7 RUM would be good, but you won't fit one in the magazine. Maybe if you extended the magazine a bit. When I had my 7 RUM I used the 180gr Berger hybrid. I had to single feed it, I hadn't extended the magazine. I got 3200 pretty easy with about 100gr US869. I got Greg Duley to rechamber it using his short throat reamer.
If you want the most velocity I'd try the 7 superfatso from Greg Duley it is based off the 338 lapua and improved. The easiest will be 7RUM.
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