First, you have to tell us best for WHAT? Self defense, slowfire target shooting, Bowling pin shoots, rattlesnake control etc.
In a general sort of answer, the 1911 was designed to shoot a 230 grain round nosed full metal jacketed bullet. In general, it feeds better than many other rounds (semiwadcutter, hollow point, etc) However, the BEST ammo is the ammo that YOUR pistol will load, fire, and extract 100.00000% of the time- and that may vary from one pistol to another.
I have one Auto Ordnance brand 1911A1 that loves the cheap, aluminum cased, non-reloadable Blazer ammo- feeds reliably shoots to point-of-aim, etc. Friend has a Colt Government model that does NOT like it.
.45 ACP means Automatic Colt Pistol. It IS .45 Auto.
Yes
45 ACP can be used in both auto and revolver. The revolver must have a 45 ACP cylinder.
9mm target ammunition is commonly less expensive than .45 ACP.
I'm not aware of a .45 Magnum. There is a .44 Magnum. The 44 magnum is typically a revolver round, very high power. The 45 ACP is typically used in semi-automatic pistols. The 45 ACP is not as powerful compared to the 44 magnum (mostly due to the amount of propellant in the 44 magnum compared to the 45 acp).
Measurements as in ammunition size? 45 ACP (Automatic Colt Pistol)
Colt M1911 chambered with the .45 ACP. No pistol has been made that is better than the original Colt. Still used today by some military and police. The Smith & Wesson K22 is the best revolver ever made.
The 45 GAP operates at slightly higher pressure than standard (non +p) 45 ACP ammunition and by this means is able to generate similar velocity and energy to the 45 ACP cartridge. The 380 Auto cartridge is not only smaller than 9mm Luger, it is also far lower pressure. The 380 Auto delivers far less velocity and energy and cannot handle as much bullet mass as the 9mm.
You can fire a .45 ACP cartridge through a .45 ACP pistol - and only a .45 ACP pistol. The various .45 cartridges (.45 ACP, .45 Long Colt, .45 GAP, etc.) are not interchangeable with each other.
.45 caliber
ACP means Automatic Colt Pistol. .45 ACP and .45 Auto are the same cartridge. .45 Colt and .45 Auto Rim are a revolver cartridge. .45 GAP (Glock Auto Pistol) is a shorter round than the ACP, and does not interchange with it.
Best left to a trained gunsmith.