There are too many way to make your guns bigger, My suggestion start to working super set, 3 to 6 set of 6- 12 reps
Best exercises are for Triceps ( Bigger group of muscle and make your arms bigger )
Dips, cable Push down, Arnold Dumbell press ( Makes sure you use variety of angle work )
Best exercises for Biceps and front arm, Hummer curl, cable arm curl, incline concentric db reverse
if you do those 3 x a week for 4 weeks your arm will be massive , when you wanna definition do more reps less weight and more body weight ( like press ups )
good luck
Murat Gecmen
Bsc( Sports science ) Personal Trainer
http://www.fitinfitness.co.uk