https://en.wikipedia.org/wiki/Colt_Walker#Problems

In addition to its large size and weight, problems with the Walker included ruptured cylinders when firing. This has been attributed to primitive metallurgy, soldiers allowing powder to spill across the mouths of the chambers, and even loading the original conical bullets backwards into the chambers. Under 300 of the original 1,000 were returned for repair due to a ruptured cylinder. Lard was loaded into the mouths of the cylinders on top of each bullet after loading to prevent the spark from igniting all chambers at once, a practice which continues to this day among black-powder revolver shooters, and although each chamber held 60 grains of powder, Colt recommended no more than 50 grains in each chamber.[6]