I wonder if anyone can help. I need to fix 15 x 2 metre long metal rails
("T" section) vertically to a concrete wall. I'll be using those 'self
tapping' bolts that require a 6mm hammerdrill bit. I will need to drill 8
holes in each rail.

Q.1. How much larger will the rail holes need to be 6.5, 7.00, 7.5mm Etc?

Q.2. Nowadays, which drill bit manufacturer is considered high quality?

Q.3. Is there a good, reliable on-line source to the above? Many thanks.