Help with 11.5" 5.45 and 7.62x39 GP uppers

[towerofpower93 22]

This is up on ar15.com as well, but it's not getting much of a response and there are smart folks here as well so....

 

I'm throwing this out there for all who are smarter than I...and there a bunch of you.

 

The lower: Del-Ton with standard power hammer spring, mil-spec buffer (not exactly sure on the weight, but it looks just like the one I had in the Army) and buffer spring.

 

Magazines: 3 x each: C-Products 30rd 7.62x39 and 5.45x39 magazines with the black, seemingly not anti-tilt, followers.

 

Ammo: 90 rounds of 7n6 surplus 5.45x39. 80 rounds of 124gn HP 7.62x39 Wolf Military Classic (desert camo box). Both steel cased, Wolf being grey in color and the 7n6 being green laquered/painted.

 

The issue of light primer strikes only occured with 10-15 of each type of round and I fully believe this was caused by the lack of a stronger hammer spring; that issue will be dealt with shortly and was only ommited initially to see if one was actually going to be needed. (The "If it aint broke don't fix it" mentality attempting to enter the world of firearms.)

 

Upper 1: Model 1 Sales 5.45x39 upper with 11.5" barrel and Adams Arms Gas Piston System installed. Bolt, firing pin, cam pin and firing pin retaining pin are all from Model 1 Sales with the bolt carrier and bolt spring (the spring on the backside of a bolt normally used in a GP set up) coming from the Adams Arms GP kit. GP kit installed by Marshall Firearms in Jupiter, FL on the upper brand new, never fired.

 

Issue: At first the upper did not want to eject, I chalked it up to brand new rifle and sure enough, half way through the first mag it began spitting cases out...but it wouldn't strip a round from the mag. Out of 90 rounds it only stripped 3 on its own. When I manually cycled the weapon after getting a "click" it would pull the rounds fine. Had a few instances where the round would eject and it would think about stripping a round but would end up riding the side of the casing and jamming the round nose high without it entering the chamber. Again, I would manually cycle the action back just enough to "unfuck" (very scientific term) the round and getting it pointed towards the chamber, let the charging handle go and it would feed fine. I would then fire the round and again, no round automatically stripped from the mag and fed into the chamber. As I stated earlier, there were only 3 instances where the rifle would fire a round, eject the spent casing and feed a fresh round into the chamber. Additionally, when firing the last round from the mag, the bolt would not lock to the rear. It seems the follower was tilting forward and not activating the bolt hold open.

 

Upper 2: Model 1 Sales 7.62x39 upper with 11.5" barrel and Adams Arms Gas Piston System installed. Bolt, firing pin, cam pin and firing pin retaining pin are all from Model 1 Sales with the bolt carrier and bolt spring (the spring on the backside of a bolt normally used in a GP set up) coming from the Adams Arms GP kit. GP kit installed by Marshall Firearms in Jupiter, FL on the upper brand new, never fired.

 

Issue: While the 7.62 upper behaved like the 5.45x39 one initially, as in not ejecting spent casings, this one never seemed to loosen up. In 80 rounds fired I got 3 to eject. It seemed to feel like the bolt would uncouple from the chamber but would slam right home again. Half of the time the bolt was difficult to pull back (had to drop the mag and ride the charging handle on a table while pushing down 20% of the time in order to get the spent casing out) and some times it would nearly get the case out, but come short and have it half stuck in the chamber or would hang up on the body of the upper receiver itself.

 

My thoughts for the 5.45 upper: 1. The upper is new and just needs some time to break in.

 

2. The bolt spring is on a bolt not designed to be used in a GP system. Maybe this is causing the bolt to stay forward in the carrier and is not getting enough clearance to strip the round. I don't think this is what's going on though since manually pulling the bolt to the rear and letting it go strips a round.

 

3. The shortened barrel on a non-5.56 weapon is causing a lack of pressure into the gas block and causing the bolt carrier to move far enough and with enough force to eject the spent case, but not far enough to the rear to strip a new round.

 

My thoughts on the 7.62 upp: 1. The upper is new and just needs some time to break in.

 

2. In line with number 3 above...the shortened gas system on a non-5.56 weapon (this one even more different from 5.56 than the 5.45 upper) is causing a lack of pressure to actuate the gas piston system.

 

The only thoughts that come to mind concern the GP system. I have a 16" 5.56 CMMG GP upper that ahs ran flawlessly from the box, so I know a bit about the system, but have never wandered in the world of custom uppers...like the ones I had ordered and then converted to GP.

 

Could it be a buffer/spring issue? I'm not smart in that area and hope some of you are.

 

Could the solution be as simple (not really, but at least it's a starting point) as drilling a larger hole in the barrel in order to allow more gas to be siphoned off into the GP system before the bullet leaves the barrel...just like the Krinkov's in 7.62 and 5.45 have larger holes to accomodate a larger volume of gas into the gas block?

