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Old January 12, 2012, 12:50 AM   #1
Nickel Plated
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Help drilling out an AR barrel.

Hi, I have a project I'm working on, a custom airsoft M727 build trying to use as many real AR components as possible.

I want to use a 14.5" DPMS barrel for the gun, however the diameter of the bore needs to be about 9mm for me to run the inner barrel through. Since having a nice finish in the bore is not a concern once I'm done, and I'm kind of limited in my selection of machinery to do this with. I'm thinking of using a drill press and one hell of a long drill bit to basically bore the barrel out to 9mm.

My concern is that these barrels are made of some serious forged steel, not the cast aluminum of your average airsoft barrel. Does anyone know what steel and hardness the DPMS barrels are and what drill bit would be best to use for this?
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Old January 12, 2012, 01:21 AM   #2
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Barrels are often made of 41XX series steels, tough alloy steel originally called "Ordnance Steel". Barrel steels are tough but relatively soft, about Rc 30.

You cannot drill a straight hole down the barrel using a drill press. Find someone who has a lathe and piloted drill bits.
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Old January 12, 2012, 10:23 AM   #3
Nickel Plated
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Aren't pilot bits used for starting a hole? I just want to make sure I clarify that this is being done to a barrel that's already bored to 5.56mm and I just need to enlarge that bore to 9mm. Since I'm boring out an already existing hole, wouldn't that keep the bit straight as long as I do it in small increments? The hole doesn't need to be very precise, just as long as its straight-ish and big enough for the inner barrel.

From the little bit of reading I've done on the subject it seems I would be better off with a core bit since those are designed for enlarging an existing hole.

Are the barrels heat treated? I read it's unadvised to do any serious machining on 4150 steel after it's been heat treated.

Last edited by Nickel Plated; January 12, 2012 at 10:30 AM.
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Old January 12, 2012, 12:52 PM   #4
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Quote:
Aren't pilot bits used for starting a hole?
I said piloted drill bits. Different.
Quote:
From the little bit of reading I've done on the subject it seems I would be better off with a core bit since those are designed for enlarging an existing hole.
Core bits are for remving a core from a larger piece. Yes, they enlarge the hole, but not in the way you are thinking.
Quote:
Are the barrels heat treated?
Barrel blanks are normalized and stress relieved after drilling, then rifled. They are not hardened. Normal state is approximately 30 Rc. Once heat treated, hardness would be about 48-53 Rc.
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Old January 12, 2012, 02:28 PM   #5
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If the barrel is chrome lined you are screwed. When I drill out barrels for liners, I use a hand drill. It keeps you from snapping drill bits off if they become chip bound. Look up the process for barrel lining somewhere and try it on a junk barrel first. You have to get a "feel " for it. If you use a piece of drill rod to extend the drill, remember you can only drill a little distance before with drawing and clearing the chips. Brownell's sells liner drills (and oil) if you can't make them at home.
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Old January 12, 2012, 09:36 PM   #6
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Scorch: Ok I think I found the piloted bits you were talking about. They are the ones with a smaller diameter at the tip that then steps up to a larger diameter correct? Why would the drill press not be able to do the job though?

Gunplummer: I think the DPMS barrel I was looking at may be chrome lined.

http://www.brownells.com/.aspx/sid=1...000021_d_10901
(it's the 14.4" at the bottom of the list.)

Brownells and other retailers don't mention chrome lining, but it says chrome lined on DPMS website. Is it going to be impossible to drill or will I just need a harder tool?

Edit: I'll probably just go with a Del-Ton barrel since I can be sure that it's not chrome lined and it costs about $30 less than the DPMS. Would have liked to keep all the same manufacturer's parts on my rifle though but oh well.
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Old January 12, 2012, 10:34 PM   #7
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Why would the drill press not be able to do the job though?
Because you can't hold the barrel straight enough in a drill press. You'll probably end up with the drill walking out the side of the barrel.
If it's chrome lined, get a different barrel that isn't. Chrome is absolute hell on a drill bit.
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Old January 12, 2012, 11:11 PM   #8
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I'll ask around but I doubt I know anyone with a lathe and I doubt there are many gunsmiths in NYC. Perhaps best option is to just send it to a decent shop that does barrel relining, might be cheaper too. Not really worth paying for one of those drill bits just to do one or two barrels.
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Old January 13, 2012, 08:40 AM   #9
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The add said "chrome lined special order only". The general specs are at the top of the page. Yes, if you buy the drill it will get expensive. If you have no one to direct you when making the drill there will be too much trial and error involved. The only way I ever drilled was with a hand drill. The drill is actually just floating in the barrel and the pilot guides it. I had a lathe, but you can not "Feel" when the drill was about to over load. Look around on line for a used "AIRCRAFT " drill. They are extra long. I just put the barrel in the bench vise horizontally and used a lot of cutting oil. Old barrels are really tough. Some of the newer barrels are a lot softer to cut. The barrel makers use steel that was re-sulpherized and it cuts like butter.

