Here's some general info about hand loading lead bullets for a 45 Colt:
Years ago I stumbled onto a load with 255 gr LRNFTs that shot real well out of my Ruger Blackhawk (8 gr of Unique). When I ran out of bullets, I bought some more but from a different company. Using the same powder charge, these shot terrible and fouled my bore badly. I ended up trying a number of different powders and charge weights ... all in vain .... so back to the books. I learned a good lesson about 45 Colts and have been "preachin' it every since.
The 45 Colt has the lowest chamber pressure of any modern gun on the market. The original cartridge was designed to shoot black powder at the very low pressure of 14,000 psi. The next part of the equation is the lead bullet. Before a bullet will reform in the cylinder throat and bore, it needs ample chamber pressure. The harder the bullet alloy, the more pressure it takes to get the lead to reform instead of stripping or fouling. This process is called "obturation" and is by no means unique to the 45 Colt, however due to the 45 Colt's low chamber pressure, it becomes a very important issue. When the bullet hardness matches chamber pressure, the bullet will actually bump up in diameter slightly until it reaches the size restriction of the cylinder throats. The bullet is then pushed through the throat and into the barrel's forcing cone. The cone swages the bullet down in size just a tad so it will fit very snugly in the bore. As the bullet travels down the barrel, a tight seal is maintained between the bullet and bore to prevent hot gasses from blowing by the bullet.
In order for obturation to take place, there are five conditions that must be met.
1. The bullet diameter must be at least bore diameter, preferably a thousandth or so larger. This is to guarantee a good seal between the bore and bullet.
2. The cylinder throat must be at least .001" larger than bore diameter. This allows enough room for the bullet to bump up in diameter a little.
3. Bullet hardness must match chamber pressure. If the bullet is too hard, it won't change shape without leaving lead fouling in the throats and forcing cone. If it is too soft, it will foul in the bore.
4. Chamber pressure must match bullet hardness (within reason) or lead fouling will occur.
5. The forcing cone must be a smooth transition from the face to the lands. If there is corruption in the forcing cone, fouling will occur.
Most Ruger 45 Colt revolvers come from the factory with throats so tight that obturation can not take place and in fact the opposite happens. Instead of the bullet bumping up in size a little, it is swaged down in diameter by the tight cylinder throats. This has the same effect as shooting undersized bullets because the bullet is delivered to the forcing cone so small that it won't achieve a tight seal in the bore. As pressure pushes the bullet down the barrel, some of the expanding gasses blow by the circumference of the bullet. This causes the lead surface to vaporize from rapid erosion and leaves lead fouling behind. As more rounds are fired, lead fouling will build up in the bore to a point where accuracy is very poor. The fix for this condition is to ream the cylinder throats to an optimum diameter of .4525".
If the forcing cone is corrupted, it can be chamfered with a reamer. An 11 degree cone angle is optimum but more importantly is the smoothness and "squareness to the bore".
Buying bullets in the right diameter is easy. If your revolver was made since WWII and is not an Italian Colt clone, it will have a .451" bore. That means .452" lead bullets will be optimum and by the way ... that's what most of them are. Larger bullets up to .454" will work fine too ... as long as they meet hardness requirements.
Bullet hardness is measured in "Brinell Hardness Number" (BHN) where a low number indicates a softer alloy. Someone came up with a formula for obturation that works well for all revolvers in all calibers when lead bullets are used. It is: BHN = Chamber pressure divided by 1440. If you know the bullet hardness and need to know chamber pressure ... Chamber pressure = BHN times 1400. The max powder charges for standard loads will run very close to 14,000 psi. In "Ruger Only" load charts, the high end powder charges produce about 25,000 psi.
With a 45 Colt's pressure so low, you need a very soft bullet to make things work. A BHN 10 bullet will work great for standard velocity loads (800-850 fps). The problem is ... most commercial cast bullets are way too hard ... typically in the BHN 20 to 25 range. No way are they going to obturate properly in a low pressure cartridge ... thus excessive fouling and poor accuracy. My "magic bullet" for a 45 Colt is a Hornady Cowboy .454" 255 gr LRNFT swaged bullet (not cast) that is BHN 10. 8 to 9 grains of Unique get the chamber pressure in the right range so the bullet will obturate properly and produce exceptional accuracy with minimal fouling at standard factory velocities.
Most reloaders think "lower is better" when it comes to chamber pressure. That is far from true when you load lead bullets in a revolver. Higher chamber pressure is needed to get bullets to obturate and that can be done two ways ... more powder or use faster burning powder. Of course you must stay under the max allowable pressure to prevent damage to your gun but I've found the loads on the high end of the chart were always more accurate with less fouling.
Lead fouling and accuracy go hand-in-hand. As fouling increases, accuracy will get worse. If you can maintain clean cylinder throats, forcing cone, and bore, accuracy will also be at it's best and that is the secret to success in any revolver shooting lead bullets. If you see excessive lead fouling, I guarantee the throats are too tight or bullet hardness is not matched to the chamber pressure and in nearly all cases with a 45 Colt, the bullet is too hard.
Just one more comment .... I've found accuracy in a 45 Colt was always better when bullet weights were 250 grains or more.