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Posted: 9/5/2009 3:53:57 PM EDT
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Ok.. i recently turned my Armalite 16" into a Mk18mod0 clone..
that meant a 10.3" barrel and a gas port modification.. (opened up to 0.075") No modifications were made to the gasrings, extractor spring, or buffer, yet.. I went to try it earlier this week, and here were the result. Shooting Swiss GP90 ammo (match grade military 5.56 ammo) , the 1st x 5 shots went fine.. Clean shots and good ejection.. after reloading 5 more rounds of GP90, I had malfunctions.. Double feed it seems.. every time. I tried to only load one round at a time, and I discovered that the bolt locked back after each shots, as it should, that the case was out ot the chamber, but still in the receiver, which obviously caused the double feed. I could reproduce that every time.. failure to eject.. Since the bolt locked back, i assume that the gas port iss wide enough, and that enough pressure was brought to the gas key.. In your opinion, would have upgrade of the extractor spring be enough to fix the problem ? 2nd test was made with Wolf ammo (cheap russian stuff, steel cases). One round at a time,and now the ejection went fine every time, but the bolt didnt reach the catch.. and closed after each shot.. Now, i think, with this ammo, there is not enough pressure, making the bolt move slower, and though allowing the extractor to do it's work properly.. but then, not cycle the bolt completely. Now, here is my question: what you guys would do ? open the gas port a little more ? upgrade the extractor spring, and maybe use a heavier buffer ? I dont want to overgas the system, but still, i want it to be reliable, and shoot a wide variety of ammo, even if I will shoot GP90 95% of the time.. WHAT would YOU do with these symptoms ? Thx a lot for your help ! |
| How much use did the rifle have before you changed the barrel? It seems like failure to extract is caused by a worn extractor or bad extractor spring or the extractor unable to grip the rim correctly. Of course working with Wolf, sugests this is not the problem. Can you measure the rims on the Swiss ammo that did not extract and the Wolf that did? Also, check that the extractor claw is not fouled with gunk. |
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I would make sure the gas port is in spec with the barrel and the carbine gas tube was properly installed. I would up grade to a heavy extractor spring with black insert and install a H3 buffer with a carbine buffer spring. Double feeds are magazine related so try other quality magazines and ammunition. Check the entire gas system including carrier key but first start off with the basics, Magazines and ammo.
I have a 11.5 Colt and use a H3 buffer and never had any issues even when I run into using under power ammo. |
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the rifle and the bolt had less than 600rounds through them, so i'd say that there is nothing too worn out in there.
I have a spare full auto bolt, so I may try it instead, next time. My guess is that the widened gas port, with GP90, is putting more velocity to the bolt assembly, and that the extractor is too weak to work with this speed. The uncomplete cycle with Wolf ammo suggest me that the gas port is not wide enough, and opening it a bit more, will increase the problem with the extractor using GP90.. So definately, i think the extractor upgrade is a must do.. My plan is to try again tuesday with a D-fender installed. if it works with the GP90, I will have the gas port opened up a bit more (like 0.080 or 0.085) and see if it also work with weaker ammo, such as WOLF or BARNAUL. If you have any other suggestion, you're welcome. |
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Double feeds are magazine related so try other quality magazines
Using pmags that perfectly work with my 4 other AR's, so I dont think it's mag related at all.. as for the gas system, I think it is perfectly aligned. I build the upper myself, and I ensured that everything was well in place. The steel case Wolf might be letting the weakened extractor still do its job,
due to the case not expanding as much? probably.. but I think the reduced velocity of the bolt is the main factor.. When the same gun was 16", I had no problem whatsoever shooting or extracting the GP90srounds... Thx for your help guys. |
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Did you shoot it before opening the gas port? Mine runs fine unmodified..............
no.. I first had it slightly opened.. But the fact that it doesnt work (doesnt cycle completely) with Wolf ammo, tend to make me believe that it's not open enough right now.. Only the good stuff in the SBR
seems a good advice ;) |
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I'd check the gas key to make sure it's not loose. Also, what buffer are you using, 9mm? Carrier?
