If the gun is rated to handle +P 45ACP loads, then it should also handle 45 Colt loads in the same pressure range.

That should get you pretty close to the required specifications.

Looking at Brian Pearce's article on the 45 Colt in Handloader #217, I see that a 255gr Lyman 454424 cast with 12.5gr of HS-6 gives 1113fps in a 5.5" barrel. This is in the 16-19Kpsi load range section. Another classic load is 20.0gr 4227 with a 250gr cast bullet for around 1000fps in a 5.5" barrel.

There are a number of other loads in the 23Kpsi category section, which is in 45ACP+P load territory, including one 300gr cast bullet load at nearly 1000fps. You do not want jacketed bullets, I would think, either for performance or to stay within safe pressure ranges.

I have no idea what the 'extra long throating' might do to velocities or if it is an issue with shooting these loads since they were developed for standard 45 Colt chambers. However, I would suspect that if S&W rates it for 45ACP+P that would be your guide and therefore anything in the 16-19Kpsi range is safely within that.

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