A Challenge

[tomc]

I've satisfied my curiosity as to the workings of a semiauto by the following mental analysis. It may not be accurate, but I'm comfortable with it.

When the gun is first fired all the forces from the expanding pressure is directed omnidirectionally outward from the closed system inside the barrel. The forces are pushing the bullet forward, pushing the cartridge backward, and trying to blow the cartridge case and barrel outwards by expanding the diameter of the cartridge and barrel. If you sum up all these forces, the only change in mass of the action is the small mass of the bullet moving outward. Being a closed system, this is why the illustrations supplied by Jim indicate the bullet moving out the barrel without the barrel unlocking and the slide moving backwards. There just isn't enough force applied in the rearward direction to unlock the barrel and push the slide rearwards. Note also that the expanding pressure has sealed the system by forcing the cartridge to expand, sealing it to the barrel and not pushing the slide backwards.

Now, however, after the bullet has left the barrel, the enclosed system opens up. The bullet stops accelerating and assumes a parabolic trajectory to the target. The huge pressure released from the end of the barrel forms a rocket blast to the gun pushing the whole gun backwards and allowing the pressure to escape. This action unseals the cartridge case from the barrel unlocking the barrel to eject the spent cartridge and reload another round. Unlocking the barrel and slide allows a lighter recoil spring to be used and reduces wear and tear of the blowback action. This is why it is easier to cycle the slide manually on my .45 than it is for my Walther PPK. Looking at the illustrations seem to back this up becouse the slide only moves backwards after the bullet has left the barrel. It is not the force of the expanding pressure that unlocks the barrel and slide, but the much larger force of the rocket blast after the bullet has left the barrel.

Excuse my rambling and quite possibly flawed mental exercise, but that's how I explain how semiautos work.

[Excaliber]

I've satisfied my curiosity as to the workings of a semiauto by the following mental analysis. It may not be accurate, but I'm comfortable with it.

When the gun is first fired all the forces from the expanding pressure is directed omnidirectionally outward from the closed system inside the barrel. The forces are pushing the bullet forward, pushing the cartridge backward, and trying to blow the cartridge case and barrel outwards by expanding the diameter of the cartridge and barrel. If you sum up all these forces, the only change in mass of the action is the small mass of the bullet moving outward. Being a closed system, this is why the illustrations supplied by Jim indicate the bullet moving out the barrel without the barrel unlocking and the slide moving backwards. There just isn't enough force applied in the rearward direction to unlock the barrel and push the slide rearwards. Note also that the expanding pressure has sealed the system by forcing the cartridge to expand, sealing it to the barrel and not pushing the slide backwards.

Now, however, after the bullet has left the barrel, the enclosed system opens up. The bullet stops accelerating and assumes a parabolic trajectory to the target. The huge pressure released from the end of the barrel forms a rocket blast to the gun pushing the whole gun backwards and allowing the pressure to escape. This action unseals the cartridge case from the barrel unlocking the barrel to eject the spent cartridge and reload another round. Unlocking the barrel and slide allows a lighter recoil spring to be used and reduces wear and tear of the blowback action. This is why it is easier to cycle the slide manually on my .45 than it is for my Walther PPK. Looking at the illustrations seem to back this up becouse the slide only moves backwards after the bullet has left the barrel. It is not the force of the expanding pressure that unlocks the barrel and slide, but the much larger force of the rocket blast after the bullet has left the barrel.

Excuse my rambling and quite possibly flawed mental exercise, but that's how I explain how semiautos work.

Well, close but not exactly.

If it isn't the force of the pressure from the gases produced by the burning powder that drives the slide rearward, where does the "rocket blast" get its additional energy from?

All of the forces in the system, including those that drive the projectile out of the barrel, the slide rearward on the frame, and the frame itself rearward into the shooter's hand (recoil) come from the very rapid expansion of gases released by the burning powder triggered by the detonation of the primer.

