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This may be more appropriate in another forum, if so the Mod is welcome to place it properly. I have question about shot velocity in a gas auto vs a break-open gun.

Since a portion of the energy of the powder is diverted to operate the action of the gun, would that reduce the velocity? That is, would the exact same shell in a gas gun result in lower velocity than in a break-open?
 

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When a semi auto cycles the shot charge has already left the barrel. That said no there is no velocity loss compared to a pump or break open gun.
 

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Since a portion of the energy of the powder is diverted to operate the action of the gun, would that reduce the velocity? That is, would the exact same shell in a gas gun result in lower velocity than in a break-open?
Yes, technically, but the difference is probably (I'm speculating here) small enough that it would not be within the measurement sensitivity of most chronographs.

This is also true of barrels with ports.

Not worth worrying about since it's not enough that you would even notice.

Here's another thing that makes it even more meaningless.

SAAMI, the organization that publishes the cartridge manufacturing standards that most cartridge manufacturers follow, allows a +/- 90 fps tolerance on muzzle velocity from what is printed on the box.

Now, truth be known, 1) most manufacturers do better than that and 2) this is manufacturing lot to manufacturing lot tolerance, not shell to shell within a box. All the shells within a box are pretty consistent. It's just that the shells you bought last year may be much faster or slower than the same shells you bought this year or next year. But you probably can't tell the difference.

Hence, my contention that the little amount the muzzle velocity is affected by the gas bleed-off when using a semi-auto is largely inconsequential.
 

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When a semi auto cycles the shot charge has already left the barrel.
Sort of. The action "starts" to cycle as soon as the shot/wad pass the barrel ports which are about halfway down the barrel. But most of what happens when the action cycles, you are correct, happens after the shot/wad have left the barrel.

A small amount of the combustion gases are bled off mid-barrel.

But it's largely irrelevant.
 
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