1) swap before you pick the ammo up or you get it reduced in a penalty.
2) take advantage of the time they have to take to switch off Gjallarhorn and back to thorn/hawkmoon/the last word and make it count.
3) Use machine guns like Jolders Hammer, BTRD345 and others.
If they wanna do it and switch to a rocket, then they are a dumbass who doesn't know how to use killing potential. If they switched to the Thunderlord, then its a viable tactic, field scout for more ammo, then perfect balance to land the shots and its an mg, its got more killing power.