Do you have gas? part two: In this last installment, Army Reserve Team Armorer Master Sgt. Joe Carlos (ret.) reviews AR-15 build experiences at Camp Perry.
To nobody's surprise, and not entirely of his own volition, he ended up leaving the Team. It was Team policy that all turned-in rifles cycle back through the Armorer for technical inspections and machine rest retesting before being reissued back out. It didn't take long to diagnose his three "bad" rifles.
All of the rifles I build have the front sight attached using four set screws that match up to milled flats on the corresponding sides of the bottoms of the barrels. This allows the shooter to move the sight from left to right and adjust it so his rear sight Mechanical Zero and his No Wind Zero are the same. Using a pointer gauge in the carry handle, it should line up with the front sight post. While not exact, it allows the gunsmith to get a the front sight roughly centered quickly without some sort of bore laser or actual range firing. These pointers used to be available from both ArmaLite and DPMS. In his book, Derrick Martin offers an alternative diagnostic method.
The gas hole in the sight housing is bigger than the gas port in the top of the barrel, so a carbon ring will form around the barrel's gas port. If that carbon ring is not centered around the gas port it shows some degree of misalignment. External checks, such as the pointer gauge, are quick and don't require the front sight housing to be removed and the front sight zero lost.
The gauge clearly showed the shooter had his front sights cocked way to the side. All three of his rifles were that way and the one that was malfunctioning was the farthest off. This guy classified High Master and I don't doubt he knew what proper sight alignment and picture looks like. I guess there was just something in his position, his eye, his head pressure, trigger action, etc. that caused the abnormality. To make sure it was a shooter idiosyncrasy and not a gun issue, I took time to fire the gun out of my shoulder and it was way off for windage for me. Bringing the front sight back up to top dead center where you would expect it to be cured all function problems and the next person to inherit it had no complaints. This doesn't happen often but I have seen it in the past and that helped with the quick diagnosis.
As extra insurance, before I reissued the gun I countersunk the top of the gas port hole a little to allow gas to flow if the sight got cocked out of whack again. Not long after, I bought some Badger Barrels from Northern Competition and found the A same kind of countersink on their ports. They said they do this with all AR barrels they sell for the same reasons. Since then I have adopted their philosophy and do it on every gun I build. If you are doing bunches of these it would pay to set up a drill press and get the cut uniform and mindless to execute. If you are doing these in small batches I recommend just using a hand drill and any bit between 1/4" and 3/8" to get a wide chamfer to catch gas if the sight gets cocked. You don't want to go deep so just a quick "kiss" is all that is called for. Remember your work is going to be hidden by the front sight and not seen so it doesn't have to be pretty. It just needs to get done and fast.
There is another way to fix mismatched sight housings and gas ports that I used to employ. The only reason I went to the chamfer method is because it is a minute or so faster. The designer of the front sight base for this rifle was smart and made the gas hole in the base fatter than the gas port so we could be reasonably sure to capture gas in the event of a less than stellar alignment of the two holes. The hole in the sight base is about 0.144" and the gas port is commonly 0.093". You can enlarge that hole in the sight base by going up from the access hole in the bottom of rear barrel band with either a 5/32" drill that will enlarge the existing hole to about 0.1562" or use a Size #21 (about 0.1590") if you want something just a bit fatter. In doing so you want to just kiss the bottom of the gas hole and there is no advantage in going all the way up to the gas tube.
While you have the base off, this is a good time to ensure that the hole in the gas tube is in proper alignment with the one in the front sight housing. This is just a quick visual confirmation. Poor drilling of the cross pin holes in the gas tube (or sight base) can result in a cock in the tube either right or left of centering up its hole with the one in the front sight housing. A small flash light is handy to see up there with a directed beam of light. Let's be very clear of what you are looking for, however. The hole in the gas tube is supposed to be smaller than the 0.144" hole in the sight housing, so it is key that you don't confuse what is correct (the smaller size of the hole in the gas tube--about 1/8") with a cocking or misalignment. You want to avoid enlarging that gas tube hole unless it fails to line up because its size is set to meter the right amount of gas back to the bolt carrier key.
The modifications suggested so far are simply designed to capture gas that the gun was designed to work with when misalignments occur. None of our modifications are designed to permit more gas than the manufacturer wanted. Drilling of the roll pin hole in the gas tube or front sight housing at an angle is such a rare occurrence that you will likely never have to deal with it anyway. If you look up in there and think you have a misalignment but you are not sure, just assemble everything and test fire it. If the rifle functions OK everything most likely is the right size and aligned correctly. Too easy!
