What kind of velocity gain is there when going from a 4 inch barrel to a 16 inch barrel?  Does the 9mm have enough case capacity to give much increase in velocity with an increase in barrel length?

I had some data on this but have no idea where it is.  Yes, there is a velocity gain with a carbine barrel.  IIRC, it's usually around 100fps faster from a 16" barrel than a 4" barrel.  

The typical gain/loss is 50 FPS/inch.

So 12" difference = 600 FPS

I do not know if this rule holds true in this situation.

Thread on The High Road about just this

~200 fps greater with 16" barrel compared to SIG 226

That is kind of what I expected.  4X the barrel length = 20% increase in velocity.

Quoted: The typical gain/loss is 50 FPS/inch. So 12" difference = 600 FPS I do not know if this rule holds true in this situation.

Not true for the 9mm.  Sort of applies to high intensity rifle cartridges.  Were it not for NFA laws, a 12" barrel in a 9mm carbine would probably be ideal.

dvo

Some years back, I did a test of 16 major types and weights of ammo through a Beretta CX4, Hi Point 995, and a few other 9mm carbines.

I'll spare you the major details, but I can tell you that the increase in velocity caused by the longer barrel ranged from 0% to about 25%.  There was no consistent correlation between bullet weights and velocity gain, but nearly all of the rounds from 135gr and up had little velocity gain.  The 115-124 grain loads would always gain some velocity, but not in any consistent way.  Federal 124 gr Hydrashok +P+ went an astonishing 1650fps out of a CX4!

I still have my old web page with the results from this.  Anybody want to host it?

It'd be interesting to see the velocity gain for 7.62x25mm in a carbine as it's already a screamer in a pistol.

Quoted: Some years back, I did a test of 16 major types and weights of ammo through a Beretta CX4, Hi Point 995, and a few other 9mm carbines. I'll spare you the major details, but I can tell you that the increase in velocity caused by the longer barrel ranged from 0% to about 25%.  There was no consistent correlation between bullet weights and velocity gain, but nearly all of the rounds from 135gr and up had little velocity gain.  The 115-124 grain loads would always gain some velocity, but not in any consistent way.  Federal 124 gr Hydrashok +P+ went an astonishing 1650fps out of a CX4! I still have my old web page with the results from this.  Anybody want to host it?

Also, most pistol rounds produce their max energy/consumed powder within 12 inches.

So the gains above 12 inches are going to be marginal. Still a 9mm going at 1300fps is pretty damn powerful.

Quoted: Quoted: The typical gain/loss is 50 FPS/inch. So 12" difference = 600 FPS I do not know if this rule holds true in this situation. Not true for the 9mm.  Sort of applies to high intensity rifle cartridges.  Were it not for NFA laws, a 12" barrel in a 9mm carbine would probably be ideal. dvo

Quoted for truth.

As a reloader I can tell you 9mm uses very fast burning powder. The advantage is that you get great velocity out of 4, 5 and 6 inch barrels because most of the propellant gas has reached full force. Increasing barrel length to 10 or 12 inches will get you optimum increase in velocity but going 16 inches or more may get you a velocity decrease over 10/12 inch barrels.

This is why militaries and police use 9mm carbines with 6, 10 or 12 inch barrels (Uzi, H&K, AR etc.) Its mostly only the civvie versions that have 16.