Not exactly. It can be done, but typically isn't. Of course, it depends on the ammo purpose (supersonic vs subsonic).

https://www.wideners.com/blog/300-blackout-effective-range-best-barrel-length/

Expanding gases continue to increase velocity, unless it is a very fast burn powder. This would depends on powder selection. 7.5" is very short to design a powder for that would complete expansion. It can definitely be done, if you take into consideration the type of barrel and the amount of friction on the projectile as it travels through engaged with the rifling. Pistol cartridges are typically designed around short barrels with extremely fast burning powder, however, even those continue to increase velocity with longer barrels, which is why pistol caliber carbines are so popular, the rounds increase velocity making the bullets typically more capable of creating catastrophic damage beyond the original intent. A 9mm becomes more like a 357 mag from a carbine. I have the chrono results to prove it with several different pistol powders.