Turning down an AR15 barrel profile, Machinists advice needed

Suchee

Suchee

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I have a "master" machinist at my disposal, 25 years on a lathe, awesome friend of mine. Only thing is, he's not into or even fired a firearm, and I have never taken metal shop let alone seen a lathe. He wants to help me out and is up to re-profiling it on his lathe. He works with 4140 all day long, mine is in 4150 cmv, how much harder on his bits is that? How long out of his weekend will this take? Special areas of concern, such as crown... cause he wont know to take any special care



i'm using ar15barrels.com/making.shtml as a semi tutorial for him. But is there any pitfalls to be aware of? I'm leaving the gas block at .750 cause i simply want to use a cut down FSB and take advantage of the pins... but if im doing it anyway... maybe 625 is the way to go?

I'm thinking .650 or .700 under handguards and .550 front of gas. I REALLY like the A1 profile, but Im leaning to 700/625/550. My understanding is the muzzle end of barrel can be .550 as rapid fire heat doesnt effect POI there?

My goal is LIGHT without giving up too much accuracy with rapid fire drills and competitions, tacrifle and the like... Ha im lucky to make 4 a year, so whatever on that point. But it would be nice to know it can be competitive through 5 quick stages if i ever sold it. I did a search and most 3 gun guys do a straight .750, thats 29 oz on 16" ....is that the "SWEET SPOT profile", diameter:heat(roundsxtime)=POI shift? What's real world data on that. The mil did choose .645 for FA bursts... but as we know the mil doesnt always make any sense...A2

Mine is at 35oz on 16", ideally id like it in the 22-26 range.

Please any gunsmith/machinist's out there that can give some gun specific tips to pass on to him is much appreciated





THIS LOOKS FUN!
 
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Really as long as you don't go too thin and you leave the functional spots (gas block area) normal I don't really see any problem playing around. I couldn't really tell you any more.
Would be cool to fin the barrel like a Thompson barrel though ;)
 
THe only reason im not touching the gas block area(journal?) is im cheap and want to cut down my fsb and use the pins.
Whats involved with that area? I'd love to have it at .625. A1 profile is so ###y
 
Just be sure that when he puts it in the lathe he protects the crown, I watched my machinist buddy do some barrel work on the lathe and he made a brass fitting for the tapered end on the bearing that holds the end opposite of the chuck.
Other than that I have pretty much zero experience with turning down barrels. Just understand that the barrel harmonic is going to change completely and you may find that the ammo it used to like won't group at all after the barrel is turned down. You never know what is going to happen with the accuracy till after you do the job and unfortunately you can't undo it. You may get lucky though and notice very little change. It's a crapshoot.
Once you turn the barrel down it's time to start thinking about coatings. Are you going to have it nitrided? cerekote'd? parkerized? spray painted? You'll want to do something with it and that will be another cost to consider.
Another option would be to sell your barrel as is and buy a lighter profile barrel instead of messing with that one.
PM Suputin if you want some pointers from someone experienced, he has made three barrels for me and they all turned out great.

Good luck. I hope it turns out well.
 
Tell him to use a live centre at the muzzle and a steady rest when making the longer cuts over thin portions or he can get chatter. He will know what that means.

Do not change the chamber area profile. Ahead of the chamber, no worries - slim to any acceptable diameter. I would not go thinner than what any real Colt barrels have been thinned to from the factory. You can slim the journal provided you have a FSB of the right diameter. But realistically, just leave it at .750" if you have a .750" FSB and cut rifle up to it with a shallow radius at the bottom of the cut. You can even thin between the two FSB rings so it will look like a narrower profile if you want.

You won't lose much accuracy by thinning it, what you will lose is the number of shots before the barrel heats up and POI changes.

No worries on tooling - HSS will cut AR barrels just fine.
 
Rather than use a steady, I prefer to support the end of the barrel on the steady, then indicate the gas block area in the four jawed chuck. Do half the barrel and then reverse for the other half. There is never more than half the barrel between the chuck and center. Leave a shoulder for the gas block to butt up against.
 
I wouldn't go thinner than A1 personally.
It's a tried and true/proven thickness.
Trust me with a barrel that short and going to A1 profile of .645 you will essentially make the rifle feel weightless.
I actually had a full A1 20" barrel I built into a A1 clone. The full sized solid stock rifle with carry handle irons weighed in at 6 lbs!
You could hold it on target standing with one hand with ease.
The A2 profile is the most useless thing to ever come down the AR15 pipe IMO. You have pencil thickness at 3/4 of the length where the barrel heats up the most and the fastest closer to the chamber, then for no reason whatsoever it gets stupid thick at the very end of the barrel making it very front heavy at the furthest point from the shooter and adding much more weight at the end of a heating up piece of thin metal. It never made any logical sense.

As far as accuracy goes I found the 20" A1 profile excellent for accuracy. My groups would open up from 10 shots at 100 meters in about a 3-4" circle (iron sights here) to 6" ish if I really started firing fast and heating it up. Not a crazy loss of consistency due to heat etc Would be considerably less with an optic. That might have even been because I was just shooting faster though not even due to heat and barrel profile.
With a shorter barrel like that you aren't going to be shooting out to 350 meters anyhow so I wouldn't worry about losing accuracy at 200 meter and less distances at all with a light profile barrel.
You'll never go back to the big beefy profiles again. They ruin all that is good about the AR15 design. Keep it light! :rockOn:
 
I have crowned barrels but never attempted to turn the diameter of one down as you propose. I would do some research to find out what may happen if you do the proposed work on a heat treated post process barrel. Again I am not certain but it may be possible that you could end up with a barrel that ain't straight. It's no big deal to machine metal but it can be tricky to have it hold the tolerances that you require.

Check out this thread fro elsewhere: http://benchrest.com/archive/index.php/t-81113.html seems to have some good info from guys who do this.

As an aside I have to say that I'm not a fan of these light barrels. On several occasions during matches we have watched guys rest light barrels on various supports and the barrels have deflected enough to really put accuracy out. One instance in particular was a real eye opener.

Regardless, good luck and let us know how it turns out.
 
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thank you gentlemen, thats excellent info.

As far as profiles go, Ive been doing searches for 3 gun profiles just for a taste of what they use. Consensus seems to be mid weight .750 for the whole barrel, 29ish oz.

How about a compromise of A1 and midweight, .700/.750/.550 ?
Would that make "that much" of a difference in SLOWING heating up having .700 instead of .645?
and as I understand it there is not much heat up at the muzzle end to effect POI change thus a .550?
 
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Suchee said:
... How about a compromise of A1 and midweight, .700/.750/.550 ?
Click to expand...

That sounds like a good idea to me.

You might try posting this question in the Gunsmithing forum.

Tiriaq's idea about clamping on the FSB seat and then flipping the barrel, is an excellent one.
 
You want to keep the FSB .750 but want a lightweight profile? That's easy, go for a C8 profile. Do a search, you should be able to find the measurements, care of Craftsman411.
 
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