It seems like with all other caliber the published data is for a 4" barrel, but for 45 ACP it is for a 5" barrel. I personnally do not know anyone that carries a full size 1911. Both of my 45 ACP guns are 4", a Kimber Compact and an XD45 compact. I always wonder if the bullet will be going fast enough out of a 4" barrel to expand, so I end up carrying +p ammo in them. I can handle the +p, but it would be easier with standard pressure loads. I saw some data that 3" barrels do not do well with standard 45 ACP ammo. but I have never seen anything on a 4" barrel. I assume since they give the velocity in a 5" barrel they are saying that is the barrel length the bullet was designed to be fired from. I just go a few boxes of Winchester PDX1's and it has a 5" barrel velocity of 920 fps. It does not come in +p, so I wonder how they will perform out of my 4" barrels.