What exactly are “nitro” cards?

[Onojutta]

What exactly are “nitro” cards and how do they differ from the ‘A’, ‘B’, & ‘C’ cards/wads that TOTW sells?

 

[ZUG]

My understanding for the term “nitro cards" refers to their use in reloading modern shot gun shells. Back before the plastic wad was invented you used paper /fiber wads in reloading shot shells. The name "nitro" came from the nitrocellulose gun powder used in the modern shot shell.

 

[Johnny Too-Tall]

a Nitro card is a thicker card typically used to go over top the powder then the shot goes in then over shot card. Some people just double up on the over shot cards using two of them in place of nitro card, and avoid nitro cards all together.