a good way to describe it.

I've made a number straightness gages for various bores, but not for rifle bores. I suppose someone sells them. I would think a laser setup could be made to work very well also.

[ar15barrels]

Another option might be to turn/grind the OD of the barrel so that the wall thickness is perfectly even all around the bore at all points along the length of the barrel. But to do this, you'd need some fancy capability where you could know where the bore is going off axis and in what direction and by how much. You'd end up with the OD looking somewhat strange, but with even wall thickness, perhaps it might actually shoot better.

It's easier/cheaper to just use a higher quality barrel which is straighter.
Bore curvature is MUCH less of an issue with premium quality barrels.
It's still there, but much less, so that it's not problematic.

As a quantified example, I cut 6" off of a BRUX barrel blank the other day.
After the bore was all dialed-in to near-zero runout, the OD of the barrel was showing 0.002" total runout.

[kendog4570]

As a quantified example, I cut 6" off of a BRUX barrel blank the other day.
After the bore was all dialed-in to near-zero runout, the OD of the barrel was showing 0.002" total runout.

This is the rule, rather than the exception, with all barrels.

[kcstott]

many different way to spot a curved bore, in the end if it's not to your liking send it back, a reputable barrel make will take back a barrel you don't like. so long as you don't cut on it.

After that few will take it back for straightness issues.

Back in the day the Germans would straighten a barrel twice, Once as a blank then once after it was profiled.

Nice fixtures for doing the straightening all shining up at a light source to pick up the reflection.

In any even you straighten to bore as best you can then machine the OD to the bore. between centers. this still will not produce a perfectly concentric bore to OD relationship, but it's a damn sight better then just leaving it alone.

[ar15barrels]

This is the rule, rather than the exception, with all barrels.

Not factory barrels.