I have HK417 and have tryed Knight's SR-25 on my friend's acc several times. I prefer HK-417 for overall Damage done. Great ROF, easy to handle ( so long as u can aim and "click" at right times ). As far as +10 Ammo mod, it sucks ( ruins stability iirc )
Note: As a Semi-sniper user, you should know that your not gonna have the best possible score, you'll be more of a "Support" for your team ( MOBA term. ), basically even if you die your job is to kill or adleast lit up ( make low hp ) several targets and let your teammates kill them.
Semi-snipers work WONDERS with lazers ( R4 Remmy, XMaid, m4 mk3, AMD65 )
But seriously if you want a Semi-Sniper with high OHKO, consider DSR-1 or even AWM or M40A5, they have really fast potential ROF and high(er) 1HKO.