hi, i want to build a spud gun with a chamber made of 4" pvc 5 feet long, converting to barrel of 2" pvc 10 feet long.....shooting a potato and using hairspray as fuel, how far do you think my potato would go???

I just made a potato gun using 2 feet of 4" pvc, 1 foot of 3" pvc and 1 foot of 2" pvc as the potato barrel. My cannon does not work well at all. Is the 2" barrel too short? im confused as to why my Big spud gun (4 feet) works worse than my Little spud gun (2.5 feet).....any answers?