If you use all of the minigun ammo, switch to mortar without reloading, then switch back, the gun won't reload unless you hit the reload button. This has happened to me so many times during games, and it's really annoying.
I had this happen once, although it was to an extend to where I couldn't even manually reload it for about 20 seconds, not sure how I fixed the problem, but I was by my base so it worked out.
I play gunner and will attest to the reloading issue, but will add that it's not just when switching to the mortar, but anytime the reload is interrupted.... ie. slam while reloading, deploy/undeploy etc. It's a real pain in the *** sometimes when you're in the middle of a battle. He takes all weekend to reload as it is, not to mention when you have to do it twice... I don't have many complaints about my main class (unlike MOST here), but this is one of them