Short cycling

[spleify]

I have been having an issue with one of my AR's short cycling(I dont know if thats the right name or not).

I load up a mag, pull the slide back and release. Pop off the first round and the slide does not go full cycle and doesnt grab the next round. I can see it stops short, and doesnt extract the spent case either.

What would cause this, and what are some tips for fixing it?

Thanks

[trlcavscout]

Gas ports not ligned up on gas block, or rings damaged, worn, all notches lined up? Or gas tube/port plugged? That would be my guesses.

[DeusExMachina]

Take it to Bowers', Rob will fix you up. But tricavscout is right.

[spleify]

Yeah this weapon is almost new, only had a couple hundred rounds through it. I can see that when I pull the charging handle back that it is not going into full battery. something is restricting it.

[DeusExMachina]

Yeah this weapon is almost new, only had a couple hundred rounds through it. I can see that when I pull the charging handle back that it is not going into full battery. something is restricting it.

Uhh if its not going into battery, don't shoot it.

What gun is it, or what are the parts?

[FireMoth]

Yeah, hard to diagnose this over the internet. Possibilities run from incorrect extractor, incorrect action spring, short chamber, incorrect locking seats, incorrect bolt, damaged or incorrect cam pin, damaged or improperly installed firing pin retainer, worn or damaged gas rings, Bent or damaged gas tube, clogged gas tube, mis aligned gas block, and even including an improperly tightened or loose barrel nut.

Just pop by the shop and we'll figure it out.

[spleify]

I'll swing by Rob, thanks!

[Hoser]

Reloads or factory ammo?

[Mick-Boy]

There already seem to be some pretty smart folks on this (aside from Hoser [Neene3]) but you might add incorrect (too heavy) buffer to the list of possible culprits for the bolt not fully cycling. For the weapon not going into battery... I'd say get it to a someone to check out before you shoot it again. But then I'm kind of attached to my skin. YMMV. ;)

[BPTactical]

Just a SWAG but I'm gonna guess the gas key is the culprit. Loose would cause the short stroke (leaking gas) and also the FTGIB if it is misaligned.
I have noticed a few new rifles with poorly fitted/staked/torqued gas keys.

[MadRabbit]

Using Wolf ammo?

[FireMoth]

OK, guys, here's a new one:

How about an unknown brand 'mil-spec' receiver extension that was too short in internal dimension?

Tried backing it off a revolution, but would not retain the buffer retainer. Replaced with a proper mil spec tube, and problem solved.

This was of the variety that has a finish that looks like paint (not anodized).

So be careful on inexpensive parts. Even some of the less regarded but known name makes at least keep this spec right.

[DeusExMachina]

So the bolt wouldn't physically go back enough, not just when fired?

[FireMoth]

Correct. Dialed out one revolution, it would go back to lock, but the lip to retain the buffer retainer was too short, and let go of the retainer the first time it was manually cycled.

New receiver extension, problem solved.

[DeusExMachina]

That must have been pretty short.

[FireMoth]

Hey, hey, hey... lets not talk about the length of a mans equipment, ok...

[SAnd]

Some numbers from my notes made about a year ago-

Measured from the long part where the buffer retainer sits.

Colt A2 rifle stock, 9 3/4 inches to the square bottom.
ArmaLite A2 rifle stock, 9 5/8 inches to the square bottom.
Armalite Adjustable A4 carbine stock, 7 11/16 to the square bottom.
Tapco Adjustable T6 stock (#1), 6 3/4 inches to the top of the bottom cone.
Tapco Adjustable T6 stock (#2), 6 5/8 inches to the top of the bottom cone.
Short "Tactical" stock, 7 inches to the top of the bottom cone.

The bottom of the hole is tapered like the point of a drill bit on the two Tapco stocks.
The bottom of the Colt and ArmaLite stocks have square flat bottoms.
The Tactical stock has a square shoulder with a pointed center of about half the diameter.

The external over all length dimensions of the ArmaLite and Tapco adjustable stocks are the same.

[spleify]

Yep, that was definitely a strange one. Rob(like always) figured it out and I bought a new buffer tube and he installed that, a new castle nut and end plate, and she in back in service again!

