The "Sergeant McHammerfist" is, while at a bar with one or more friends, one or more attractive females chat you up as a ploy to buy them drinks. Instead, once drinks are ordered, you slam the shots with your buddy in front of the chicks. If the barkeep gives you free drinks, you have just McHammerfist'd those two females.
Ladies (after some time of conversing): So, you guys want to buy me and my gal here a few shots?
You and friend: Sure!
*After slamming the shots in front of them*
Barkeep: Nice job, boys! Here's a free beer for each of you for your Sergeant McHammerfist!