 

As I said in the beginning, there are many out there who are smarter than I and I'm seeing what you guys have to offer.

 

Sorry for the research paper's worth of info, but I thought it would be best to get as much detail in the initial write up as possible.

 

Thanks much

 

Eric

 

ETA: Some may ask why would you get 11.5" uppers and put some GP kits on them. My answer is this is America....land of cheap surplus ammo in these two calibers and when my M16 registered receiver gets approved in a few months it will be much cheaper to feed if it's shooting com-bloc surplus. Plus, the 7.62 gives a fantastic fireball out of an 11.5" bbl:)

 

Additionally: both bolts had their gas rings removed to operate in the GP bolt carrier.

[Casp 119]

Here's my take on the problem, others may or may not agree. There are a lot of things to consider here.

 

You may (probably do) already know most of this, but it bears repeating in general: The pulse of gas that hits the piston lasts for a certain period of time which we call dwell time. Dwell time starts when the bullet passes the gas port and ends when it leaves the muzzle. As the distance between the gas port and the muzzle decreases, dwell time gets shorter and reliability goes down.

 

One possible fix is to enlarge the gas port, but a more-powerful-but-still-too-short gas pulse can make it overgassed and open up another can of worms. That in turn can be compensated for by a heavier buffer, and then it will generally be much more finicky about ammo than it would be with a longer dwell time; a much narrower sweet-spot for what ammo it will run with. The other fix obviously then is to make the barrel longer. Colt did this on the XM-177E2 with a miniature sound suppressor (called the Colt moderator.) Unfortunately, the BATFE actually did classify it as a sound suppressor, but Noveske's KX3 "fire pig" flash hider does the same thing and is BATFE approved as not being a suppressor (and they make it in 7.62mm also.) However, the added length from the expansion chamber begs the question "why am I not just using a longer barrel?" It may be easier for you to attach a KX3 than to swap out the barrel; that's up to you.

 

If adding a KX3 (or using a 14.5" barrel) doesn't fix it on it's own, and enlarging the gas port doesn't fix it on it's own, then adding the KX3, opening up the gas port, and using a heavier buffer should do the trick. Which route you choose to try first is up to you. The KX3 isn't cheap (but then you're getting a reg'd M16 lower so your definition of 'cheap' is probably MUCH different than mine ) but it doesn't involve permanent alterations to the barrel.

 

I'd also like to add that it's usually a good idea on gas piston converted guns to use Slash's Anti-Cant heavy recoil buffer; they seat into the back of the bolt carrier and prevent the tail of the carrier from digging a trench in the bottom of the buffer tube. Downside to those is that disassembly requires you to pull both receiver pins and remove the upper in a forward motion; you can't just pull the rear pin and swing it open like usual.

 

Your recoil spring shouldn't be causing any problems. If you do feel that a heavier spring may be in order for fine tuning after everything else is said and done, Wolff is your best bet as usual.

 

On the subject of light primer strikes, a heavier mainspring will help but another thing to consider is that MGI makes a bolt and firing pin designed specifically for the x39; it duplicates the larger pin diameter and greater protrusion (I think) of an AK.

 

If, after all that, the x39 still doesn't feed reliably, then it's probably the magazine. It's long established that you just can't force (more than 6 rounds of) 7.62x39mm through the AR-15's magazine well with any reasonable amount of reliability; it demands a much more pronounced curve to the magazine, as is visually evident just by looking a standard AK mag. C-Pro I guess claims to have solved this issue with their x39 mag, but I'm highly skeptical and wary of that...

 

[Azrial 1,091]

Good Response CSM!

 

But, I might add, trying a pig tail gas tube as a possible solution in adding dwell to the equation!

[Casp 119]

Az: That is true but if he's using a gas piston conversion that doesn't really apply...

[towerofpower93 22]

Thanks for the help gents.

 

Funny you talk about the anti-can't buffer...I had a 6.5 oz one and a wolff xp spring on the way before intook these out for the first time.

 

My plan now is to try the heavier buffer, thinking the longer lock time will cause better recoil impulse, and if that doesn't work ten to try a sligtly larger hole in the bbl. He hole in the gas block is roughly 1/3 larger than the hole in the bbl, so I have room to play with.

 

Thanks for all the help gents, funny that I get a faster response for an AR issue on a Russian shotgun forum than on AR15.com...though a couple guys in the piston section are givig advice on bbl hole size.

 

Eric

 

ETA: csm, no worries on explaining dwell time. I know what it is, but it may help others who look at the topic:) Knowledge is never a bad thing.

Edited December 6, 2010 by towerofpower93

[whitetrashrn 74]

Tower, a little heads up. CSM is a very bright, gun savy, female. Brings a lot to the table here especially in the AR area.