I find .45 Colt bullets are sold in sizes from .452, .454 and .456. I usually buy the .454 for my guns.
.451 and .452 are normally used for .45 ACP, or so I have been told. Call the bullet manufacturer and ask what is up, if possible.
I made some rounds for my wife to shoot that range from "Hey, I can see the bullet when I shoot" to "Wow, that thing must have gone in sideways." I think Shootest got it right, you might have to try something that will give you a higher velocity.
I never trim my pistol brass. I am not a competition type of guy and found I do not need to. Try to load a half-dozen of them as they come out of the package.
As far as the primers, I have no idea how the machine you are using works. I use a hand primer and I can feel them when they seat. Is there an adjustment you can make that will seat them flush or just under flush?
Here is my concluding question. Everyone stresses to check and recheck, measure and remeasure in order to avoid mistakes. How can this be done with such variation in factory equipment and factory specs? Am I missing something? Help please.
As mentioned before, SAAMI specs are maximum. You'll find that some of the things there don't work for your ammunition. The biggest is OAL (Overall length). I have some bullet/powder combinations that when set to book OAL perform horribly, won't feed, etc.. Biggest thing to keep in mind is not to exceed the specs.
Usually the first thing everyone says when a new guy asks a question is "Read the manual." You, sir, have obviously read the manual(s). Your asking good questions. Welcome to TFL.