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04-27-2010, 01:23 PM #1
AR-15 Gas Port in Barrel Question
This is almost Off Topic since I don't plan on having to do this, but I am really curious. I was reading in an AR-15 book (may have been The Competitive AR-15) about where the hole should be in the barrel for the gas port. The author said it should be in a groove between the lands. Of course this is possible if the timing has not been selected yet on the barrel.
First question: Is this true? Do barrel manufacturers do this and if so how do they line up the barrel to drill from the outside? Or, as a home shop machinist am I clueless and the answer is CNC?
Second question: If I were building the barrel I think I would get the timing right for the barrel extension first and then drill the gas port hole. Of course if I did that I couldn't guarantee the hole goes in a groove of the rifling.
So what's the answer?
Thanks in advance,
04-27-2010, 02:16 PM #2
I have heard the same thing about gas-port placement, and have dismissed it for several reasons:
1) I have seen no evidence to prove that a port drilled right through a land is any worse than trying to locate it in the bottom of the groove. I have borescoped hundreds of barrels, and where the port is located doesn't seem to make a whole lot of difference.
2) In some port locations, with certain port sizes, and land/groove configurations, the port could be larger than the groove. In that case, you would disrupt two lands.
3) Locating and drilling the port is very easy once the chamber is cut, the shank is threaded and the barrel-extension is installed. Drilling the port first, even if you could consistently drop the port right between lands, makes it absolutely mandatory that your chamber and threads turn out perfectly the first time, or your barrel is scrap. It is my opinion that you are far better off to make certain your chamber is perfect first, then drop in your port afterwords.
The guy that built the winning AR at Camp Perry the last two years in a row doesn't attempt to locate the port according to land/groove placement.
04-27-2010, 02:20 PM #3
It is true that some of the better smiths try to put the port in groove. Doing so prevents a sharp edge on the edge of the land that will collect bullet metal over time. It also can help eliminate the burr when drilling the port.
The way to do it is determine the exact twist of the barrel you are working on( do not trust the manufacture as they may not be exact), find one groove and figure out where it will be at the distance that the gas port is will be. then figure out if it was at 12:00 what timing point it will be at the breech. Some build a guide rod with pointer that they align to 12 oclock and fit the barrel onto it and line the groove up. These guys seem to add about 1/4 inch to the location and mark the barrel for the port location then they fit the extension onto the breach using the port as their index point.
Some also use a bullet in the barrel to drill into which helps eliminate the burr. There are those that do not consider the burr anything to worry about as it will burn and break off with the bullet passing.
There is another group that fits the extension and lets the port fall where it may.
I think it is a matter of how picky you want to be and what you consider is good work. I have done it both ways and do not see any major differences between them. I did everything with calipers, scales and dial indicators so it can be done by in the homeshop.
I do recommend that you read Derrick Martins book on the AR. He goes into alot of detail on how he does it and why.
Best of luck.
04-27-2010, 02:33 PM #4
You will generate burrs no matter where the port gets located. I drill the port just a few thousandths of an inch smaller than the intended port, then ream it to size with the desired sized straight-flute reamer. The reamer cuts away the area that the burr left from drilling is anchored. Any burr left can only be a couple of thousandths of an inch long. If you take a tight fitting brass bore brush and run it thought the bore, it will break off any remaining burrs.
04-27-2010, 02:53 PM #5
I agree with Dtech but get there using a different method. In a 223 I tend to shoot very light loads with the cws bolt weight and drill it out to function. It ends up being assembled and tested multiple times before I get it to function with my standard load. I have found that with what I use, I end up being larger than standard. I seem to have good luck doing it that way and until my eyes went and I can no longer see the sights, I was right on the edge of master across the course.
04-27-2010, 03:20 PM #6
AR gas port location
I've shot AR's with the gas port in the groove, and had one that had the port partially in the groove and land. The one that had the partial gas port fouled very badly, right past the gas port. That location simply ripped copper jacket off the bullet and left about a 2" streak of copper in that barrel at that location.
The "easy" solution to locating the gas port hole without knowing any geometry or three decimal twist rates is to take your handy-dandy angled lens bore scope (I use a Hawkeye) and mark off the proper length from either the muzzle or breach on the bore scope shaft with a sharpie or piece of tape. Put the bore scope in your barrel to that depth and rotate the mirror until you are looking at the bottom of a groove. The notch at the base of the rotating part of my bore scope points in the direction that the top of the barrel should be when you drill your hole. Align barrel, drill your hole and don't look back...
I'm not sure how well I explained this, but it works for me.
04-27-2010, 03:43 PM #7
Locating the spot to drill the port between lands is not the difficult part about it. Yes, it is more difficult than just drilling the port where ever it ends up, but not that big of deal to locate a spot inside the bore where you want the port. As I said earlier, chambering, threading, headspacing and getting the barrel-extension to time-up is the bigger challenge.
How the port creates fouling has much to do with how the first bullet crosses the port. I have seen ports that, just by chance, are right in the bottom of a groove that fouled tribally, and I've seen ports that cut half-way through a land not cause problems. when the first bullet crosses the port, it is not uncommon to damage or distort the far side of the port. I borescope the barrel after the first shot and often have to run the reamer back down through the port because of metal being pushed into the port.
I chamber and port roughly 300-350 AR barrels a year and have had ports that have "behaved nicely" and ports that have "behaved poorly" in every imaginable location in the bore. Just because the port happens to fall in the groove doesn't get you a "free pass"
I can't even imagine my disappointment if I went though all the work of placing the port between lands only to have a port that produces fouling anyway.
04-28-2010, 07:31 AM #8
Gentlemen, thank you for a very informative exchange. I really appreciate you all taking time to share your techniques and experiences.