Lathe a heavy barrel to lighten?

[entoptics]

Not exactly a "long range" question, though the principles should apply.

I have an old AR15 barrel, Rock River Arms, 16" HBAR profile. She was a dandy shooter 5000 rounds ago, and still holds 1.5 MOA "all day long" with good stuff.

I'm planning to build a "run-n-gun", light-weight, short-range, beater AR, with the rest of the rifle, and the HBAR was going onto my "barrel wall of shame", because it weighs about 300 lbs and doesn't owe me anything.

Anyway, my question...Any thoughts on just turning the barrel down on a lathe myself, to make a light weight option for my beater? She shoots as good as any 1.75 lb "cheap" barrel I'd buy for $125. If pealing a bunch of metal off would save 8 oz and keep me at 3 MOA with cheap ammo, it would be money well NOT spent...

Anyone tried this sorta project before? I've got large lathe access/experience for free. And I like tinkering...

 

[birdiemc]

What's the worst that could happen? You trash a barrel that you were going to trash anyway...

 

[entoptics]

What's the worst that could happen? You trash a barrel that you were going to trash anyway...

Sorta my thought. Was hoping for tips/tricks/advice that might keep me from actually achieving "worst that could happen"

 

[2 the Plug]

sounds fun, can't wait to hear the result.

 

[biggen]

Main thing I would watch is, don't create any hot spots while turning it down.
Heat=stress=distortion

 

[entoptics]

Main thing I would watch is, don't create any hot spots while turning it down.
Heat=stress=distortion

Having not worked on anything this big, made of this type of steel, I'm hoping to get some feedback on how much of a bite I can take, how fast to go, etc.

I'm sure I could turn it down to a pencil and keep it cool, but that could take 8 hours and 6000 feet of tiny spirals, which I'm assuming isn't necessary...

Also wondering just how much is wise to shed.

The barrel isn't really "trash", but it's too heavy for my intended use, so I if this works, it would probably save me $150 on a replacement. And it would be fun to try if it's not too intense and/or likely to fail.

 

[AZShooter]

If you could run coolant over the barrel as you machined it would help. Not all lathes are setup for coolant. Cutting oil will help but it will smoke.

As for a way to get close to correct cutting rate:

CS X 4 divided by diameter of metal

CS for steel is approx. 100
CS for alum is approx. 300

Example:

¾" dia meter steel rod:

100 X 4 = 400 Divided by .75 = 533 RPM for facing or turning a diameter.

Naturally you can still screw things up by making too deep a cut or too fast a feed rate. If you notice brown or blue chips coming off tool bit, especially a high speed tool bit, you will have to reduce either depth of cut or feed rate.

There are other things that will make it interesting to do. If barrel is supported by centers how do you remove metal where lathe dog clamps on? Or if done between jaws of chuck in headstock and a live center at tailstock, I recommend you put a shoulder on that end so metal won't walk into headstock as you cut. At some point will have to reverse barrel to reduce diameter of barrel that was in headstock.

One thing that may show up in middle of barrel is some flex and resultant harmonics. It will make an interesting pattern in the metal instead of a smooth cut. Your barrel may be short enough to keep this from happening.

I turned down a #7 profile to approx. #3 profile, 24" barrel and had to sand the devil out of it afterwards to take out harmonic cutting pattern.

======

My gunsmith has a special machine that is setup for reducing barrel diameters. It is a grinding machine that follows a pattern. It has coolant pumped through bore as it is operated.

 

[cohunt]

If you've never profiled a barrel before, your eyes will be opened on just how long it will actually take to do it.

