reraising rule? (NLHE)

so at the club today this happened, which I thought wasn't quite right.

blinds are 50/100

SB 50, BB 100, fold fold fold ... raise to 300, guy behind reraises to 500. Then another guy said that, minimal raise is to 600 since the other guy raised to 300. so the reraiser put in another 100.

Now I thought that the minimal raise is the BB amount - in this case, to 400 (although that wouldn't accomplish much). I didn't say anything about it because by then, I was getting kinda uncertain and confused.

so could the guy legally raise it to 500 or did it have to be 600?

500 is the correct min-raise amount. The original raiser raised 200 on top of the 100 BB. So the bet increment is effectively 200, making 500 the minimum raise in that situation.

This is assuming that the original raiser raised to 300 total.

Yup its legal. The minraise amount is the amount the last person raised BY on that betting round, NOT the amount they raised TO.

In this case it went from 100 to 300 so thats a raise of 200. A minraise is a raise of another 200 so thats to 500.