Last edited by Gunplummer; January 13, 2012 at 08:46 AM.
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Old January 13, 2012, 12:43 PM   #10
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Brownell's sells barrel liner drills (drill bits). I'd recommend a lathe for drilling out barrels

http://www.brownells.com/.aspx/pid=6...L-LINER-DRILLS


AR barrels are no different. Years ago I use to shoot a lot of CNGB postal matches. That's where you used M16s with sub cal devices shooting little targets (same as used in Appleseed shoots today).

I converted a Colt SP1 barrel to 22 only by drilling it out with one of Brownell's barrel liner drills and installing a 22 liner, then chambered it for 22 RF. It was quite a bit more accurate then 22s out of a 223 barrel.

It's not really that hard, the drills have a built in pilot on the front of the drill bit. Once drilled out you can use Accra gel to glue the liner in or silver solder it in. Never had one come out and I've re-lined lots of 22s besides the AR barrel. You do have to use chamber reamers and head space gages to finish the job.
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Old January 13, 2012, 01:09 PM   #11
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BTW, your barrel is NOT chrome lined. This is from the specs section of the page you linked to
Quote:
Not chrome lined; chrome-lined barrels available by special order only.
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Old January 13, 2012, 01:09 PM   #12
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While the liner drills can be used, you may not find one the size you want.

Gun drills are used to drill barrels.

The have a special design (a single flute) and coolant passages in the drill.
A pressurized system is used to deliver coolant/lubricant to the cutting tip and flush out the chips.

The other type feeds coolant between the drill body and the hole and flushes the chips back up the passage in the drill.

The barrel is normally rotated in a lath while the drill is held fixed.

Holding the barrel while preventing flex are vibration are another problem.

Do a search on 'gun drills' and you will see how things work.

Last edited by brickeyee; January 13, 2012 at 01:25 PM.
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Old January 13, 2012, 03:32 PM   #13
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Old January 13, 2012, 03:35 PM   #14
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Just an observation: Why do the drill bits have cutting edges on the piloted part if it's not supposed to be cutting anything (and would be bad if it did) Wouldn't it make the bit less likely to walk if the piloted end was a smooth shaft and the cutting edge started on the larger diameter at the back? That way the pilot can't cut into the side of the barrel and go wandering off. It would have to go straight down the bore and the rest of the drill would follow along.

I may just try my hand at making the piloted bit I need. Regular drill bits are cheap so I can buy a few and try it out. Like Brickeyee said, I'm having a hard time finding a specific bit that goes from .22 to 9-ish mm.

Edit: Sorry for double post, the first one kept coming out blank for some reason o_O
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Old January 14, 2012, 10:23 PM   #15
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The "pilot part" does not cut. You take a drill and spin the pilot on. What you are looking at is what is left of the flutes after spinning it down. The shoulder area does not even have to be that sharp. Best if it is not really. Too sharp and it will try to pull itself into the hole and break.
A gun drill only works well drilling a solid piece. There are two different angles on a gun drill and the sides have relief ground on them. The head of the drill kind of floats by itself.
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Old January 17, 2012, 06:39 PM   #16
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If I was going to do this I would run through it with gradually larger drill bits ending with a 11/32 drill making sure to clear the chips often. Then I would run a 9mm reamer through it with LOTS of cutting oil, running as slow as you can and making sure the chips are being cleared. A lathe would be optimal but that is hard to come by if you do not have a machine shop avaliable.
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Old January 17, 2012, 08:32 PM   #17
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I might be a lazy bastard, but why not just order a 9 mm barrel?
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Old January 17, 2012, 11:46 PM   #18
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I would never put a reamer through a barrel. (Again)
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Old January 18, 2012, 03:57 AM   #19
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A reamer may be a bit much for this. The bore doesn't need to be perfect. Just drilling should do fine. If the bore comes out too big I can use some electric tape wrapped around the inner barrel as a spacer.

As for the 9mm barrel, That was my first thought, however finding one that's 14.5" non-chrome lined and under $200 has so far proven fruitless. It's still an airsoft gun after all, I don't want to spend as much for just the barrel as the rest of the gun. Especially since the actual AR barrel on these guns is really just a cosmetic piece and nothing more.

There are plenty of 11.5" 9mm barrels though. I'll probably use one of those when I start working on an M733 upper for this gun.
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Old January 21, 2012, 08:00 AM   #20
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Nickel plate
You need a long 9 mm drill bit. You can make them by silver brazing or welding on drill rod extension.

I do use a lathe but other than that just regular jobber drill bits and aircraft extension bits.

I have drill out .22, .32 and 9 mm barrel with this way.

And if you use a drill motor, clamp the barrel so it does not move and drill half way then drill out other half.
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Old January 21, 2012, 08:08 AM   #21
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As previous posts mention stepping up to a 9mm is a every good idea
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Old January 24, 2012, 06:45 PM   #22
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I don't think I'll need an extension. I have some drill bits that are long enough by themselves to go halfway down the barrel, and then halfway from the other end. Anyway the barrel is coming in tomorrow so I will see what needs to be done in detail.
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Old January 26, 2012, 04:49 PM   #23
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Oh fun, looks like the barrel Brownells sent me is chrome lined. I looked online to see how to check chrome or not and i hear that if it's chromed, it will have some silver/gray overspray around the crown of the muzzle. Yep, sound like this is gonna be a fun time
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