How much force is applied to 9mm bolt in AR15 after cartridge is fired?

[arakboss]

I have been reading up a little on bolt plus buffer weight used in 9mm ARs. It seems around 20 ounces give or take is the norm?

One question I have is how much buffer plus bolt weight would be required to keep the 9mm case from extracting at all, 30oz, 40oz, 80oz?

Secondly if a barrier was placed behind the bolt to prevent it from opening how much force (in pounds) would that barrier have to withstand to hold the bolt closed?

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[arakboss]

I have resigned to the fact that I will need to purchase a commercially produced 9mm single shot rifle if I want a bolt action or break open action firearm. They appear to be a rare item and costly from what I've found so far.

To fill in, while that search is ongoing, I have come up with a simple solution. I am going convert an AR15 9mm blowback upper to fire in single shot mode. The solution is to drill a hole through the side of an AR15 upper, then through the tail end of 9mm bolt and out the other side of the upper. A cotterless hitch pin will be inserted through all of the holes to lock the 9mm in battery during firing. The cotterless hitch pin will then have to be removed to extract and eject the spent case and load the next round. The CHP will then be reinstalled to fire the next round. If I want to fire the upper in semi-auto mode I will simply leave the pin out.

The 3/16" stainless version of the CHPs are rated at 2400lbs breaking strength. The 1/4" versions are rated at 4300lbs breaking strength. I believe this far exceeds the force they would be under, holding a 9mm bolt back during firing.

My desire for this type of operation is pretty much for fun. It would have some practical uses for working up loads and firing a wide range of loads from mild to wild without have to change out buffer components.

The downsides are minimal as the only part of the project that would not be reversible is the holes drilled through the receiver and bolt. That would be a cosmetic only problem.

Drilling will commence this week and after pics will be posted here. If you have any advice on where to locate and drill the holes, feel free to chime in.

 

[arakboss]

Any last words before the drilling begins



X marks the spot, if you have any reason to believe I should change the hole location speak now or forever hold your peace.

 

[arakboss]

I believe I have room to use a larger diameter pin like 5/16" or maybe even 3/8". The 5/16" is rated at about 6700lbs breaking strength and the 3/8" is rated at about 9700lbs breaking strength. I have no idea what force the bolt will exert on the pin except that the addition of a few ounces of buffer weight can make extraction sluggish. I am guessing a 1/4" pin would be sufficient but I may go big for the overkill effect.

 

[arakboss]

Any last words before the drilling begins

View attachment 1250338

X marks the spot, if you have any reason to believe I should change the hole location speak now or forever hold your peace.


View attachment 1250344


View attachment 1250345

I have the upper in the fixture and vice but I am going to sleep on it until Wednesday before firing up the drill. Tomorrow I will swing by ACE and figure out what diameter pin to buy. Two inches in length should be able to go in one side and come out the other side beyond the spring loaded ball point of the hitch pin. I might pick up a cobalt drill bit too, since I have to drill through the bolt and some type of weight metal that is added inside the bolt. It could be tungsten which may present a drilling problem?


Edit: I don't think the extra weight stuffed into bolt center is tungsten. It is strongly magnetic and I was able to drill a small divot in it with a regular drill bit. I will pick up a cobalt bit to speed things up but I don't anticipate any problems except drilling the hole clean and straight with hand drill.

 

[Will_White]

A 5.45 bolt and standard extension works for a locked breach 9mm AR it doesn't have enough gas for DI but manual operation works. You'd just need a headspace gauge set for 9mm.

 

[arakboss]

I believe you mentioned that before. Do all 9mm barrels have separate barrel extensions that can be replaced? Is that something I should be able to replace myself?

 

[Will_White]

I believe you mentioned that before. Do all 9mm barrels have separate barrel extensions that can be replaced? Is that something I should be able to replace myself?

I don't know if all do but Faxon barrels do. I haven't replaced a barrel extension but I have put one on, a good bench vice with V blocks and a pipe wrench is what I used, Brownells has a special tool for it that would probably work better I definitely didn't get to the proper 150lbs torque spec.

 

[MACE]

Keep in mind that the smaller the pin will be, the higher the bearing stress your aluminum upper receiver will see. The shear strength of the pin is probably much higher than what your receiver will withstand without "damage".

