My guesses are:
a. Sports Rifle (so Feinstein doesn't assume it's an assault rifle),
b. Short Rifle (20" barrel compared to the Axis' 22" barrel),
c. Short-action Rifle (hyphenating three words may be cheating) and
d. Suppressor Ready (referring to the threaded barrel).

SavageArms.com doesn't say why they gave it the SR designation.

Is there a way to find out for sure?