You do know that the SCAR 17 is known as an optics killer. It has something to do with the piston spring recoil.
I own one and have had good luck with both accuracy and optics. The optic I used most was a Lucid L5 4-16 x 44. You can spend a fortune for an optic, and you probably should if your life depends on it, or competition. For occasional use the Lucid worked very well for me.
I have a nightforce 2.5-10 on my scar 17s it works great. What are you shooting at 300-400? Are you going for iddy bitty groups on paper or ringing steel? Do you already shoot at these distances? If so what magnification do you prefer? Once you determine a magnification range then start looking at which scopes in that range are the most durable and fit your budget.
what are you doing with it? what distances are you shooting? whats is your budget? lots of good options out there, my personal favorite is the 2.5-10x42 Nightforce, I set up a buddy of mine with a 2.5-10x32 before he deployed a few years ago and it's stuck with me ever since.