1. In real life, expansion from pistol cailber ammo fired from pistols with barrels of the proper length (standard original design barrel lengths: 5" for .45 acp, 4" for 9 mm, etc.) is iffy.
2. A 3" barrel is really iffy for .45 acp.
3. To get higher velocity needed to expand when using a shorter barrel and when you can't up the powder load unsafely, you must use a lighter projectile, say 185 gr.
I am looking at the March 2001 Shooting Times issue where they were specifically addressing the issue of smaller 1911s and selection of defense ammo. The barrel sizes they used were from 3 (Kimber 3.16") to 6" (Springfield longslide).
Of note, the .45 acp was really designed for a 5" barrel. That does not mean other lengths of barrels won't work. What it does mean is that the standard of 850 fps is based on using X amount of powder that produces 850 fps out of a 5" barrel for a 230 gr slug.
That is part of the reason .45 acp does not make for a good rifle round. You might get slightly better accuracy with a rifle or carbine chambered in .45 acp, but little, none, or even less velocity depending on the barrel and round use.
In Shooting Times, there was not much gain of velocity with the 6" over the 5" barrel and in a couple of cases, there was loss of velocity. Several of the velocities between the Commander 4.25" and 5" barrels were not great at all, usually around 30 fps. The difference in the 3" to the 5" barrel was much greater. They ranged from 75-125 fps. What does that mean? According to Forker's "Ammo and Ballistics" book, loss of that much velocity is roughly equivalent to hitting your target 50 yards away for much of the 230 gr ammo he chrono'd at various distances.
Using the Speer Gold Dot 185 gr hollowpoint, out of the four barrels the velocities were 901 (3.16" barrel), 1012 (4.25"), 1024 (5.0"), and 1046 (6.0") fps. This resulted in expansion of .527, .592, .597, and .612" but it should be pointed out that expansion was based on shooting into water, not gel and not animal parts.
Given that many .45 acp do not expand reliably in real life situations, the shooter may not want to choose to shoot a 230 gr hollowpoint from a 3" barrel. For the Fed. Hydrashok, that round will only be going 769 fps, a full 132 fps slower than if fired from a 5" barrel. If it is going to have trouble expanding a full speed, then it will have even more trouble traveling at 132 fps slower. The 3" shot is 15% slower than the 5" shot.
Barrel lengths do make a difference in velocity and that difference can be significant between 3" and 5" barrels. However, there will be diminishing returns with increased barrel length over 5" on the .45 acp.
Personally, I liked the information on the Speer Gold Dot 185s, not the +P ammo. As such, you would get essentially a higher velocity round without the extra POP of +P and still get better performance from a 4" barrel than some other 185 gr. ammo does from a 5" barrel.
Majic is right. Even if all goes wrong for expansion, you still get your .45 hole(s) and that isn't too bad at all!
For additional consideration, I carry my Kimber Ultra Elite quite a bit, but not because of the Speer Gold Dot 185s and their potential expansion. The Kimber and cartridge are a compromise to be able to carry in situations where I can't carry a full-size. The gun is reliable and the cartridge is reliable from the gun and is reported to be a good performer, but a 3.16" barrel is far from ideal for terminal ballistics.