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Posted: 5/20/2005 1:11:48 PM EDT
[#1]
Call DPMS, the gas port opening did not get enlarged.
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Posted: 5/20/2005 3:10:56 PM EDT
[#2]
I'm with pighelmet; it sounds like it's shortstroking due to a small gas port.
As for the buffer spring's tension, factory carbine springs should all be the same. It's the weights of the buffers that are different. A rifle's buffer is really heavy compared to most carbine buffers: Standard CAR 2.9 oz. H buffer is 3.8 oz. H2 buffer is 4.6 oz. Rifle buffer 5.2 oz. 9mm/tungsten CAR buffer 5.5 oz. |
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Posted: 5/20/2005 6:04:52 PM EDT
[#3]
Dude, you're not having much luck with these Dissys are you.
 I'm building a rifle length Dissy right now and my gas port needed to be opened up some. |
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Posted: 5/20/2005 6:23:13 PM EDT
[#4]
Howdy Mongo,
You got that right: I am not having much luck with these Dissys. I had CMMG build it for me with a ventilated DPMS free float tube. John and Jeff were great talking with over the phone, and had fast email responses during its build. I am certain that all will work out with them. It could be that DPMS did get one out without making the hole big enough. Upper was originally delayed because DPMS sent a barrel (?) that was bad. I really don't remember what the part was, but 'barrel' sticks in my mind. John or Jeff said that the new part would be in the next week; it was; and I got the upper very soon thereafter. Its nice to deal with folks who tell you the truth. They patiently answered all my questions. I have it on a Mega lower, with J&T Dist M-4 stock and lower parts kit (bought at Knob Creek). I'm still stoked about it though. I hit the black at 200 yards with it. |
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Posted: 5/20/2005 6:25:23 PM EDT
[#5]
In case you didn't see this link, I'm trying to sort of make a diary of my findings during the build and tweaking process.
Dissy Build Link |
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Posted: 5/20/2005 6:36:11 PM EDT
[#6]
Good thread.
Is it possible to over-gas the system? What type of damage could occur? |
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Posted: 5/20/2005 6:39:06 PM EDT
[#7]
I believe it is possible to overgas a system, but I'm not sure what damage would happen. Stress to the lower receiver and the buffer tube comes to mind. I've seen cast lowers break there, I wonder how much of a beating a forged unit could take? I'm very slowly creeping up on the size of the gas port only 0.001-0.002" at a time. As you can see, I've got quality ammo running fine, but not Wolf. I'm wondering if it's worth my effort to tweak again to get Wolf to run or leave it. |
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Posted: 5/20/2005 6:47:19 PM EDT
[#8]
That's a tough call. I don't know how much work it takes to open the port correctly. Plus, I don't know how much risk is involved.
What someone needs to develop is an adjustable gas port for the AR15. Then, no matter what ammo we use, we could find a setting that will work. |
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Posted: 5/20/2005 6:51:12 PM EDT
[#9]
There is an adjustable, rifle length gas tube. MGI, or somebody, makes one. |
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Posted: 5/21/2005 1:27:26 AM EDT
[#10]
That may be what I need. Time will tell. But then, it won't do much for me if rifle has an insufficient sized gas port.
I am heading for the fire department in a few minutes. I don't have time to search for MGI, but I have a not to myself to do so when I get home. |
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Posted: 5/21/2005 4:33:43 AM EDT
[#11]
Yes. Too much gas causes the bolt to unlock too early, while the pressures in the chamber are still too high. This makes for tough extraction, and usually results in ripped case rims. You can treat the symptom by using a heavier buffer to slow the timing down. The fix is to use an adjustable gas tube. But I doubt that you are having a problem caused by too much gas. The spent brass would definitely show problems. The best way to use an adjustable gas tube is to drill out the gas port in the barrel to oversize, the regulate it back down with the adjustable gas tube. I use one on my M16. MGI |
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Posted: 5/21/2005 5:42:51 AM EDT
[#12]
I did that one time with a 11.5"/5.5" upper I had one time that just didn't want to run. I drilled/reamed the gas port to .105 +/- and put an MGI tube on it. It ran fine after that after I got the tube tuned in. |
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Posted: 5/23/2005 5:02:43 PM EDT
[#13]
I emailed CMMG and DPMS. I'll keep you posted on situation.
