is it some sort of gun or something?

It’s deltarune

What's deltarune?

some sort of special forces?

No

An RPG created by Toby fox Formally known as the creator as undertale

*starts explaining chapter 2 lore*

BIG SHOT is used as a term by Spamton, a character appearing in Deltarune Chapter 2.
BIG SHOT is used as a compliment and [[Little Sponge]] as an insult.
His use of this term was actually based off a commercial for Big Shot soda.
The actor's name in the commercial is Mike, therefore the reason why Spamton keeps saying Mike.

Are you being srs or /j?

srs

It's a word that spamton uses in the game "deltarune" by Toby fox. Asriel knows everything about deltarune talk to hrt

*him/her

Its a long story