#021 is the viton size(same as factory made)
#33 is the size for plain old neoprene rubber plumber's o-rings.

15/16" by 1-1/16" by 1/16" thick

Of the two, you looking for the viton ones, since they will last the longest.  
Hence it all about the ability to last due to nicking, and not the actual sealing/heat properties instead.

A 50 pack is only around $8 but truth be told, all you need is around 3 to last you the life of the shotgun (receiver is good for around 100K before the carrier rail channels wear out and the entire receiver needs to be replaced).

http://www.ericwesselman.com/store/RemmyORing.html