My Mod. 94 .356 Winchester has a 20" barrel and the full length magazine tube holds 7 rounds. If I put a 24" Douglas barrel on it about how much of a velocity increase could I expect with the extra 4" of barrel launching 220 to 250 grain bullets? Assuming a full charge of what ever powder would be best.

I am just trying to safely get as close to possible to true .358 Win. ballistics.

I suspect very little...as a matter of fact the larger calibers leave large volumes for gas to fill upon firing and that requires very large cases.....I wouldn't expect more than 50 FPS for the entire additional four inches.

There's no real way to know unless you try it, but barrels vary enough that with that sort of larger-bore, smaller powder-room round, you might not gain any velocity.

I believe that if the 356 was load in a bolt action you would have 358 velocities it is the gun action not the cartridge that is the controlling factor. Both have the same volume!! Cheers NC

I'll soon know, built a 24in 356, going to the range soon to chronograph.

The .356 is a 52,000cup cartridge just like the .358 Win so no trouble there.
The interesting thing about the Winchester Big Bore cartridges is with appropriate powders they run out of case capacity as they reach their maximum pressure. Easy cartridges to load. The Model 94 Big Bore rifles have excellent case life with high pressure loads.

Shooting Barnes 180 & 200 gr Tipped TSX my Kimber 358 WIn (rebored 308) with 18.5" barrel achieves nearly the same velocity as my 22" barrel 358 Win.

Have not tried 250 gr bullets so there may be additional benefits when shooting heavy.

If it were a 35 Remington (low pressure and 38-ish grains of propellant) maybe 50 fps. But 356 Win is way closer to a 358 win (higher pressure and 46-ish grains of propellant) than a 35 Remington. So I'd guess minimum 100 fps and quite a bit less muzzle blast to boot. You'd be in 358 territory for sure.