Semi is way better than auto.

However, auto beats semi ONLY and ONLY if the gun is really fast, like the minigun.

You cant really use minigun like a semi auto gun, well, you can, but it will be horrible.

You can use the PDW SD like a auto gun, but the RoF will be pretty slow, and if you repeatedly tap, like semi auto gun, itll be faster, the RoF will.

So basically,

Auto = High RoF guns.

Semi = Normal/Slow RoF guns.