A friend was having mysterious cycling problems too, we swapped out the buffer, magazine, tried different ammo. Eventually, a Young match M16 carrier solved the problem. |
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Quoted:
I just put my spare m16 carrier it it.. it seems that the bolt that was with it even has a better extractor spring. A will also try tuesday, with a D-fender.. My buffer is a H buffer- Is this full or semi-auto? Like another poster said above, don't try to make this run with Wolf. Save that for carbines or full length guns. Shorties require premium ammo. Or at least decent brass cased stuff. You need to bump up your extraction force as you stated. You could also use a heavier buffer as long as it will cycle ok. With ammo that has the proper port pressure, 10.5s will run with a .070 port and whatever buffer is in the lower. They are just soooooo picky on port pressure and other factors. There just isn't enough dwell in the system for much tolerance of important factors like port pressure, lubricant, temperature sensitivitys, etc. They can be reliable, but if anyone thinks that they are as reliable as a full length gun, they need their head examined. They do look really freaking cool tho'  
Brett |
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Is this full or semi-auto?
this particual build is semi, but as I own also a Colt M4 built on a M16A1 lower, it "could be" full as well ;) Shorties require premium ammo
That is a good thing to know. I will stick to that then. Don't wanna overgas the system. You could also use a heavier buffer as long as it will cycle ok.
I will bring some heavier buffer with me to try. With ammo that has the proper port pressure, 10.5s will run with a .070 port and whatever buffer is in the lower
Yeah, that is what I read in the NAVSEA documents as well.. thank you for your help, much appreciated- Picture attached 
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Quoted:
Ok.. i recently turned my Armalite 16" into a Mk18mod0 clone.. that meant a 10.3" barrel and a gas port modification.. (opened up to 0.075") No modifications were made to the gasrings, extractor spring, or buffer, yet.. *snip* i believe the correct port size for guns in the 10.5" range is 0.093". either way, you might want to open it a little at a time until you get it to function 100%. |
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When you say that the ammo that works "ejects fine", how do you mean?
Where is the brass ejecting to? Does it eject straight out the side, to the rear, to the front? If the muzzle is 12'oclock on a watch, and straight out the side is 3'oclock, which direction does the brass fly out of the gun (when it does eject). Also, how far from the gun does it go? 3 feet? 2 feet? 6 feet? You also say you havn't done anything with your buffer. Is it a "standard" buffer or is it already an "H" buffer? More info is needed before proper diagnosis can be made. p.s. I have a 10.5 LMT upper (on a pistol build) that runs every ammo I put through it just fine, including wolf. |
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I am a pilot.. I know how to work with clock directions ;)
When I said the ammo ejected fine, I meant they were going about 3 o'clock, as they always do with my AR's. How far, i couldnt tell, since they were hitting the wall on my side. I'd evaluate it to more than 5 feet without the wall. My buffer is a standard buffer. p.s. I have a 10.5 LMT upper (on a pistol build) that runs every ammo I put through it just fine, including wolf.
that's great, and it is how it should work. But keep in mind that mine is a modified upper, with a barrel brought to 10'3".. it sure needs some tweaking.. Thank you for your help ! |
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I tried to only load one round at a time, and I discovered that the bolt locked back after each shots, as it should, that the case was out ot the chamber, but still in the receiver, which obviously caused the double feed. I could reproduce that every time.. failure to eject.. Break down the bolt and check for carbon build up or brass shavings in the ejector channel or around the ejector itself. Lube per the -10. If that doesn't work, start with a carbine buffer and work your way up the weight chain with a single type of quality ammo. Don't use wolf. |
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I you are looking for an adjustable A2 style front sight this is your best chance: MidwayCH
I believe that the very short AR15 works best with a muzzle device like the Noveske. But I have no idea where you could find one in Switzerland. |
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I am NOT paying 231chf for another a frontsight, I tell you, even adjustable !
