Home Forums Hunting & Shooting AR and Tactical Rifles DPMS AR15 DCM Barrel Problems

Jerry,

What is the diameter of the gas port on your barrel?

I know what mine are on both new and used take off barrels from various manufactures that I have on hand. This is another rabbit I failed to flush on this thread.

As I said previously on this thread I own and have installed several DPMS barrels without a problem. Another upper I will assemble on Friday.
You have talked to several DPMS reps and they told you to change the extractor. Unknown before your last post, they did test fire the rifle with their ammo along with another bolt/extractor. It functioned. As it all turns out now, they have been more than fair. After all, the only component that is their's is the barrel. Now you have over 200 rounds fired. If you were "over gassed" you would have over 200 cases with the rims pulled off!

Instead of taking their advice, several others suggestions or a number of things I suggested, you just ordered an adjustable gas tube.

Best of luck;

Dave McGrath

Here's the latest. Today is 11/24/2007.Went dove hunting this morning and took the AR with me to just shoot a box of ammo, and see what would happen. It functioned all 20, without a hitch. Yesterday, while dove hunting I fired 6 rounds without the gas tube, and they were easy to extract, so at the urging of my friend, I reinstalled the gas tube to try as a test. What scares me, is it the cooler temperatures, or some unknow work DPMS did while they had it. I guess I'll just keep shooting it and hope for the best.

Jerry

Cooler temps will cut down on gas flow.

As great a friends as Dave and I are... you are still overgassed...

I Re-barreled another AR for a club member on Friday with a DPMS barrel (BL-DCM from Midway USA item number 550-750). Shot it on Saturday for break-in and zeroing the adjustable front sight tower. Again, no problems!
That makes 10 or 11 that I have done without any problems and I know a dealer/smith about 40 miles from me that has done several others with the same barrel, all without a problem. Very accurate barrels I may add too! No copper fouling either!

Jerry, your gas port diameter should be .0995" which is a #39 drill. I have new barrels in my shop and several shot out take offs from RRA etc. and this seems to be the industry standard diameter. I'm sure the take off's have all seen their day with different weight bullets, powders, extreme barrel temps on hot days during "rattle battle" competition etc.

Here in Pa. we shoot high power when there is snow on the ground to temps in the high 90's. May not be as hot as what you and Jeff encounter but I know of no AR problems relating to "over gassed" or rims pulled off the cartridge cases.

Hope everyone had a nice Thanksgiving.

I don't know what else to say. Ya'll take care!

Dave McGrath

Dave:

I didn't take DPMS' advice about the extractor, because it was a crook of S---. As I mentioned earlier, I had changed the extractor during all of my previous testing, and didn't believe that was the problem. First of all if DPMS would have told me the FIRST time, what the had discovered, I would have told you all about it then. Telling me after the fact, just created another waste of time and effort. The extractor must be ok, as it pulled all 27 rounds out of the rifle this weekend, just as it is designed to do. As a matter of fact one of the guys who observed me shooting the 20 rounds, mentioned they 'sounded" KIND OF LIGHT. 24.5 GRAINS OF imr4895/69 SIERRA hpbt IS A "LIGHT" LOAD, In fact under min, in the Sierra book. I ordered the adjustable gas tube, as a way to insure I could use the barrel, without any worries, I hope. I have not removed the front sight to check the port size, as I locktited the set screws, and didn't as yet want to lose my front sight zero. I'll let you know how it goes. And thanks a lot for your help and advice.

Jerry

Dave

Have seen that standard size show rim lifts with charges of RL15 that in another barrel and smaller port would show nothing....

My episode of gas ports an the AR started circa 1990 with standard port size 995 in Oly barrels, I'm leary of it ever since. For 55s probably ok, but heavy bullets, 75 and 80s at that point, and different powders, it did not work well at all.

