Step 1: DISASSEMBLE CLOTHES PEG
Disassemble clothes pegs in two by pulling two halves apart. you'll end up with 3 parts two wooden halves and spring.
Step 2: CARVE OUT MECHANISM
Draw with a marker onto one half as shown. onto another one make place to hook spring. Cut off two ends so match sits in there freely.
Step 3: ASSEMBLE AND SHOOT.
Put two halves together and secure them with cable ties. And attach spring as shown. then push matchstick into a barrel till spring clicks in place. Then pull string and shoot. :)
More detailed instructions in THIS VIDEO