I will tomorrow try again, with a heavier bolt carrier, a D-fender'ed ejector spring, and GP90 .. gas port untouched yet, at 0.075. From there, I will then maybe try a lighter buffer with weaker ammo than GP90, like wolf and Barnaul.. But I'll stick with GP90 with this gun, as often as I can.. Thx for your help guys ! |
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Quoted:
Quoted:
Ok.. i recently turned my Armalite 16" into a Mk18mod0 clone.. that meant a 10.3" barrel and a gas port modification.. (opened up to 0.075") No modifications were made to the gasrings, extractor spring, or buffer, yet.. *snip* i believe the correct port size for guns in the 10.5" range is 0.093". either way, you might want to open it a little at a time until you get it to function 100%. No offense, but don't open it up. Yeah, you can open it up to .093 and then throw the heaviest fucking buffer in there that you can find just so that it won't rip the rim off the casing. Yeah, you are going to need the super duper extractor spring so that it doesn't suffer from dropped extraction. In a lightweight gun like a MK18 or whatever you want to call it, a .093 will slam the bolt carrier group into the stock so hard it will feel like you are shooting a 7.62. 10.5s are always on the verge of either running too fast or not running at all. The system is completely out of balance. That is why they are so ammo sensitive. You have no dwell and thus no sweet spot. As short and hard running as most 10.5 are used, they burn the port out fast enough as it is. .075 is plenty. It is just going to run harder and faster the more rounds you put through it. If you want a shortie that will run with a variety of ammo, get a 11.5 or better yet a 12.5 so that you have enough duration to close the port down. A colt style 9mm buffer like PK Firearms sells has a steel body on it, which is the same thing as adding carrier weight, is one of the best things you can do for semi auto use. However, they have the same sliding weights in them as a carbine buffer, so they do not work that great for full auto because they do not have the dead blow effect like a H2 or H3. You could try it, but you will probably have carrier bounce and the gun will stop hammer down with a live round in the chamber. You can always try an Enidine hydraulic if you can get one and do not mind the extra force it takes to lock back in manual operation or the extra force it takes to drop the bolt when you hit the bolt catch. They smooth things out dramatically, but some people like the brapp-brapp high cyclic rate they get with a standard H buffer. The first two shots will hit very clost to each other if you can get off the trigger fast enough. For mag dumps, the enidines rule. Just to put in a little different perspective, when you get that short, the chamber sidewall dimensions become just as critical as the gas port size and the condition of the gas rings in the bolt group. I do not know how you can get either one in Switzerland. I am speaking from experience, I've built and test fired more AR15/M16 tops than most people on this board have ever seen, other than vendors and some of the people that take a lot of classes. Brett |
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thx for that complete answer.. it confirms me what I thought after reading through this post.
I will not open it anymore, and usge GP90 ammo with a good extractor.. maybe I will use a lighter buffer with weak ammo's when i have only them.. As for all the stuff you mentionned (ex: Enidine hydraulic) they are not readily available here, but I can always get them from the USA, one way or another. no big deal. i'll keep you up about my testing. next range trip, tomorrow. |
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Ok, as promised here are the results of my latest test shooting.
I took the older bolt assembly out, and put in a new one (M16) with a stronger ejector spring. no change made to the buffer or gasport. 30 GP90 were shot with only 1 double feed due to bad extraction.. I than installed a D-fender on the ejector.. 30 more GP90 were shot, without any malfunction. 100% ejection, and bolt locking on empty mag. I than tried Barnaul steel ammo. 30 more rounds- 100% ejection and functionning. Only problem, the bolt doesnt cycle far enough, and the bolt assembly doesnt lock when the mag is empty.. I guess the Barnaul, like the Wolf, isnt powerfull enough to make my bolt cycle completely. I won't open the gasport anymore, but will try to get a spare lighter buffer, to shoot crap ammo, if I need to. Next test will be on thursday.. I will put 200 rounds through it, and see how it goes... Anyway, with the good ammo (GP90) the D-fender seems to have made miracles ! I LOVE THIS GUN ! |
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This is still with the H-buffer?
