Re: bullet weights.
There will usually be a bit of variation in bullet weight, it will make a bit of a difference, but probably not as much as other things,ie neck tension and velocity difference between shots.
The 7mm mag is a great long range rifle, but to get the most from it you might like to try the higher BC bullets, like the 162 a-max and the 168 berger VLD both have a G1 BC greater then 0.6.
You won't shoot these as fast and you might get more drop (but that is easy to calculate and allow for) but you WILL get less drift, and that can be hard to judge at times.
For example at 600 yds with a 10mph crosswind my friends 7mm mag with 150 Sierra gamekings drifts 3.7 MOA, my 7 mm mag with 168 bergers drifts 2.7 MOA, that is 1 MOA or 6 inches difference, thats enough to blow you off the shoulder into the guts if your estimation of the wind is wrong.