I was making a big batch of .38 loads today and something confused me.

[Ole]

Loading the same exact bullet with the same exact powder charge, the 9mm produces about 200-300fps more velocity than a .38 special.

Powder is 3.7 grains of Bullseye and the bullet is Lee 125gr RNFP lead. As cast in the .38 and sized .357" in the 9mm. Pistols are 4" SW model 59 and a Ruger GP100 4".

Extra case capacity for the .38 and higher pressure limit for 9mm is what's going on here I take it, but how does it work to a layman? How do you get an extra 200-300fps out of a smaller case simply by running a higher pressure limit?

Maybe the revolver cylinder gap accounts for part of it?

Just wondering to myself how this could happen. I mean the same powder charge should propel the same bullet the same speed, one would think.

Does running higher pressure out of a smaller case automatically mean more velocity out of the same powder charge? :veryconfu

[bhn22]

The 9mm case is half the size of a 38 Spl. You really don't lose that much from the barrel/cylinder gap.

[Gunsmoke4570]

Look at some of the load data for wad cutters in a 38. You'll see the smaller charges will provide similar velocity as heaver heavier charges with a SWC of similar weight.

[danski26]

Just case size.....no replacement for displacement. If you seated the bullet deep enough on the 38 sp to match the 9mm para's volume i am confident velocities would be similar.

[MtGun44]

Combustion chamber volume. Ever mill the heads on a small block Chevy to increase
compression ratio and get more HP? Same principle.

Bill

[Willbird]

For some uses you could use 38 short colt brass, closer to 9x19 in internal volume. Some shooters use that brass for shooting games that require quick reloads...shorter rounds go in easier :-).

[runfiverun]

you jus crammed the same gas volumn into a smaller area like putting a turbo on your car.
same everything more air shoved in there, it pushes harder on the piston/boolit.