Thanks again Rob, and I'm looking forward to running this baby!

[FireMoth]

Some numbers from my notes made about a year ago...

Good data. Thanks. We'll have to measure the offending tube to see what was different, though i should say it was at least .250" too short, and maybe even .5".

Where did you measure out to? bottom retaining lip, or the short part of the threaded ring? I want to have a realistic comparison

[ldmaster]

Given that if they made ONE like this, they made a LOT like this.

How many people have this on a rifle, and don't know of the problem because they have never shot their rifle?

[DeusExMachina]

It would be good on a 9mm upper. Bolt doesn't need to recoil as far.

[FireMoth]

Tube was pretty soft too. I'd use it for a stock mount on a dedicated .22, but little else.

[SAnd]

Good data. Thanks. We'll have to measure the offending tube to see what was different, though i should say it was at least .250" too short, and maybe even .5".

Where did you measure out to? bottom retaining lip, or the short part of the threaded ring? I want to have a realistic comparison
I stuck a 1 inch wide ruler that barely fit down the hole where the spring/buffer go. When it wouldn't go in further I masured to the area where the buffer retainer pin notch is located. At least that is what I remember. The difference between the long and short areas isn't much, maybe 3/32 inch, especially compared to the 15/16 (+) difference between the ArmaLite and Tapco stocks.

[FireMoth]

Cool, i'll give it a measure. I also found that the 'spoon' (my own term for the extension that retains the buffer retainer) is a pretty widely varying appendage. so i'll have to take that into account.

[spleify]

I measured mine from the inside bottom to the retainer lip, using the 1"tape measure like above and the one we pulled off is exactly 6 13/16"

[cstone]

More of a question for FireMoth; what extensions would you recommend and are there any you would stay away from? They seem like fairly straight forward choices, either mil-spec or commercial, but like most things, not everything is equal.

[FireMoth]

That's a tough one.

I prefer mil spec because in my experience the more square rib at the bottom results in less stock wobble, they are a little shorter, and a little lighter. I also feel that the larger threads (not a different thread dimension, just a larger tolerance .185" v. .170") gives a stronger mount and holds single point adapters better.

However, Commercial function the same from a cycling stand point, so its not like its a huge deal.

I would stay away from anything that isn't properly hard anodized. The cheapy ones tend to look spray painted, and some actually are.

Sadly, most of the time an offending part shows up on my bench, its of an unknown origin, so i cant damn any one manufacturer. I might hazard that Hesse/Vulcan/Blackthorn might have something do do with it, but that's speculation based on their other products.

To play safe, just use known name manufacturers. Doesn't matter if they are regarded as high speed by the 'list'. If they are known enough to be on the list, they are probably ok.

Inexpensive parts like those from DPMS and Bushmaster work fine.

Other than mil spec vs. commercial, there are differences in both that might or might not matter.
There ARE flat backed commercial receiver extensions.
Its diameter is what makes the difference, so that's what you should use to judge. Mil spec is 1.148", commercial is 1.168" on average. There are some minor dimensional differences in each type, so a couple thousandths wont be enough to fuss over,a s long as the stock slides

Another common difference is locking positions. A true Mil Spec M4 stock has only 4 positions. One fully collapsed, and three at the back end. Ther are commercial extensions with this configuration as well.

More commonly, we find a six position stock. Using the same spacing as the last 3 holes on the M4 extension, they simply fill in with 2 more positions close tot he front. This is common to both mil spec and commercial diameter extensions.

Finally, we have the 5 position stocks, such as DD and Vltor. These just have a larger spacing.

All 3 work.

I wish i had a comprehensive list of offenders, but the best i can say is if no one knows where it comes from, its probably not worth the risk.

[cstone]

Thanks for the information FireMoth.

I just finished putting together a lower and will probably be picking up a stock for it in the next few weeks. Based on your reputation and a recent visit to Bowers, I believe I know where I will be picking up the receiver extension and stock.

Maybe you could take a look at it to make sure it won't short cycle on me. [Flower]

[spleify]

Good information Rob! I wish the one that I had issues with had a name on it so we could steer clear of this model.

I think the best advice you gave was; "the best i can say is if no one knows where it comes from, its probably not worth the risk."