Charging handle cycling with bolt on AR

[Fuel Fire Desire]

I'd like to thank Doug for letting me come by the shop and taking time out of his day to help me out. We stared at it and talked for probably 30-40 minutes and still couldn't figure it out. No evidence of mechanical interference anywhere. We both decided the next step it to reset the gas, and try a mil-spec style handle.

I'll try that first and see where it goes. I don't have a scope hanging over the latch, so it doesn't NEED to be oversized.....but I liked the red. I might just swap the latch itself.

[Fuel Fire Desire]

The plot thickens.....

I finally had a chance to take it out again. Different ammo, regassed the gun, same issue. The charging handle even jumped with the gas system OFF.

I took some slow mo video with my iPhone to see the sequence of events. It surprised me.

1. Trigger is pulled
2. Gun fires
3. Charging handle latch moves completely to its rearmost position
4. Charging handle begins to move backward
5. Bolt unlocks and begins to move
6. Charging handle moves rearward and stops on its own at 1.5" rearward, moving independent of bolt
7. Bolt reaches rear most position of travel (does not strike buffer)
8. Bolt moves forward, catching charging handle and sliding both home.


I put my thumb behind the charging handle while firing. It hit my thumb with almost no force, even just 1mm behind the closed position. It did not move my thumb.

Here are stills from the video....


These first two are with the gas system completely OFF. No bolt movement.

Gun at rest, charging handle fully latched at home.

IMG_3137.jpg


One frame after firing, before recoil impulse moves rifle rearward, charging handle latch moves to its rear most position, striking its stop. Charging handle just starting to move rearward. (It only takes 1/4 the distance the latch has moved in this photo to unlatch the handle)

IMG_3136.jpg




Another post coming with more images....

[Fuel Fire Desire]

This is with the rifle properly gassed. Bolt does not strike the buffer stop, but reverses direction on its own and catches the hold open.


Rifle at rest.




Immediately after firing. Bolt is still locked and closed, charging handle latch unlocked, charging handle beginning to move.





Charging handle moving rearward. Bolt now unlocked and moving rearward.





Bolt and charging handle moving freely rearward. Bolt moving 4x's the speed of the charging handle.





Bolt nearing its rearmost position, ejecting brass. Charging handle still moving rearward.





Rifle at rest. Bolt locked open, charging handle resting where it lies 1.5" aft of home.

[10x25mm]

Charging handle latch appears to be vibrating due to resonance in the second image from the top. Resonance is probably due to recoil impulse since bolt carrier does not appear to have started moving. Latch is initially drawn forward by the recoil impulse, then reverses as it elastically bounces off of whatever is its forward limit stop (latch nose bearing against upper receiver lock slot floor?).

Higher force charging handle latch spring, or lighter charging handle latch will probably keep the latch in place. Grinding down the 'nose' of latch so it does not contact the 'floor' of the upper receiver latch slot would alter the latch's resonance response, but not necessarily stop the latch from bouncing.

Monroe Rifle & Pistol Club?

[Fuel Fire Desire]

Charging handle latch appears to be vibrating due to resonance in the second image from the top. Resonance is probably due to recoil impulse since bolt carrier does not appear to have started moving. Latch is initially drawn forward by the recoil impulse, then reverses as it elastically bounces off of whatever is its forward limit stop (latch nose bearing against upper receiver lock slot floor?).

Higher force charging handle latch spring, or lighter charging handle latch will probably keep the latch in place. Grinding down the 'nose' of latch so it does not contact the 'floor' of the upper receiver latch slot would alter the latch's resonance response, but not necessarily stop the latch from bouncing.

Monroe Rifle & Pistol Club?

I think you're exactly right. The initial impulse is springboarding that latch open.


The latch originally had a sharp pointed nose that sat right on the bottom of the detent. It has since eroded into a semi circular nose.

The spring is fairly stiff. More stuff than what a standard mil spec handle is. The latch is also a rather thin aluminum casting. Would you think a steel or thicker aluminum cast latch would stop the springboard effect?

