Critical hits are house ruled as max damage plus a die roll.
When critical hits and fails are rolled, the d20 is rolled a second time. If another critical is rolled, it is taken into account and effect.
Eg: You roll 20, then you roll 20 again, so you blow the dragons head off and it dies in a spouting gasm of blood. You now have inspiration.
Eg2: You roll 20 then you roll a 1. Your gun barrel bursts as the round leaves, taking out the eye of the dragon. It now has disadvantage on all attack rolls. Your gun has disadvantage on rolls until the barrel is replaced.
Eg3: You roll 1 then you roll a 20. The dragon chomps down on your gun arm as it goes off, resulting in the dragon being stunned and your hand is chewed. You have disadvantage on attack rolls until your next short rest.
Eg4: You roll a 1, then you roll another 1. Your gun explodes in your hand. You have no hand. You have no gun. You're stunned for one round.