When the bullet leaves the barrel, the release of the highly pressurized gases allows the expanded cartridge case to contract away from the chamber walls, freeing it for extraction and ejection. The magic here comes from the designer's engineering genius, which specifies the design and weight of the slide (which determine its inertia characteristics), the resistance of the recoil spring, etc. so the rearward impulse imparted by the cartridge case to the breech face of the slide will cause the slide to start moving at just the right moment, and the barrel to unlock from the slide at just the right point, to make the thing cycle as it should. The fractional second delay in slide movement after the powder has been ignited is due to the inertia from the slide and the force of its uncompressed recoil spring. These parts are starting from rest and have to be accelerated into motion. Although it happens really quickly, this does take a measurable amount of time that is accounted for in the engineering.

[jimlongley]

Okay, here's my synopsis of what I had been trying to find right from the original post.

Early designs for the Thompson Submachine gun involved a device patented by James Bell Blish. Blish was a Navy Commander who recognized that the breech of a "bag" gun that was fired with a light load would sometimes open on their own, but never when fid with a heavy load. Of course the gunners were aware of this, but Blish figured out why, metallic adhesion - the tendency for metals under pressure to adhere to each other beyond a level accounted for by fricton.

The '"Blish Lock" was used in the (unsuccessful) Thompson Autorifle and in the early model Thompson Submachine guns, but there a great deal of conjecture that the actual lock did not function as designed and that guns equipped with it merely functioned as "retarded blowback" and would work as well without it.

It was this Blish Priniciple that I thought was what part of what the original post was about.

Sometimes it's hard to remember stuff you learned more than 40 years ago that never had a real practical application.

BTW, later Thompsons were simplified greatly and fired from an open breech where the first models fired from a closed breech and had a much lighter bolt.

The M1 Thompsons I had in my armory in the Navy had a relatively massive bolt with the firing pin just machined into its face, and was much less complicated than the M1928A1, which I had one of.

[Embalmo]

metallic adhesion - the tendency for metals under pressure to adhere to each other beyond a level accounted for by fricton.

Hmm. Not as easy as I had initially thought, but this metallurgy explanation totally makes sense. Here's what I am believing right now-If I were Superman and could rack the slide externally with the exact pressure and speed of a round the moment it is primed by the firing pin, and maintain that precise pressure, I believe that the barrel and slide would remain "locked", "stuck", or "adhered" until a slight reduction would allow the slide to be racked; I call this my seat belt theory where a fast hard tug makes it catch, and a slower tug with less pressure will allow it to move.

Still I'd like to know if it has to do with omnidirectional internal force, which really makes a lot of sense, 'cause maybe it would push up for a little while which would explain the moment of "catch" also.

Embalmo

[jimlongley]

metallic adhesion - the tendency for metals under pressure to adhere to each other beyond a level accounted for by fricton.

Hmm. Not as easy as I had initially thought, but this metallurgy explanation totally makes sense. Here's what I am believing right now-If I were Superman and could rack the slide externally with the exact pressure and speed of a round the moment it is primed by the firing pin, and maintain that precise pressure, I believe that the barrel and slide would remain "locked", "stuck", or "adhered" until a slight reduction would allow the slide to be racked; I call this my seat belt theory where a fast hard tug makes it catch, and a slower tug with less pressure will allow it to move.

Still I'd like to know if it has to do with omnidirectional internal force, which really makes a lot of sense, 'cause maybe it would push up for a little while which would explain the moment of "catch" also.

Embalmo

I don't believe the Blish Principle is used in any modern firearms.

Your seatbelt probably depends on a centrifugal clutch assembly combined with a weighted pendulum to lock up when subjected to stress above a certain threshold. Notice that most seatbelts will not move if already locked, unless ALL tension in removed.

[Embalmo]

I'm not suggesting that the mechanics are in any way related, I'm suggesting that the cause and effect of a seat belt may be similar.