First, what ammo you using?...
Quality stuff or mil-surp? Match shooting require consistent QC for ammo, don't buy the commercial match, roll you own.
Variation in case length, neck thickness affect groups.
A decent group @ 100 yds (presume using the SR-21 (reduced 200 bull @ 100yds), you should be within the 10/X-ring, occasional flyer into 9 ring are you slips.
Is there a smith that can go over your rifle to make everything tight, any part loose will vibrate and that detract accuracy.
What type barrel & chamber? NATO, Wylie, or SAMMI?
Continue in the matches you are doing, ask and listen to experienced competitors as we love to help.