Jeff

I think a lot of the problems I have been having, are my "LACK" of experience with the AR. My impression was, bolt it on and go shoot. I didn't expect a problem, and no one out here mentioned gas port size, as Dave and Jeff had, until I talked with John Martin, father of Derrick, and a good friend of mine. He told me of guys that were "plugging the gas hole, and then redrilling to a smaller size. Dave the barrel is accurate, lots of groups under 1" @ 100 yds, that is/was not my complaint. If it only tore off one rim in 4, how could I shoot across the course with that performance. Nevertheless, if regulating the gas solves the problem, I will call Dillion back @ DPMS, and report to him, as he asked me to do, his idea, if one guy, me, runs into this problem, at least they will have enough info to help another out.

Jerry

Derrick Martin saying a smaller port??? Wow, I was almost sure he was one that said go larger if anything... I also thought in reading all his articles/books, that he came upon pressure signs sooner than normal on loads generallys peaking and I always attributed that to big gas ports for him.

Problem with smaller gas ports is sometimes the guns don't run 100% till they are shot and broken in some. Heck my custom M14 didn't run right for a couple hundred rounds, but it was tightly built and needed to break in.

You cannot afford one failure in match shooting. I hate alibis anyway....

Dave knows enough about XC shooting, he's an icon at Perry and always has been since I can recall. His wife volunteers there too!

Jeff

Jeff:

Derrick didn't suggest I plug the gas hole and redrill, his dad did. If you know John, or know about him, you know he is a high master, and in his time, was really tough to beat. Of course he prefers the M-1 and the M-14. He comes over to my home, and coaches me, on position, trigger pull, timming etc. and I am 62. John is only 79, lol. I still feel like I am pissin in the wind, and dang it, that barrel should work without all this messin around. Don't you all agree?

Jerry

I agree on the barrel

What I found hard to believe is that Derrick IIRC, is a proponent of more gas... Interesting to see his dad take an opposite view.

I don't know John, I do know Derrick a bit though. Both being HMs, and I'm HM from service rifle only in both XC and LR.

Be glad ya got a coach of that caliber!

Jeff

Jeff: I am real glad Ol John is close by. He is an asset to all shooters, and his goal is to promote more shooting. I received the adjustable gas tube last night, I am going to load a few of the cartridges, which "always" tore the rims, and go shoot it. I bet it will work fine. I'll let you all know.

Thanks again, and Merry Christmas.

Jerry

Jerry,
The best money I ever spent was a book called BLACK MAGIC by John Feamster. He likes H335 and 52 Sierras for close range.
FWIW I shoot 77's at close range and 80's at 600. Varget works for me. I have an Olympic Arms barrel on my DPMS gun and it has never malfunctioned in 10 years. They give us barrels to put on the junior shooters guns and have donated rifles to our State Team to take to Perry. DPMS is OK in my book.
My instinct tells me that if you mix and match parts from different manufacturers you could experience tolerance stacking which will not help anything.
I currently earn a living as a machine shop inspector, and I can tell you that whatever can go wrong will go wrong at some point.
Good Luck.
whelennut

Whelen

One of my favorite books.... wonder why ... grins....

Jeff

Next to the 270 Winchester, the 35 Whelen is my favorite. I have killed many critters with both, for elk to rabbits. Interesting, that both of these cartridges are based on the 30-06. I also have 6mm-06, and a 6.5mm-06, oh yeah a 338-06 as well.

As to the AR, now that I have the adjustable gas tube, and DPMS has had their chance to look at the upper. The only report I can now give, is after I go to the range, and run the loads over a chronograph. I somehow have the feeling,that the velocties will be lower than when I started out, that resulting in no rim tear offs. The original problem could have been a short throat, large gas port etc., but I can guarantee it was not the extractor, or head space.

I'll let you all know soon.

