semi-auto question

[ditchman]

2 types of semi auto

 

• gas operated
• recoil operated

 

im talking about GAS OPERATED

 

 

is there any drop in muzzle velocity as a result of gases being ported/diverted to the piston.....say if you are using a cartridge that has been proved (on a chronograph) to deliver say 1100fps in a 26" barrel ......would the muzzle velocity (fps) be different from a 26" gas semi-auto

 

 

 

i ask this for a particular reason

[theshootist]

Logically yes, there would be since like a rifle, gas is being tapped from the barrel before the shot column has left the muzzle. So if the gas is expanding into a larger volume than it would do on a pump gun and whilst it is still accelerating the shot column, then we can infer that it will achieve a lower velocity. Whether it would actually show up on a chronograph I don't know. Obviously it wouldn't make a difference that a shooter would notice.

[ChrisAsh]

With One or Two holes in the ring about 0.5 mm each I doubt it effects the measurable velocity

[bruno22rf]

I always thought that ignition and expansion of the powder was decreasing in ferocity once the wad/shot stated to move up the barrel-as the room for expansion increases then the pressure in the breach area must, surely, drop? If this is the case then the propulsion properties of the propellant are almost negligible by the time the shot/wad have reached the venting ports on a semi-auto-in fact with the possibilities of the wad/shot causing a vacuum behind them-the venting holes may even help increase velocity as the wad/shot nears the end of the barrel?

Edited April 4, 2014 by bruno22rf

[theshootist]

Although most of the powder from a modern cartridge will be burnt in the first 18" of barrel, the column is still accelerating. When espousing the benefits of his 25" barreled guns Robert Churchill said that the difference in velocity between a 25" and the then conventional 28-30" barrels was negligable. This is true but there is still a difference.

 

What difference an auto makes over say a pump I just don't know. The difference could be small enough to be swallowed by the shot to shot variation of the cartridges.

[ditchman]

thanks for the replies.........im switching over from a 26" sxs to my old beretta a303 semi.....ive always stuck with one cartridge, and i need to start looking /trying other cartridges, my suspicion is that the semi will be far more "forgiving" on my choice of shell, which will allow me to try some cheaper makes of cartridge than what i have been using for the sxs

 

cheers again

[Harnser]

The gas that works the action on a self loader is the latent gas in the barrel after the shot and wads have left the barrel . The guns timing will not allow the action to open the breech until the load has left the barrel. As for a drop in velocity , I very much doubt it . If the timing is out then the gun may shoot out of battery which can be a problem . This very rarely happens unless the gun is very ,very worn or abused . Shooting out of battery will be consistent with a flash and gas coming out of the breech face when fired . Harnser.

Edited April 5, 2014 by Harnser

[Greenhunter]

thanks for the replies.........im switching over from a 26" sxs to my old beretta a303 semi.....ive always stuck with one cartridge, and i need to start looking /trying other cartridges, my suspicion is that the semi will be far more "forgiving" on my choice of shell, which will allow me to try some cheaper makes of cartridge than what i have been using for the sxs

 

cheers again

Personally I would be far more concerned with the quality of the cartridge caseing. Particularly the lip on the brass. Poorly defined lips have caused missfeeds in nearly all the self-loaders I've had.

 

GH

[Underdog]

The gas that works the action on a self loader is the latent gas in the barrel after the shot and wads have left the barrel . The guns timing will not allow the action to open the breech until the load has left the barrel. As for a drop in velocity , I very much doubt it . If the timing is out then the gun may shoot out of battery which can be a problem . This very rarely happens unless the gun is very ,very worn or abused . Shooting out of battery will be consistent with a flash and gas coming out of the breech face when fired . Harnser.

Good point!

 

The tapped off gas through the ports is only to start the unlocking process once pressure has dropped to a safe level to just about thrust the bolt back enough to start the cycle. When the wad was a piston and nearly gone the cartridge case now becomes the piston acting on the bolt.