Opening gas port

[Boolit_Head]

I have a unique AR15 barrel, it's a 10.5 inch 6.8 spc. There were only 4 made by the manufacturer and they had undersized gas ports. I need to open it up from .075 ish to .086. Now I have the bit and was thinking I should fixture it up in my drill press but was considering putting a sub caliber brass rod in the bore to prevent hitting the far side. That way when I am peck drilling If I see brass shavings I know I am through. Sound like a plan or how would you do it?

[roverboy]

That should be fine. Just make sure your drill point comes all the way through. I know that is a no brainer. Lol.

[DCM]

Sounds like a very good plan. I have used wood dowels in the past but like the brass plan Much better!

[jcren]

Haven't done it myself, but was told to drive a lead slug to the port position and drill slowly and the slug would prevent internal burrs. Just a thought.

[country gent]

the brass rod wil probably work but lock it in place or it may walk and break the drill. I would also recommend setting the drill presses depth stop as a primary stop.

[Moleman-]

I'd get at least one more size between .075" and .086" Perhaps a #46 (.081") or a #45 (.082") and test fire it before opening it up .011" unless it's not even cocking the hammer. Figure out your barrel thickness and just put a piece of tape on the bit to keep you from drilling too deep, or mark it with a sharpie which is what I do. Don't worry about any burr as it will disappear with the first or second shot.

I generally drill mine on the mill with the barrel held in an indexer. After the gas port is drilled the barrel is indexed and a divot milled with a ball end mill opposite of the gas port for one of the set screw type gas blocks.

[Boolit_Head]

I'd get at least one more size between .075" and .086" Perhaps a #46 (.081") or a #45 (.082") and test fire it before opening it up .011" unless it's not even cocking the hammer.


Known issue with this barrel. Manufacturer agrees upon the size it should have been. At the existing size it does not get enough gas to lock back..086 is pretty much standard for this caliber and length. These were pretty much the first 4 barrels in this caliber this manufacturer made.

[JSnover]

Does anyone else think a reamer is more appropriate for removing that small amount of metal and at the same time is better at avoiding a burr?
Worth a try, chuck it up and turn it by hand but I might go a size smaller as I've had new reamers cut oversize. The trick was to turn them slow, feed them fast and pull out after one or two revolutions at depth but... I never reamed a hole that small.
Travers Tool
https://www.travers.com/solid-carbide-decimal-size-chucking-reamers/p/20-565-124/

[popper]

I used a wood dowel and hand drill, works fine. Run it very slow. A few jacketed will remove any burr.

[B R Shooter]

I like the lead slug idea. Or you could put a cleaning rod in and basically pour a lead lap, pull it to the gas port, drill, then stroke back and forth to remove any burs.

[nekshot]

I will agree with the others, it will work. I tried opening up a gas port in a chrome lined bore and that was a chore!! Of course I can plead stupid because I am a newbie at metal work!

[akajun]

Does anyone else think a reamer is more appropriate for removing that small amount of metal and at the same time is better at avoiding a burr?
Yes when I profile a blank for an ar I always drill about .010 undersize and step ream at least twice for the finished hole size. I could always see the burr when I drilled them, no matter how new or how many steps I took to get to the finished size. Im sure my reamers still leave a small burr but I cant see it, cant feel it with a brass brush, and it shoots out very quickly.

[country gent]

There is a small drill sensitive spindle I use to drill with gives much finer feed rate and less pressure, this helps reduce the burr left. its a sliding spindle with chuck that is mounted in chuck or collet. a small ring under the chuck is gripped and used to lightly feed drill.

[popper]

It's not rocket science nor will 0.001" make any difference. I would NOT (and didn't) use a drill press as that would require a more precise alignment to prevent 'bending' the bit. Let the twist drill follow the 'pilot' hole and go slow, plenty of oil so as to not snap the bit. Use the dowel and wood will show up when you are through. Speed up a tad 'clean' the hole a bit. Did it to a BO barrel and all is fine, took about 2 minutes. Yes I was a bit nervous to begin but it's not a big deal. The original hole should be in a groove, not on top of a land. If not send the barrel back and get one done right. A lot of barrel turners used the 223 size which doesn't always work.

[B R Shooter]

The original hole should be in a groove, not on top of a land. If not send the barrel back and get one done right.

OK, I'm not an AR guy, but that seems like a tall order. I assume the position of the gas port has a very specific distance from the breech to be at.

[popper]

Port is at a specific distance due to length of gas tube, otherwise not a critical distance. If the port is on a land, will nick a boolit passing through every time. OP is 6.5CM which is high pressure HV long range accurate gun, don't want to spoil it. I used a hole drilled through 2 2x4 held in a big vice laying on my work bench and a worn out variable speed hand drill - lets everything 'free float' while drilling, bit follows the existing pilot hole. Only 'trick' is getting the bit to stay in the existing hole when starting. After that, go slow, oil, done. Run a few jacketed down the bore to eliminate any burrs left from drilling. I did put some tape on the bit as a depth indicator to visualize the task.