There are new NRA Service Rifle Rules in place for 2016.
Some of the notable changes are:
- Optics up to 4.5 power, fixed or variable.
- Metal or synthetic (polymer) magazines(20 or 30 round).
- Butt-stocks may vary in length and be either fixed or collapsible.
- Quad rails or similar hand guards may be used.
for official details from the NRA Web Site.
Look for "Rule 3.1(c) Service Rifle"