I have rounded many bolts of before and i have found a way to get them out. Basically just take a pair of strong mole grips or pipe grips if you have them and just tighten them and twist them just as you would with an allen key etc. If you grip the head of the bolt tight enough and twist then it will come out with ease. I would also recommend that if you tend to keep rounding more bolts of then buy a good ratchet, with adaptor, with an extension bar and this gives you more leverage and you tend not to round bolts of as much as you would with an allen key. Hope this helps