the approximate damage per bullet (ADPB) is, if you were to fire a singular bullet at an enemy(s) with near infinite health how much damage would it do after a near infinite amount of time. you can see this if you ever get 'brn'; if you put 'brn' on a gun the ADPB will go up, the damage of your bullet never changes, but now you do damage over time, ADPB tells you that after infinite amount of time you will do ((damage + burn dmg x burn time) + damage x how many enemies you can hit - 1).
If you get 'brn' on a gun and then add 'ti' the ADPB will change a lot because 'ti' increases how long 'brn' lasts. if you want to know your true damage output, go test your weapon, he made a button for doing said action for a reason. if you get 'sca' your ADPB does not go up because it is about a singular bullet. The weapon mod 'fra' seems to be weird and chooses whether or not it gets added to the ADPB because its multiple bullets from a singular, same with 'tra', but that's all a topic for another time. i hope this has cleared things up. ( @kurt_c0caine, please correct me if i'm wrong about any of this, and pleas confirm if i am right! Great game, just wish there was a sandbox to play around in! )