Some good advice above.

I would invest in a set of - pin gauges to accurately measure throat diameters. You can find them on Amazon for sale. These have been a great investment and I use them on all my revolvers.

After you measure your throats and have the proper diameter, order some sizing dies from NOE, they are cheap and work well.

45Cal Push Rod (45PR) | NOE Bullet Moulds

45Cal .456 Body Bushing (B456) | NOE Bullet Moulds

45Cal .455 Body Bushing (B455) | NOE Bullet Moulds

001 Push Through Size Die Body | NOE Bullet Moulds

You can also buy the Lee Sizer but you're limited to the sizes they offer, in this case .454.

Lee Bullet Sizing Die Kit 452 Diameter

Play with a few different sized bullets. I have typically gotten the best results at or slightly larger than throat diameter.

I would worry less about bore diameter and focus on chamber throat diameter.

The typical expander die included in a 45 ACP or 45 Colt die set will size the case too small since they are set up for a .452 bullet. You will be sizing your bullet down further when you seat it in the case which defeats the purpose of correctly sizing your bullets in the first place! I first read about this in Dave Scovill's book "Colt's Single Action Army, Loading and Shooting the Peacemaker" but struggled to believe it so I ran a test.

I sized a lead bullet to .456 and then seated it in a case that had been sized with the regular expander plug in my die set which measured .449. Once seated, I used an inertia puller to pull the bullet and the .456 bullet now measured .454.

You can call RCBS and have them send you an expander from their 45 Colt Cowboy set or you can use an expander die from a 455 Webley die set.

This sounds like a lot of work and $$$ but it's a pretty small investment to get the most out of these great old guns.

I also use a 16-1 alloy for my bullets and powder coat them prior to sizing them.

Some results from my Colts with .456 chambers, I use .456 sized bullets.