Tam has started posting again. Here's a range report on .45ACP and the effect of different barrel lengths on bullet velocity.
It's good to see the posts, but she has turned off comments. Even though I understand the reasons, it takes something away from a blog to not have the interaction that comments create.
You can load .45ACP specifically with a longer barrel in mind and see some increase in velocity, although it is not going to be that great. The case size is also a limit on how much pressure you can develop as the powder begins to burn.
The volume in a larger case, like a rifle cartridge, allows the pressure generated to have some place to begin it's expansion before the bullet has moved. Once the bullet is moving, the ongoing burn continues to generate pressure behind the bullet so that it continues to accelerate. In handgun cases, that volume isn't there. Trying to generate an ongoing pressure behind the bullet to take advantage of a longer barrel results in a dangerously high pressure in the early moments when the volume is limited.
This is what limits carbines in .45ACP to lower velocities even though they may have barrels of 16 or 18 inches. In some cases, the bullet velocity peaks in the barrel and begins to slow down before it exits, the powder having been consumed and no additional pressure being created. Here's a link to ballistic tables for various commercial .45ACP loads in barrels from 18" down to 2". It's the same sort of testing Tam was doing, taken to 11.
Ballistics by the Inch is a great resource on the effects of barrel length on handgun ammo and I am adding it to the sidebar.