Maximum Pressure For Straight Blowback Firearm?

[arakboss]

What is the maximum pressure a cartridge could operate at and still be safe in an AR15 or other straight blowback system?

For example could the 350 Legend cartridge be made to operate in a straight blowback AR15 set up (with sufficient bcg/buffer weight)?

Read More

 

[arakboss]

Cool slow mo animations of blowback systems.

View: https://youtu.be/aw8TREL_Yf0?feature=shared

 

[arakboss]

If internet is correct, UZI bolts weighed as much as 24oz. Not sure what their spring weights were. That's a lot less than the typical combined weight of an AR9 bcg & buffer. Perhaps the AR9 spring does more of the work compared to the UZI recoil spring.

 

[lucusloc]

I think in theory you can make any cartridge straight blowback if you have a bolt heavy enough. So the only limiting factor here is how much mass you could cram into an AR upper bolt carrier group. A very quick back of the napkin math says you could get that up to 5.5 pounds if you used solid tungsten (estimating about 8 cubic inches of space for the reciprocating mass). This guy did some calculations on bolt weighs using some basic assumptions, and assuming his calculations are correct you could stuff up to a 500 S&W into an AR 15 platform as a direct blowback action (we are also not considering anything else like feeding or ejection or how it will all fit, just if the weight can fit in a stanrdaish sized upper).

If all you care about are rough numbers pulled together using lots of assumptions there is your answer. Past that I think you need to get into some actual engineering.

 

[CamoDeafie]

@arakboss

Hi points are straight blowback and they are heavy, bulky and widely considered ugly

 

[lucusloc]

@arakboss

Hi points are straight blowback and they are heavy, bulky and widely considered ugly

There is a joke in here somewhere, I just know it. . .

 

[TTSX]

I think in theory you can make any cartridge straight blowback if you have a bolt heavy enough. So the only limiting factor here is how much mass you could cram into an AR upper bolt carrier group. A very quick back of the napkin math says you could get that up to 5.5 pounds if you used solid tungsten (estimating about 8 cubic inches of space for the reciprocating mass). This guy did some calculations on bolt weighs using some basic assumptions, and assuming his calculations are correct you could stuff up to a 500 S&W into an AR 15 platform as a direct blowback action (we are also not considering anything else like feeding or ejection or how it will all fit, just if the weight can fit in a stanrdaish sized upper).

If all you care about are rough numbers pulled together using lots of assumptions there is your answer. Past that I think you need to get into some actual engineering.

You linked it before I could. What I liked about that article is at some point the bolt weight becomes ridiculous, such as North of 50lbs for a .50BMG where my rifle weighs about 32lbs. I can't imagine trying to handle the recoil involved in stopping 50lbs of reciprocating mass

 

[arakboss]

I think in theory you can make any cartridge straight blowback if you have a bolt heavy enough. So the only limiting factor here is how much mass you could cram into an AR upper bolt carrier group. A very quick back of the napkin math says you could get that up to 5.5 pounds if you used solid tungsten (estimating about 8 cubic inches of space for the reciprocating mass). This guy did some calculations on bolt weighs using some basic assumptions, and assuming his calculations are correct you could stuff up to a 500 S&W into an AR 15 platform as a direct blowback action (we are also not considering anything else like feeding or ejection or how it will all fit, just if the weight can fit in a stanrdaish sized upper).

If all you care about are rough numbers pulled together using lots of assumptions there is your answer. Past that I think you need to get into some actual engineering.

Good stuff thanks for the link.

Edit: I'll stick with 10mm for my black powder AR15 project idea. I will plan for a 2.5ish pound bolt/buffer combo. That seems excessive but better to start high and walk the weight back to where it will cycle.

 

[JDubDub]

The Oerlikon 20mm AA cannons are straight blowback. Might work in an AR platform, but you'd need one heck of a recoil spring…

 

[lucusloc]

You linked it before I could. What I liked about that article is at some point the bolt weight becomes ridiculous, such as North of 50lbs for a .50BMG where my rifle weighs about 32lbs. I can't imagine trying to handle the recoil involved in stopping 50lbs of reciprocating mass

Yeah, an actual locking mechanism has become a must-have feature on even my PCCs, they just make shooting them so much nicer. Even with a 9mm the difference in recoil from a straight blowback to something with a delay is amazing.

Good stuff thanks for the link.

Edit: I'll stick with 10mm for my black powder AR15 project idea. I will plan for a 2.5ish pound bolt/buffer combo. That seems excessive but better to start high and walk the weight back to where it will cycle.

What are you expecting from the velocity of that combination? I assume we area talking about a black power stuffed cartridge, instead of using smokeless?

The Oerlikon 20mm AA cannons are straight blowback. Might work in an AR platform, but you'd need one heck of a recoil spring…

The recoil spring is really just to slow down the reciprocating mass, it does almost nothing to manage the actual action opening delay. That was something that surprised me when I firs read about the math behind blowback. But having said that, holy hell yes a 20mm spring must be massive. I am not doing the math here, just extrapolating from the smaller cartridges, but the bolt mass of a 20mm blowback must be like 200lbs. The spring has to be able to return that much mass back into battery with enough force to chamber a new round, so a minimum of 400lbs of spring pressure? I do not think you are cocking that sucker by hand. Better go get the winch. . .

