I'm about to buy a small mill for small jobs and I'd like to know how hard it is to make a picatinny rail, and what instrument I need to copy the curvature of the top of the rifles' reciever.
How difficult is something like this to make?
Can anyone shed any light on the matter?

Thanks.

More than one wat to skin that cat. A horizontal mill would be my first choice.
A vertical mill using a fly cutter would be my next choice. Or you could use a lathe,bore out the center, and slit it in half or wear ever you like above the center line.
Again, bigger machines are better, more rigid.

To copy the top contour, I'd have a custom cutter made to the radius I wanted......possibly cost $200 for each cutter.....find a tool grinder and get a quote.

The rest of the picatinney rail should be straight foreward endmilling and chamfering tools.

You may need to make a special set of vice jaws to hold the rail properly and the cost of aluminum might surprise you as a lot of places have minimum order pricing.....

The idea of making a lot of things can seem simple until we try to make things that are made in high volumes that are extruded.

Not trying to discourage you but I see a very expensive rail here unless you're intending to make hundreds of them.

You might look for a rail that is already available--one very close to what you want and then modify that for your purposes. I bet it would be more cost effective; unless, like Vapo said, your going to make hundreds of them.

Are you wanting to make rails out of steel or aluminum, or both?

If your time is worth anything, I'd consider buying the rail blank and only machining the underside to contour. A simple way to do that (not the best, but one of the simplest for limited use) is to get a carbide router bit and grind it to the contour you need.

Machining a picatinny rail out of steel on a small mill (like the Proxxon FF400) would be an extremely difficult and time consuming operation. I would never recommend trying this. If time and money are no object go for it.