When I became Armorer for the Army Reserve Service Rifle Team I inherited 85 National Match uppers built for the Reserves by a competing military team. The Reserves were too broke to have their own armorer so they paid another team to build uppers for them! I wonder if you could find any race car driver who would allow another race team to tune his engine?
One of the first things I did was machine rest the whole lot for accuracy and found most to shoot in excess of one MOA, which is too fat in group size to be competitive. I started tearing the whole lot of them down and rebuilding them from the bottom up. Since the Team had no parts I simply recycled what I had inherited and put the rifles back together right. Eventually, I was able to cut group sizes in half from what I had inherited and spent very little money in doing so.
I had noticed when I had the guns on the machine rest during initial evaluations that function was often sluggish but bolt ring and other quick tests revealed no obvious reasons. Obviously, I took the front sight housings off during the rebuilds and noticed something odd with the gas ports that were now visible. Some appeared to be undersized and I started to gauge them. I found some ports to be spot on at 0.093" (those were the guns that functioned properly) while about equal numbers were either one or two full drill sizes too small! The ones that were one drill size small functioned sluggish and the ones two sizes small were the rifles I was getting function complaints.
I never figured out why some of these gas ports were so far out of spec. It is common to drill the initial hole for the port a little on the small size and then ream it out. Perhaps whoever did the work on these barrels got distracted and forgot to go back and perform the reaming. Or maybe someone did the work who was confused about what the finished size was supposed to be. There are at least two good reasons that I can think of to keep gas ports on the "skinny" side.
First, the smaller the gas port the less chance you are going to have of hitting a land. Second, felt recoil in Stoner platform guns comes from two components. Acceleration of the bullet (the same as in any other firearm) and "secondary recoil pulse" coming from the bolt carrier pushing the buffer to the rear until it hits the rear of the buffer tube, halting its movement. Metering less gas will soften secondary recoil pulse because the carrier and buffer will be moving to the rear with less speed. Go too small on the gas port, however, and one runs the risk of not moving the carrier to the rear far enough to reliably eject spent casings, pick up the next round, or lock the bolt to the rear on the last shot in a magazine.
Every military armorer that I have ever known has carried some kind of kit with him down range. These kits can be in some sort of canvas bag or an old fishing tackle box. Each kit contains an assortment of basic hand tools and spare parts to quickly get a shooter up and running again in a hurry in a field environment. One of the gas related items that I always carried was a light weight bolt carrier assembly, mostly relating to the amount of metal at the bottom rear of the carrier. The commercial carriers have varying lengths of metal at the bottom and I always selected one with the shortest section to lighten the carrier. This can amount to a good ounce of weight reduction and that, by itself, will sometimes get a gun functioning again. My spare assembly was one that I had field tested in a number of rifles. I always made sure it had new rings and a properly staked key so I could drop it in a shooter's rifle and check a bunch of potential problems in seconds. If the bolt carrier swap works, in addition to checking the original carrier's key and bolt rings, you might also want to check the size of the "tail" of the bolt. Derrick Martin says this should measure between 0.2500" and 0.2506". The corresponding hole in the carrier should measure between 0.2520" and 0.2526" resulting in no more than 0.002" of clearance. He says that more clearance than that will allow gas to escape, resulting in possible malfunctions. If you have customers complaining about getting gas back in their faces and eyes you might want to check those dimensions on offending guns.
The second item I carried was a low mass buffer for problems more severe that weren't cured by the lighter carrier. JP Enterprises sells a nice low mass buffer that is red for easy identification. If you want to save money you can simply disassemble a regular buffer and pour out the innards, lightening it about four ounces. If you are concerned about maintaining rigidity of the unit, cut a section of half inch diameter wooden dowel and put that in the hollow space. Back in the van I also had larger items like reduced power buffer springs and the like.
Any parts change mentioned can cause zero shifts. In an emergency I am pretty sure most any shooter would far rather accept ten shots in the 9 ring, forfeiting 10 score points, as opposed to having to sacrifice the full 100 points because his gun wouldn't function and he had to sit out a string. The field expedient repairs are just band aids to get the shooter back up and running quickly. You still need to do a real diagnostic and repair of the underlying cause when you follow up off range later.
I am sure I have missed problems, diagnostics, and repairs that you may know of. Feel free to email me (NCC1701@penn.com) and I'll try to work your tips in at some point. AG
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|Date:||Feb 1, 2014|
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