 

[arakboss]

Yeah, an actual locking mechanism has become a must-have feature on even my PCCs, they just make shooting them so much nicer. Even with a 9mm the difference in recoil from a straight blowback to something with a delay is amazing.


What are you expecting from the velocity of that combination? I assume we area talking about a black power stuffed cartridge, instead of using smokeless?


The recoil spring is really just to slow down the reciprocating mass, it does almost nothing to manage the actual action opening delay. That was something that surprised me when I firs read about the math behind blowback. But having said that, holy hell yes a 20mm spring must be massive. I am not doing the math here, just extrapolating from the smaller cartridges, but the bolt mass of a 20mm blowback must be like 200lbs. The spring has to be able to return that much mass back into battery with enough force to chamber a new round, so a minimum of 400lbs of spring pressure? I do not think you are cocking that sucker by hand. Better go get the winch. . .

"What are you expecting from the velocity of that combination? I assume we area talking about a black power stuffed cartridge, instead of using smokeless?"

Not near as high as smokeless 10mm loads but higher than this guy got with his 9mm loads. I want to experiment with duplexing powders using a pinch of smokeless with black powder to help keep fouling down.


AR9 action starts at the 1:30 mark.

View: https://youtu.be/g8JlFVEt0mk?feature=shared

 

[TTSX]

"What are you expecting from the velocity of that combination? I assume we area talking about a black power stuffed cartridge, instead of using smokeless?"

Not near as high as smokeless 10mm loads but higher than this guy got with his 9mm loads. I want to experiment with duplexing powders using a pinch of smokeless with black powder to help keep fouling down.


AR9 action starts at the 1:30 mark.

View: https://youtu.be/g8JlFVEt0mk?feature=shared

 

[lucusloc]

"What are you expecting from the velocity of that combination? I assume we area talking about a black power stuffed cartridge, instead of using smokeless?"

Not near as high as smokeless 10mm loads but higher than this guy got with his 9mm loads. I want to experiment with duplexing powders using a pinch of smokeless with black powder to help keep fouling down.


AR9 action starts at the 1:30 mark.

View: https://youtu.be/g8JlFVEt0mk?feature=shared

Be sure to document what you find, I am not sure there is all that much info on this on the internet. If you just want to dump a bunch of notes on me I would be happy to collate it into something more digestible for the masses. Or really anything to help get the workload down low enough to make it worth your effort, just let me know.

 

[arakboss]

View attachment 1875061

The AR platform with 10mm brass should be fairly forgiving with pressures of duplex loads.

 

[arakboss]

Be sure to document what you find, I am not sure there is all that much info on this on the internet. If you just want to dump a bunch of notes on me I would be happy to collate it into something more digestible for the masses. Or really anything to help get the workload down low enough to make it worth your effort, just let me know.

Thanks but my interest in these types of projects rarely last long enough to take notes. Usually a few shots to prove it works and then I call it a day.

 

[lucusloc]

Thanks but my interest in these types of projects rarely last long enough to take notes. Usually a few shots to prove it works and then I call it a day.

A yes, the ADD style of research. I am familiar with the concept. Maybe I will explore the idea myself at some later date, now that you have given it to me. One more think to look forwards to during retirement.

 

[Nosferatu]

Thanks but my interest in these types of projects rarely last long enough to take notes. Usually a few shots to prove it works and then I call it a day.

Wait, when did you start shooting?

 

[JDubDub]

The Oerlikon 20mm AA cannons are straight blowback. Might work in an AR platform, but you'd need one heck of a recoil

The recoil spring is really just to slow down the reciprocating mass, it does almost nothing to manage the actual action opening delay. That was something that surprised me when I firs read about the math behind blowback. But having said that, holy hell yes a 20mm spring must be massive. I am not doing the math here, just extrapolating from the smaller cartridges, but the bolt mass of a 20mm blowback must be like 200lbs. The spring has to be able to return that much mass back into battery with enough force to chamber a new round, so a minimum of 400lbs of spring pressure? I do not think you are cocking that sucker by hand. Better go get the winch. . .

I got interested in the Oerlikons after visiting Pearl Harbor. The mechanism is a bit more complex than a normal straight blowback system. The firing pin ignites the cartridge while the bolt is still moving forward, so the pressure has to overcome the inertia of the bolt as well as the weight. Still a heavy bolt, but no where near 200lbs. The whole gun weights about 200lbs unloaded.

The Brits used a system of pulleys to cock the action. The American mounts had a bar that hooked in the bolt know, then the operator tilted the gun up using their body weight.

 

[Certaindeaf]

View: https://youtu.be/2FoWpog5KU4?feature=shared

 

[P7M13]