Knob Creek's website is www.machinegunshoot.com. Yes you can rent weapons there. They have the rentals on the lower range. There are also jungle walks, shotgun matches, pistol matches etc. The website has a schedule of events.
I may be wrong on this point but my understanding is that leo can buy with supervisory letter and signature on dept. letterhead. If you leave dept, weapon stays with dept. or you must sell. Problem is limited market for leo-only weapons. Mil must go through personal same procedure as civilian when purchasing FA.