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Posted: 5/25/2005 7:42:16 AM EDT
[#14]
UPDATED: 05-25-2005
I inspected how the bolt carrier slid through the upper receiver. The bolt carrier key hits a high spot (for lack of a better term) on the cut-out for the rear sight assembly (or darn near it). This is most notable when upward pressure is applied to the bolt, such as the hammer does during bolt cycling. This causes the bolt to STOP! This is where the bolt stopped during test firing. Also noted are UNEVEN wear marks on the bolt carrier key: front right and left rear: NOT EVEN WEAR. I tried THREE other AR bolt carriers with the EXACT same results. I checked my THREE other AR upper recievers for this problem, and ALL were smooth as silk: no hangs noted. I will do my best to post photos later today, when time allows. The gas port MAY be the correct size, because the bolt cycles to exact distance where the high spot is noted. I suspect that this upper is not machined correctly. Bolt retraction should be smooth. DPMS said to send it back. I will update as things progress, or not progress, as the case may be. This is also posted above in my original post. |
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Posted: 5/25/2005 7:44:22 AM EDT
[#15]
By the way, I am less stoked now.
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Posted: 5/25/2005 9:51:54 AM EDT
[#16]
Please forgive my ignorance. Is the reason the dissipators have a hard time cycling due to the shorter buffer tube? I see that it is a gas tube length/port issue but what is the difference between this rifle and a full length? Thanks for the edification.
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Posted: 5/25/2005 9:58:10 AM EDT
[#17]
There is less barrel length after the gas port, therefore there is less time for the gas pressure to build up in the gas system, while the bullet is still in the barrel, acting like a plug, holding the gas pressure in the barrel and the system. This is commonly known as "dwell time". Once the bullet leaves the barrel, it essentially pulls the plug on gas pressure. A certain "amount" amount of gas is needed to cycle the gas system. If there is less time for the pressure to build up, you need to allow it to build up faster. That's where opening up the gas port comes in. It allows the gas system to pressurize "faster", allowing proper gas system operation. |
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Posted: 5/25/2005 10:34:53 AM EDT
[#18]
Thanks Mongo, that helps a ton. So a carbine length tube would alleviate the problem?
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Posted: 5/25/2005 11:05:27 AM EDT
[#19]
A carbine length Dissy would run just like a carbine - just fine. It does require a second gas block at the end, though if you want that true Dissy look. Bushmaster sells their Dissy in this configuration. 
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Posted: 5/25/2005 4:27:10 PM EDT
[#20]
This afternoon, I worked the bolt carrier back and forth through the upper, for two episodes of the X-Files. The high spot went away, and now the complete bolt assembly slides into battery while holding the upper at a 45 degree angle.
 With this problem gone, I hope to get to the range tomorrow to test fire it again.
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Posted: 5/29/2005 7:16:43 PM EDT
[#21]
Test fired it the other day with several types of ammo. Still short-cycling. I sent it back to DPMS. DPMS gave me UPS account number to send it back on their dime. That's a hell of a lot more than Model 1 Sales did with my three returns. DPMS was very good to talk with over the phone too.
I was surprised at the groups I got using GP90 and Hornady 77 grain Match: about 3" at 100 yards with iron sights. Remember too that this was shot single fire, under duress. Do I have to work the fire department tomorrow? That place sucks a$$. |
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Posted: 6/10/2005 2:44:06 PM EDT
[#22]
Upper came back today from DPMS. Looks great. Invoice stated that gas port was adjusted and was fired 20 times without malfunction. DPMS had me send it back on their dime, and sent it back to me: no cost involved.
 This is fantastic service. I look forward to range time with it.
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Posted: 6/10/2005 2:59:44 PM EDT
[#23]
good news :)
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Posted: 6/28/2005 6:27:35 PM EDT
[#24]
I took it to the range this afternoon. At least the weather was nice, and the mosquitoes were away.
