Unfortunately, when Colt resumed production of the SAA after WWII, they changed the bore to 451, as used in the 45 Auto, but did not change the cylinder's chambering to match. The chamber throats are still sized to the older generation's 454 bullet.
The proper throat diameter for the Newer made Colts would be on the order of .4515 to 454.
Two solutions, one is to use softer lead 454 bullets, which will swage down in the bore, it might affect pressure but the caliber is a fairly low pressure cartridge anyway.
Other, but expensive solution is to replace the cylinder with a third party make, or have a 357 or other smaller caliber cylinder rechambered to 45 Colt with to a proper throat diameter for 450/452 bullets.
The oversized throat has led to claims of poor accuracy in the Colt and it certainly promotes leading and velocity loss as propellant gasses are passing and overheating the base of the bullet on its way to the forcing cone.
Many or most just live with it. I found that Moly coated bullets, such as Bear Creek brand don't lead up the Colt as badly, the Moly must protect the lead from the torch behind it.