Jerry

Jerry,
I was digging through my loading manuals last night and was reminded that there have been three different chamber variations in recent times. I would guess that you have a 223
Remington chamber but it could be the NATO chamber?
That might make a difference. I have a 1/7 twist and assumed that I was good to go with the 80 gr Sierra's but found out that I need to have a special chamber with a different leade angle
into the rifling. (very frusrating)
I also share your frustration that a person can't just go buy an out of the box rifle for competition. You need to replace the trigger and sights in most cases. More than likely the factory barrel is not acceptable either. A sad state of affairs and tough on a new shooter or an M1A shooter swithing over to the "dark side" Why can't it be simple?
whelennut

Whelennut:

You'r right, why can't we just buy one out of the box, well maybe we can. Bushmaster's DCM rifle, is as accurate out of the box, as I have seen. Excellent trigger too! Bench testing the bushmaster, with the loads I have been having all the trouble with, gave groups under.75" @ 100 yds. That's the kind of accuracy, I want to start with, and build from there. I am a "freak" about shooting groups with differnt ammo, before I go to shoot a match. Probably too darn anal for my own good. The guys that I shoot acroos the course with, say go load this load, or that one, and go shoot. I realize there are certain loads that "everybody" shoots and has success with, but my regimen, is work up loads, even if I reach the same conclusion as everyone else. It's part of my fun. I spend a great deal of time shooting form the "bench", with all sorts of different rifles and cartridges, and this is the first time, I have not been able to solve the problem, so far, myself.

Whelen

RRA comes factory ready with a much better trigger than a bushy will ever dream of having...

Jerry

Shoot it with the new tube yet?

The old days you had to swap all parts, these days they come match ready!
Of course I remember issues with new M14s years back too!

Jeff

Jeff,
Some of the guys around here had good luck with out of the box Armalite if I remember right. I bought a DPMS because I was on a buy local kick a few years back. Now I say buy the best you can find no matter where it was assembled! (If you want to win)
I am giving up on post front sights and want to build a match rifle on the AR-10 platform with a .260 barrel. Once I tried match rifle sights and a match rifle trigger............
whelennut

Yep, I've heard same on match sights.... I've shot them a few times... I could really get used to them. I'm 42 with some P100 tabs etc..... but not shooting since 03. Not sure what will happen though I'm on a mission to shoot a 200 at 1000 with the AR service rifle.... that means a special build and post sights again... I doubt I'll shoot XC ever seriously again. But that 1000 yard mark is tempting!!! Once I learned how to shoot 200s at 600 the 1000 became the next logical goal!

You'll like the 260 I suspect.

Jeff

Finally, today 12/6, I got to the range, and had a chance to shoot some groups, and run the ammo over the chronograph. Here's the results: last year,and before I sent the upper to DPMS, I shot 22.5 grains of IMR4895/69 Sierra, and got 2788 fps, Extreme spread 31, today same load 2641fps/43es. 23.5 grains last year 2895fps/75 es, today 2817fps/53es. 24 grains gave me 2911fps/95es, sierra says 24.5 grains should give 2700, and 21.5 grains of Varget, with a Sierra 77 grain, last year 2510fps/18es, this year 2444fps/57es. All of this means nothing however, if the gun won't work. I ran 33 rounds thru the gun today, and 32 worked fine. I did have one rim tear off, and thats when I quit. The load was 22.5IMR4895/69 Sierra.I can attribute the little bit of lower velocity to the ambient temperature, so that is not a concern. Here's where I got the rim tear off, after shooting about 28 or 9 rounds, I noticed the action appeared to be moving a little slow, so I opened the gas tube up an 1/8th of a turn, I started at 1.5 turns as was suggested by the mfg., within the next 4 or 5 rounds I had the rim tear off. Also to note, the rim was torn clear down to the extractor groove, and was the width of the extractor, so much for a bad extractor. I really wish I had'nt increased the gas flow, as I would have liked to shoot a few more rounds. I really am trying to to be fair here, but I am almost sure, I have a bad barrel. All this "screwin" around should not be necessary. If it works thur another 100 rounds, after I readjust the gas tube, and it will group, I will use it next season, if not DPMS can have it, if they think it is so great, I will donate it to them, and they can sell it to someone else, LOL!

Jerry