For 45 Colt in Revolvers and carbines I use 460 brass trimmed to 45 colt COAL (1.51) and put a small bottleneck with a 44-40 die. WLP primer, 5.5gr of WW231, cork or cardboard over powder card, 1/2 oz of #7.5 shot, another card over shot, and seal with Gutter Seam Sealer (from Lowe's roofing department.) Let cure a few days.

Still have some of my first batch of 61, They have held up to chamber and some pocket carry for 12 years.

Velocity is about 750 fps and pattern is 12" @ 12 feet from 5.5" Vaquero.
Tears up snakes on your trail like a lawnmower!

Ivan

memtb: to reduce your pattern, reduce your speed/powder charge. (your charge of 18 grains of 296 (or something similar) is probably a flame thrower too! (It was intended for a 26" barrel or longer!)

ITB