13

All of the numbers on the train cars shown in various parts of the film add up to the number 13.

The cylinder in a properly functioning revolver will not spin freely once closed. While normally correct, the double-action revolvers used for the competition were owned and distributed by the holder(s) of the competition, and were likely modified for use in the competition. The modification is a simple one, and considering the stakes and as a necessary element of the competition, it is very likely the modification was made. It is obvious during the competition, that the cylinders are, in fact, able to spin when closed, despite common functional design.

Edited into The Clock (2010)