Semi or full? You could probably even step up to an H2 with a .075 port, with the GP30 ammo. Just depends if you want to mess with it or not. It will slightly slow the cyclic down, but it might not even be noticable without a shot-timer. Still, it is suprising though how small changes can make a difference in the feel of the gun. As far as the Barnaul goes, most of it seems to cycle harder than most wolf,however, you probably will not get good full auto operation with less than a H-buffer. Just not enough dead-blow effect to control carrier bounce. Semi auto only use, sure, lighten the buffer up to get it to lock back if you want, then again, is it really a big deal if it cycles fine, but just doesn't lock back when it is empty? Glad to hear that you consider your chop-job a success! 
Brett |
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New parameter, and NEW failures... Help needed-
Ok, yesterday, I got my new handguard.. It is a Free float KAC RAS, and I installed it in place of my old KAC RAS (non freefloat) . To do that I had to once again remove the flashhider, frontsight pins, frontsight, forend cap, and gas tube.. Than I removed the barrel nut... I put the new KAC RAS FF barrel nut with the screwing cap, put the handguard in place, screwed it in , put that gastube and frontsight back in place, and finally the fronstight pins and the flashider.. And today, I was back at the range to test all of this.. I was using only matchgrade military GP90 ammo.. after a few rounds, I got double feeds again.. i feared an extractor problem, so I checked one round at a time.. and extraction and ejection always went fine.. but problem, the bolt did not LOCK BACK on the mag.. which means I had cycling problems... the bolt assembly didnt go far enough backward.. problem that i did NOT HAVE the day earlier, using same ammo, carrier, buffer, spring etc.. So I thought.. I am loosing pressure somewhere.. checking near the fronsight, I noticed that the barrel around it was all dirty.. I had a GAS LEAK around the frontsight (gasblock) - By touching it, I noticed that even if the pins were fully in, the fronsight was still slghtly moving... The only difference with the day earlier (in which I had no problems anymore) is that I dont have the forend cap anymore (not used with a free float handguard) and it leaves a little space behind the fronstight (gasblock) and where the barrel grows bigger (something like half milimeter) and that is where the gas is coming out I guess... Why do I have a leak here ??? I never had, and I still use the same frontsight, same frontsight pins, and same barrel on the same position.. Can the removing of the forend cap have produced a leak that was sealed by the forendcap itself ? How can I seal this leak according to you ? thank you for your help.. I am lost here.. I don't know how i could seal the frontsight better than that. If you need pictures to make things clearer, I can do them. Just let me know ! Thx for your help ! |
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Oh my...
Stupid is as stupid DOES.. And I did... my frontight pins were mounted the wrong way.. right to left instead of left to right, like my other AR's.. put them back the proper way, and th frontsight is solid fixed now... if it's not enough, i'll use spare conical pins that I have, because these were straight, and I am unfraid the number of times I had them in and out might have damaged (used) them. if you have any other things to make me notice (except that I am stupid) please go ahead :) |
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You're not stupid, I think you've done an excellent job at troubleshooting these issues and I look forward to hearing about your next range report! I have a feeling that your gas port might be a tad too small for cycling some of the underpowered loads but If you are satisfied with it then I'm sure it's fine.
I opened the gas port on my 7.5" pistol because the bolt wouldn't lock back, even after tweaking with everything that was un-doable. I was worried about over gassing the bolt so i went by drill bit sizes with the backup option of buying an adjustable gas block. It was originally .075 so the first drill bit bigger was #48, .076. I intentionally only loaded mags with 1 round because I knew it cycled I just wanted to make it lock back, unfortunately the first size larger wasn't enough to help the system. . Next was #47, .078 with the same result, wouldn't lock back. Then #46, .081 Almost! It occasionally wouldn't lock back, approximately 1 out of 10 times. So #45 or .082 which was only a thousandth larger... it worked! Opened the port a total of .007 which made all the difference. I've been shooting it for a while now and haven't had to replace the gas rings yet so I'm pretty sure the gas port isn't too big. Painfully slow process but I'm glad I did it. |
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