Slugs can be shot through it, but they will not be accurate at all. Saboted slugs are a must, and even then they will suffer a loss in velocity, as a great deal of the propellent gasses produced from the powder will escape around the slug and sabot. I'm curious, however, as to who said it's overbored to 10 guage? I'd wager it's probably only opened up to an open bore, which would only be an open 12 guage bore. Otherwise, the barrel on a standard Mossberg would be paper-thin and unsafe to shoot. The best, quickest explaination I can give, would be to redirect you to some articles some fellow shooters have in an ongoing webpage,
http://en.wikipedia.org/wiki/Gauge_(bore_diameter)
It's a good read, and gives a great deal of insight as to the history of shotgun bore guaging..... as for your open bored 12 guage, try some buckshot or heavy shot. Practice will help overcome flinching, and if it's a real problem, you need to seek another weapon, perhaps in another, smaller, guage.