Charging handle cycling with bolt on AR

[Fuel Fire Desire]

I'm not sure exactly what's going on here and am looking for opinions.


I have an ultralight AR build who's charging handle is cycling with the bolt while firing. It becomes unlocked, travels about 1.5" rearward, then closes with the bolt. When forward and locked, it's locked tight. I can apply quite a lot of force to the handle and the latch will not come undone. I can cycle the bolt with my fingers in the upper and feel no dragging on the gas key in the handle. The handle glides freely in the upper and over the bolt.

The pertinent parts are a Faxon ultralight DI BCG, SanTan billet Pillar upper, and a SI charging handle. The gun does have an adjustable gas block, and it is tuned correctly (last round barely catches the BHO).

This is the position of the handle after the last round locks open. This is the farthest rearward travel it has on its own, but glides freely if pushed/ pulled.
IMG_2871.jpg

The notch in the receiver is still crisp, though cut shallow.

IMG_2872.jpg


The latch, while peened (aluminum catch), still has a crisp latching surface.

IMG_2874.jpg

The peened strike face of the latch.

IMG_2876.jpg


There is a small amount of chatter where the end of the handle sits in the receiver when it's at its rearmost position when cycling on its own. The handle has a square sliding/ bearing surface at its tip so the whole handle isn't dragging, and this is what's chattering. I don't know if this is a symptom or not.

IMG_2880.jpg
IMG_2878.jpg
IMG_2879.jpg



The handle is not hitting me in my face while shooting, and it is not impeding performance. I could care less about the wear on the handle latch, but am concerned about wearing out the latching surface on my not cheap upper.

The rifle has 300-500 rounds through it so far. I believe this started to happen around 200 rounds ago, but it happened on just about every round in today's 100 put down range.

There is no evidence of the back of the gas key striking the back of the charging handle or dragging inside it.

Any thoughts?

[10x25mm]

Is the back of the bolt carrier key or its sloped tube section making contact with inside rear of the latch at full bolt carrier travel? Your buffer may be too short [or have shortened due to impacts], or the inside rear of your latch may not have been milled back far enough. It could be a dimensional stackup problem.

The only previous cases of this I have seen have been due to gun gas leaking at the back of the bolt carrier/key interface and pressurizing the inside of the upper receiver. But this is pretty obvious because the entire interior of the gun is filthy.

[dougwg]

bring it by the shop, I'll take a look for you.

[Fuel Fire Desire]

There is no obvious wear on the gas key, or rear underside of the charging handle. Bolt over travel was my first thought too, as I do not have a standard buffer. It has a TacCom Rifle Recoil System, where the bolt sits on a stubby piece of Derlin and the bump stop at the back of the tube is 4" long with a soft rubber insert.

I'll have to do more bench testing beyond what range observations I've made today to rule that out though. It's still on my mind.

bring it by the shop, I'll take a look for you.

The Taylor address in the link below? I appreciate it. It will probably be early next week since I'm not in SE MI until then. I'll shoot you a PM when I have a handle on what day it would be.

...as long as you don't make fun of my meager AR building skills.

[10x25mm]

Dry cycling the bolt carrier group to full rear travel with a little modeling clay applied to the back of the gas key, or inside the rear of the latch, might tell the story.

[dougwg]

20224 Lorne St
Taylor, MI 48180

[Fuel Fire Desire]

I was able to take a closer look at the gun today. The bolt carrier stops a 1/2" short of the gas key striking the rear of the charging handle, and there is no peening marks on either the key or the handle. This really has me puzzled. I hope to stop by Doug's this week.

[10x25mm]

If there is no evidence of a carrier key gas leak, you are left with the possibility of resonance. A higher force charging handle latch spring, a lighter/heavier buffer, or a lighter/heavier charging handle latch would decouple your latch from whatever vibration generator is driving it.

Resonance is unusual in small arms, but not unheard of. The typical remedy is to make springs out of braided wire so they internally damp. The MG.42 drive and extractor springs were the first such resonance uncouplers; the AK hammer spring is the most common example. You can't do this with your latch spring, so fiddling the charging handle latch spring force, the recoil spring buffer, or the charging handle latch mass are your best options.

[Fuel Fire Desire]

If there is no evidence of a carrier key gas leak, you are left with the possibility of resonance. A higher force charging handle latch spring, a lighter/heavier buffer, or a lighter/heavier charging handle latch would decouple your latch from whatever vibration generator is driving it.

Resonance is unusual in small arms, but not unheard of. The typical remedy is to make springs out of braided wire so they internally damp. The MG.42 drive and extractor springs were the first such resonance uncouplers; the AK hammer spring is the most common example. You can't do this with your latch spring, so fiddling the charging handle latch spring force, the recoil spring buffer, or the charging handle latch mass are your best options.

Im kind of leaning towards this way. The charging handle and latch are ultralight, with at least 80% of the latch's mass opposite the fulcrum from the latching surface with a standard weight spring. It wouldn't at all surprise me if some weird impulse is causing the latch to jump. The point of this build was to make a near recoilless ultralight AR, of which I succeeded. There is almost zero rearward recoil, and very little forward impulse with as light as the carrier and buffer system is. Id almost say the rearward recoil and forward impulse is about equal. There may be some strange physics happening in between the two.

I thought the forward impulse may have hopped the latch, but in battery theres not enough clearance for the latch to move off its home. Something has to be happening while the bolt is in motion.


It was just a cheap ultralight handle. I might try to source and use a standard handle without the oversized latch if Doug cant figure it out either.


Yay race guns....

[10x25mm]

Here is a writeup on a new buffer design which dramatically attenuates AR vibrations:

http://www.thefirearmblog.com/blog/2...recoil-buffer/

The money shot is:



This buffer increases x-axis primary resonance frequency from the .39 Hz of a stock 'H' buffer to 8.9 Hz. Resonance vibration displacement is inversely proportional to resonance vibration frequency, so this is a dramatic reduction in excitement to other components.

Should dramatically reduce your latch jumping problem. Of course, a GI charging handle with its minimalist steel latch would be cheaper.