Re: best long range 7mm for 1200 yrds+?
For the distances and bullet you are looking at, I would do a 7 STW, 7 RUM, or 284 JAZZ.
The Jazz and Rum will be about equal in performance, but the brass for the JAZZ is much better quality and is done in a standard magbox length of 3.60 with the 180 Bergers.
The Rum and STW will need a longer magbox, which can be done.
There are a few 284 JAZZ rifles out now, and ES are very low with this caliber, which will help at the distances you are looking at.
If you want the simplist solution, do an STW, use 8 mm Rem brass, RCBS dies, and Retumbo powder. Will do what you need it to, with alittle less powder than the other two.