I'm still probably going to try a milspec handle to see if there's a difference (I have to order one still). I might even email Strike Industies and show them what I posted here and see what they say. See if it's a known thing with this cheap-o handle.


Monroe rod and gun btw.

[10x25mm]

The latch originally had a sharp pointed nose that sat right on the bottom of the detent. It has since eroded into a semi circular nose.

The spring is fairly stiff. More stuff than what a standard mil spec handle is. The latch is also a rather thin aluminum casting. Would you think a steel or thicker aluminum cast latch would stop the springboard effect?

The two key properties at play here are Young's elastic modulus and density. Steel has three times the elastic modulus of aluminum, but about 2.85 times the density of cast aluminum-silicon casting alloys. Elastic modulus in this instance is the ability of the metal to store energy for a given displacement and the critical area being displaced is the small section of the latch which surrounds its vertical hinge pin. The high polar moment of inertia of your latch geometry creates larger displacements of the outboard (release trigger) section of the latch as metal density increases. These two differences should offset each other, except for the fact that the 'stop' for the latch in either case is the aluminum floor of the latch slot in the upper receiver.

The high polar moment of inertia of your latch geometry is likely the bad actor here. If the outboard release trigger section of the latch were reduced to a flat plate, as in a GI issue latch design, its polar moment of inertia would be dramatically reduced and it would not be able to respond to recoil impulse with anywhere near as much vigor.

I have a cheap M4gery with an aluminum charging handle latch cut from sheet to the exact steel GI latch geometry. Never experienced this kind of unlatching during recoil. This strongly suggests that latch geometry, rather than the specific metal, is the culprit.

[Fuel Fire Desire]

The two key properties at play here are Young's elastic modulus and density. Steel has three times the elastic modulus of aluminum, but about 2.85 times the density of cast aluminum-silicon casting alloys. Elastic modulus in this instance is the ability of the metal to store energy for a given displacement and the critical area being displaced is the small section of the latch which surrounds its vertical hinge pin. The high polar moment of inertia of your latch geometry creates larger displacements of the outboard (release trigger) section of the latch as metal density increases. These two differences should offset each other, except for the fact that the 'stop' for the latch in either case is the aluminum floor of the latch slot in the upper receiver.

The high polar moment of inertia of your latch geometry is likely the bad actor here. If the outboard release trigger section of the latch were reduced to a flat plate, as in a GI issue latch design, its polar moment of inertia would be dramatically reduced and it would not be able to respond to recoil impulse with anywhere near as much vigor.

I have a cheap M4gery with an aluminum charging handle latch cut from sheet to the exact steel GI latch geometry. Never experienced this kind of unlatching during recoil. This strongly suggests that latch geometry, rather than the specific metal, is the culprit.

This is why this plane is awesome, with technical informative posts like this.

[Fuel Fire Desire]

What are the thoughts on a latchless charging handle, like the SLR or Blackhawk? Both used a spring loaded ball detent instead of a lever/ latch assembly. Or, the SI phantom that uses two internal leaf spring detents that bite on to the internal charging handle tab insertion area inside the upper. There would be no risk of vibration bouncing the latch out of the locked position.

The reason I ask, is I've found the Blackhawk on sale for not much more than a standard GI handle, and really would just like to buy once and not worry about another latch acting the same way. And since this is supposed to be a "race gun", I'd really like something "faster" than the small GI design. Ambi is just a bonus, and makes sense on my ambi receiver.

[DrScaryGuy]

from the Strike Industries webpage about the "phantom latchless ambidextrous charging handle"

WARNING: For standard mil-spec AR15 only. Not rated for suppressors and /or full auto fire. Not for use on SBRs or pistol length barrels (<14.5")
that really makes me wonder about their ability to stay in when you want them to.

[Fuel Fire Desire]

I'm actually in contact with SI right now via phone and email. I'll post the chain of conversation, but basically it's him blaming tolerance stacking on my end, and me trying to explain the latch is springboarding open from the physics 10x25mm explained.