It ejects cases now, but will not go back far enough to consistantly strip another from the magazine, nor does it go back far enough to hold the bolt open after the last round. On average, it would strip about 2 shots out of 10. I tried the following ammo, 10 rounds each. Swiss GP90 Radway Green Georgia Arms SS109 XM-193 (not PD) Wolf 62 grain Hornady TAP (60 grain, if memory serves me correctly) I stopped trying after six brands, totaling 60 rounds. I figured if it aint going to work with these, it aint going to work with much of anything else. Accuracy was 2 inches at 25 yards! Not acceptable! I have no explaination why the accuracy went to hell after being repaired at DPMS. If anyone wants to inherit this headache, I will be selling the complete upper on the Equipment Exchange sometime when I get back from the fire department on Thursday. I'll post photos too. I consider the barrel to be JUNK, but the rest of the parts show no defects: bolt, carrier, charging handle, upper, vented free float tube, FSB, flash suppressor, etc. I bought it from CMMG for $490, with the DPMS vented FF tube installed. I figure $250, delivered, is a fair price. DPMS, and probably CMMG, are off my list. To be fair, I borrowed a Bushmaster 16" CAR upper and used the same mag, lower, and ammo to rule them out as being the cause. It worked and grouped better...with a shorter sight radius. Shots were on top of each other at 25 yards. Swiss GP90 grouped right at 3 inches at 100 yards. The Bushmaster upper shot very well. I noticed that I wasn't covered in smoke each time I shot either. |
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Posted: 6/28/2005 6:34:26 PM EDT
[#25]
This is nearly unbelievable from a major manufacturer. Hell, I'm just a garage monkey gunsmith and my first rifle length Dissy project came out worlds better than theirs did. Goo dluck with whatever you do next.
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Posted: 6/28/2005 6:40:12 PM EDT
[#26]
Mongo, I think that I like my M1A's and Garands. I can tune these rifles to shoot MOA. I tend to shoot better with these rifles than I do with an AR: match or otherwise. I had a Bushmaster XM-177 (?), and that thing shot like a house afire! It was fairly accurate too. Put an AS-IV on it, a 30-round mag, and you got a handfull of fun. I sold it at the height of that pre-ban buying craze during the AWB.
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Posted: 6/28/2005 6:48:47 PM EDT
[#27]
Sorry to hear about the disappointing service you received from DPMS.
 You can always drill that gas port out HUGE and then adjust it back down to a functioning pressure with an adjustable gas tube or gas block. |
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Posted: 6/28/2005 6:56:46 PM EDT
[#28]
That works. I did that once, on accident. I used an MGI tube to correct it. On my Dissy, I drilled the port with very small increases in size until I reached a size where all ammo worked well, from junk Wolf ammo to semi-hot reloads. |
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Posted: 6/28/2005 6:57:42 PM EDT
[#29]
But how do you get the accuracy back?
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Posted: 6/28/2005 7:01:54 PM EDT
[#30]
Not sure about the accuracy. I'd be wondering if your crown isn't dinged or something like that. |
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Posted: 7/1/2005 12:35:04 PM EDT
[#31]
Mongo, you were right. The crown was hosed.
I removed the flash suppressor. The crown had been nicked. I gave it an 11 degree crown. Took it back to the range today. Swiss GP90 grouped 3.5 inches at 100 yards, using the small aperature sight. It did not short cycle with Swiss GP90, but short cycled with the rest. I left XM-193 at the house; so, I was unable to test it again today. Temp was in the low 90's or high 80's. I am pretty convinced that with anything with a 16" or less barrel NEEDS the carbine gas system in order to function 100% with just about any ammo. |
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Posted: 7/1/2005 12:53:32 PM EDT
[#32]
Good to hear that you got some accuracy and better to hear that I was right. The rifle length Dissy I have is 16.25" and runs fine with all ammo, from junk Wolf ammo to semi-hot 77gr handloads. I think that less than 16" can work and has worked with rifle length gas systems, but you need to bring up the gas port size gradually until you get optimum operation. Too much gas is bad also. I feel the problem lies in the fact that somebody thinks that, say a 0.100" gas port that works in one will work in all. This is truly a custom build and each should be treated that way. I think the gas port could very well be unique to each setup, especially if you are swapping out bolt carriers, buffers, etc. Changing the moving mass can change gas requirements, to a certain degree. |
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Posted: 7/1/2005 1:10:04 PM EDT
[#33]
I think that you are right on the money: moving parts will change gas needs, and each build is unique.
