Always carry a spare ...
In my skydiving days I always carried a reserve parachute. In 2000+ jumps I only had 9 emergency chute rides but I was glad to have a spare on those occasions.
My primary weapon is a revolver AISW for fast access and comfortable carry. Even a revolver can fail, my brand new 640 Pro locked up tight after the first round fired. A tiny burr of metal got into the works and rendered the gun inoperable.
Taking off the side plate and blowing it out restored functionality but hardly what you would be able to do in a gunfight. Not everybody can do a revolver reload as fast as Jerry Miculek, and I wouldn't want to try it under stress.
For me a NY reload is the best option for staying in the fight. Here is my often posted picture of my carry when "going light":
360PD AirLite and 3914 DAO
Optionally with back-up carried OWB:
Drawing with the weak (left) hand is awkward in this configuration.
Having just acquired a 547 has given me the option of "going heavy" with an 18 round 915 taking the place of the 3914 DAO:
The 915 goes behind the back for concealability.
I went "heavy" all day yesterday with just an untucked open shirt to cover both weapons and it worked well but concealing two larger guns in lightweight clothing demands more attention to what you are doing.
digiroc