Storm racks aren't a perfect weapon, but you can eke out their best with the right decision-making. You might already know some of this, but here's some stuff that can help.

1. The rest of your squad determines what kind of enemies you're facing. If you're facing rank 5 and above enemies most rank 3 weapons will do very little damage even if they are "legendary" :-).

2. The projectile speed is slow, so don't aim directly at the enemies (they can just move out of the way and you'll miss). Instead turn off aim assist and point at their feet ahead of their path. Even if they move slightly away, you'll get them with splash damage, and with luck you might score some back hits. Or if they're right against a wall, aim at the wall next to them.

3. At rank 3 the splash radius is 4.4m, which is actually pretty good at low levels, but as you start to face higher tier mechs they will start to get faster (esp the ever-present scouts) so the 7.5m radius at rank 5 becomes important.

4. Pick mechs that mount their weapons at the shoulders instead of lower. The height allows you to aim downwards and increases the chances of hitting near a mech when you aim at their feet. Either that or use a mech that can jump. In early game my top mech was a rank 2 lancer with a rank 5 missile rack 6. The jump plus 4.5m radius helped get some fun kills even when opponents were tougher. I also used ares with two missile rack 6s very successfully.

5. Similar to point 4 above, take the high ground when possible so you are mostly shooting downwards.

If possible, try all this before upgrading, in case you find it's simply not your style at the moment. All the best!