 

[shortgrass]

A 16" AR barrel? Do 1/2 at a time, then you only have 8" sticking out of the chuck at a time. If you chuck it up on the barrel extension and leave all 16" outside of the chuck, you'll need to install/use the steady rest to keep from inducing chatter. You are planning on using a live center, right? What dia. gas block journal? If you are going to reduce that dia., chuck-up there first and turn the 'big' end first. When you get the 'big' end where you want it, turn the barrel around, and turn the muzzle end and gas block journal. Will you be using HS or carbide turning tools? If using a HS turning tool on a chrome/moly barrel, I'd probably calculate RPM from SFM @ 75. .005/rev shouldn't be too fast of feed @ .020" to .025" depth of cut. Feed rate and depth of cut will be determined by how ridged, how much mass, the lathe you are using has. Barrel work, be it profiling or whatever should always be done at reduced speeds & feeds to keep the heat and possible induced stress to a min. HS would be preferable over carbide. HS cuts by sharpness of the tool, modern carbides 'cut' by tool pressure. The barrel makers can contour fast, because they are using CNCs and have special, pneumatic follow rests to keep the work from 'springing away' from the tool, and they are working under flood coolant. It'll take awhile on a manual lathe, but it can be done.

 

[entoptics]

Great info all. For reference, it's a 16" HBAR, chrome moly.

Haven't pulled it off the rifle yet, so can't measure under the hand guard, but the barrel is 0.74 past the block, and eyeballing it, around 0.9" up to the block. No significant taper I can see.

I'm thinking I'll leave the gas block at 0.75" and turn the back half to ~0.77", to leave enough shoulder for indexing the block. I was thinking of taking the front down as much as "feasible" to lighten the front of the gun. Perhaps 0.65"?

By my calculations, going from 0.9 to 0.77 on the rear 7" of HBAR and from 0.74" to 0.65" on the front 5" will remove about 7-8 ounces of weight. If things work well, I might try to taper from the gas black back, and shave it down to about 0.70" or so in the middle 5" between chamber and gas block. That's still fatter than a government profile I think.

The lathe I have access to is an old Monarch. It's a big, old-school, hoss of a thing from the 1960s most likely. It's pretty solid and true in my experience, so I think it will suffice. There's not a ton of "accessories", so I'll likely use a 6 jaw headstock and a live center on the end. Not sure what cutters are available, but I'm pretty sure they're HS steel. No auto coolant, just me and a squirter bottle...

Clamping around the gas block and working both directions is a great idea, and should keep wobble to a minimum. Not sure how the live center will interface with the barrel extension though...

 

[shortgrass]

Clamping around the gas block and working both directions is a great idea, and should keep wobble to a minimum. Not sure how the live center will interface with the barrel extension though...

In other words, you're not a machinist. This work needs to be done in an independent 4 jaw chuck or a "set-true' style chuck. If the gas block journal is .75" and you want to leave it that diameter use aluminum 'pads' between the jaws and the barrel so you don't mar the gas block journal. Any chatter induced stress into the steel.

 

[KyCarl]

How are you going to stress relieve it after you turn it?You need to "relax" the stresses caused by machining.Then straighten it. It might work?

 

[xsn10s]

I'd probably just sell the barrel and get one that is profiled to your liking.
https://www.brownells.com/rifle-par.../ar-15-m16-5-56-pencil-barrels-prod79356.aspx

 

[entoptics]

How are you going to stress relieve it after you turn it?...

Couple of beers and a hot bath?

Seriously though, what steps would be required to "stress relieve"?

In other words, you're not a machinist. This work needs to be done in an independent 4 jaw chuck or a "set-true' style chuck...

Definitely not a machinist. I've done a number of small parts for various work/personal projects, but nothing serious.

The chuck that's normally on the machine is similar to the images I get by googling "independent 4 jaw", but I think there's six, and all the jaws move at once. I'm pretty sure they can be adjusted independently to ensure center, though. There might be a 4 jaw independent one in the accessories though. I'll investigate. I'd planned to use a dial indicator to check runout before I gave it a try regardless.

I'd probably just sell the barrel and get one that is profiled to your liking.
https://www.brownells.com/rifle-par.../ar-15-m16-5-56-pencil-barrels-prod79356.aspx

Something along those lines is my fallback option. I don't think I could sell my barrel for much, as it's been duracoated and used hard.