12:00 rail sizing - JP Enterprises Tactical Rail Kit Accessories

Quality is good, but figuring out what you need is really confusing. Spend some time on the JP website figuring it out. For the 12:00 position:
JPTR-R Rifle length is 12"
JPTR-M Mid-length is 9"
JPTR-FS FS is typically for the front sight and is 2".

Please remember these are only for the 12:00 position and as far as I know are compatible on all MKIII style including their Signature series. Shorter ones are compatible with longer rails if you want to save a little weight. Please remember that one end is designed to bridge the gap over the barrel nut and does not have a hole for a screw.

There are 4" and 2" that DO NOT fit 12:00, but will mount in ALL other positions. I suggest you find your handguard on the JP website and click through the options in the upper right corner to figure out what you need. Signature series needs a backer plate to fit in the slot. Good luck!