The Wife's got an M7, 18.5" bbl that's been (stock) shortened to her size. I did a glass bed & trigger job (actually the best trigger in our whole "arsenal" - absolutely sweet & anybody can come by & feel it - the trigger that is) ... 140 Rem Core-Locks factories & she did a 4 shot ~.75" 100 yd group w/it. Has a 2X7 Luepold compact & is one of the sweetest li'l shooters & lightweight too boot. 120 Nosler ballistic tips @2600fps, shoot very nicely, thank you very much, as well.
.308 Win-based catridges (.243, 7-08, etc.)are all inherently accurate catridges.
7mm has great ballistic coefficients & sectional density - good caliber all 'round.
The velocities are not blazing by "today's standards" but can be compensated for by knowing the bullets/muzzle velocity you get & adjusting accordingly.
But too, anything you'll shoot to 400 yds will require a scope adjust (or hold-over, or ... ?) or long range zero. Even "the blazers" start to fall off a bit past 300yds if sighted in for the "more normal" "to 300 yds zeros.
As an example, a 7mm 120gr bullet at (even) 3000fps w/a +3" 100yd zero will (off the top of my head) print ~-6" at 300yds &(probably -remember - off the top) minus 12-18" at 400yds. Quite an adjustment - can be done, but it won't be the flattest shooter around, but none really are at that range.
You'd really have to range-find & compensate (& really know your muzzle velocities, etc.) to consistently dial in at the longer ranges - for all practicallities.
Get your 7 - you'll be glad. Understand the limitations, as with everything/anything else you'd buy.