I'd also use 5.5mm drill bit if 6mm make a too large hole . With 5.5mm Usually it need to be wiggle a bit to enlarge the hole but at least I can control how large I want the hole to be.

Btw, I've purchased rm130 Mr DIY rotary hammer and find it quite adequate for my once in a while drilling. So far no failed attempt to drill any hole. I'd recommend it compared to B&D impact drill.

It looks like this:http://www.mrdiy2u.com/rotary-drill/

It got 2 sizes, I picked the smaller one. If you need to drill actual hole through concrete then you might need the bigger one. But I'm not sure how reliable it is for making a hole.