I did note that the FSB leaks gas all around the band where the gas tube is pinned. Well, I think that I will dissassemble this thing and sell it for parts. I would use the barrel for a tomato stake, but its too short. Its too small for a boat anchor. It is a rather useless piece of steel. Edit added: I got my money back from M1S. My $490 is lost on this one. From here on with AR15 stuff, I will ONLY buy from vendors who GUARANTEE the accuracy of their barrels and INCLUDE a test target to prove it. |
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Posted: 7/1/2005 2:37:45 PM EDT
[#34]
I bought one of the DPMS 16" Barrels with the 20" sight two weeks ago from Midway and it shoots fine. Shot couple hundred rounds through it no problem. Barrel quality has been a real problem for me lately also.
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Posted: 7/18/2005 5:05:45 AM EDT
[#35]
Since I have $490 in this thing, I figured that instead of throwing the lemon away, I may as well try to make lemonade. I drilled the gas port to 7/64 inch (0.109"), but it still short cycled...just swell. Last night, I drilled it to 1/8 inch (0.125"). 0.125" is over half the bore diameter! Its already broke. How much more broke could it get?
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Posted: 7/18/2005 5:10:44 AM EDT
[#36]
My experience says that 0.125" is excessive, but I've only done three to date, and only one has a significant number of round through it to mean anything. My gut reaction with this is that you may have some other issue, but I can't say what. Overdrilling is no big deal anyway, as you can always add an MGI adjustable gas tube to tame it down if you go too far with it. Good luck. |
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Posted: 7/18/2005 6:09:09 AM EDT
[#37]
IF I can get this thing to cycle, how will I know that I have excessive gas.
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Posted: 7/18/2005 6:24:13 AM EDT
[#38]
Just a guess, as I don't have too much experience with this, but you may see FTEX (fail to extract), leaving a fired case in the chamber, because the bolt is being flung back too fast by excessive gas. If you have more info, correct info, or detailed info, please add as appropriate. The way I built mine was to start with the original gas port and bring it up slowly in size until the short cycling stopped with all ammo. In theory, I think that I successfully went from undergassed to properly gassed, at least that's what I am hoping and seeing right now. |
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Posted: 7/18/2005 8:22:28 AM EDT
[#39]
Try this test:
Remove the bolt carrier from the upper receiver. Keep the bolt in, but push it in so that the gas rings seal the chamber under the gas key. Spray/pour oil around the gas key-to-carrier mount. Using an air hose or a straw, hold in the bolt while blowing air into the gas key. Look for bubbles around the gas key seal area. if there are bubbles, that indicates the gas key is not sealing to the carrier properly and requires lapping. The only other thing I can think of is worn out gas rings or the gas tube mushroom is too worn out. I found my gas key was not sealing properly (RRA assembled bolt carrier group, purchased from RB Precision, a vendor on ARFCOM) and I did not bother to check the gas port. It was short stroking on Wolf but worked fine with XM193 and M855PD. I do not blame the vendor, but do in fact blame whoever assembled the piece. I don't think anyone currently checks for this type of leakage and they just assume it will seal. Many people with AR15s don't use Wolf and stick to the domestic brass cased stuff. |
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Posted: 7/18/2005 11:14:31 AM EDT
[#40]
+1 |
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Posted: 7/18/2005 11:57:01 AM EDT
[#41]
If I went too far with it, too bad. This upper was messed up from the start: short cycle city! If its too big, I doubt that I will put close to $60 MORE in it to make it work, when it should have arrived in proper working order. If a heavier recoil buffer will do the trick, then MAYBE. If it doesn't work, the barrel is trash, and I sell the parts for what I can get. I am done with this damn thing.
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Posted: 7/18/2005 12:14:16 PM EDT
[#42]
Try the gas key leakage test. |
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Posted: 7/25/2005 9:46:07 AM EDT
[#43]
It worked smoothly.
10 rounds each: Wolf Swiss GP90 Radway Green SS109 Georgia Arms SS109, "canned heat" Hornady TAP Hornady Varmint Express, 55 grain. No FTE or FTF! Felt recoil was same as WOA SPR upper with the same rounds. Case web unfired=0.374+"; after firing, 0.375" I have no answer why these loads would not cycle when the gas port was 7/64", but will work at 1/8". Edited to add: I do not appreciate having to fix something that should have worked from the start. 
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