If you want a subjective idea of the force, you could hold a 9mm pistol slide forward with your thumb and fire it. (I'm serious.)

 

[MACE]

YouTube Similar Example

 

[arakboss]

Keep in mind that the smaller the pin will be, the higher the bearing stress your aluminum upper receiver will see. The shear strength of the pin is probably much higher than what your receiver will withstand without "damage".

If you want a subjective idea of the force, you could hold a 9mm pistol slide forward with your thumb and fire it. (I'm serious.)

Great points. Starting small would allow for using larger pin later if the receiver holes become enlarged.

 

[arakboss]

YouTube Similar Example

That demonstrates why adding a little extra buffer weight can cause sluggish extraction.

 

[arakboss]

I don't know if all do but Faxon barrels do. I haven't replaced a barrel extension but I have put one on, a good bench vice with V blocks and a pipe wrench is what I used, Brownells has a special tool for it that would probably work better I definitely didn't get to the proper 150lbs torque spec.

I am going to try the pin option first since the cost will be under $10. If it doesn't pan out I will look into the barrel extension swap and purchasing a 5.45x39 bolt. Cheapest one I have found so far is $80 for bolt only.

 

[CountryGent]

I have resigned to the fact that I will need to purchase a commercially produced 9mm single shot rifle if I want a bolt action or break open action firearm. They appear to be a rare item and costly from what I've found so far.

Some options to explore that aren't particularly onerous cost-wise and/or super difficult to track down:

• Spanish Destroyer carbine. Most are in 9㎜ Largo, but they can be found in 9㎜ Luger and .38 Super. (IIRC, conversions from 9㎜ Largo to the more common Luger is feasible, but I'd have to defer to a gunsmith.)
• Ruger 357/77. Bolt-action and can run .38 Special and .357 Magnum. Not a 9㎜ Luger, of course, but in that territory.
• The H&R Handi Rifle could be had in .357 Magnum. Not sure about the auto cartridge, though it is a single-shot, break-action, so presumably a chamber adapter could be an option.

Hope that helps.

P.S. I know @Reno did a neato conversion from a .22LR to 9㎜ Luger; maybe he can fill in details.

 

[arakboss]

Some options to explore that aren't particularly onerous cost-wise and/or super difficult to track down:

• Spanish Destroyer carbine. Most are in 9㎜ Largo, but they can be found in 9㎜ Luger and .38 Super. (IIRC, conversions from 9㎜ Largo to the more common Luger is feasible, but I'd have to defer to a gunsmith.)
• Ruger 357/77. Bolt-action and can run .38 Special and .357 Magnum. Not a 9㎜ Luger, of course, but in that territory.
• The H&R Handi Rifle could be had in .357 Magnum. Not sure about the auto cartridge, though it is a single-shot, break-action, so presumably a chamber adapter could be an option.

Hope that helps.

P.S. I know @Reno did a neato conversion from a .22LR to 9㎜ Luger; maybe he can fill in details.

I liked Reno's conversion but it looked expensive. I do have a 357 lever action so that base is covered. I will look into the Spanish Destroyer rifles.

 

[gmerkt]

I will need to purchase a commercially produced 9mm single shot rifle

Thompson Center made 9mm barrels for the Contender at one time. I think maybe there are aftermarket firms that make barrels for the Encore in 9mm.

 

[CountryGent]

I liked Reno's conversion but it looked expensive. I do have a 357 lever action so that base is covered. I will look into the Spanish Destroyer rifles.

Out of curiosity, I checked Gunbroker and they have at least a couple Destroyers, including one in the more rare 9x19㎜ Luger. Cheers.

 

[edzz]

Keep in mind that the smaller the pin will be, the higher the bearing stress your aluminum upper receiver will see. The shear strength of the pin is probably much higher than what your receiver will withstand without "damage".

If you want a subjective idea of the force, you could hold a 9mm pistol slide forward with your thumb and fire it. (I'm serious.)

However "most" 9mm pistols fire from a locked breach, so...

 

[Will_White]

However "most" 9mm pistols fire from a locked breach, so...

Yeah holding the slide forward only has to overcome the unlocking force not the full force of the cartridge firing. I haven't seen anyone do it with a hi point.

 

[arakboss]

However "most" 9mm pistols fire from a locked breach, so...

Good point.