I have a 45c that I am installing on my friends AR, but I don't have the instructions.  I installed one many moon ago, but I can't remember any of the details.  Does anyone have some instructions that they can email to me or post?  I would greatly appreciate it.

I dont think there was any that came with it. All you have to do is take the old handguards off and mount the top of the SIR to the top rail on your AR. After that mount the bottom handguard to the top rail. There is a plastic thing that comes for it for light barrels but you should not need that at all once tightened down.

I've got a couple SIRs including a 45C the only "plastic thing" included is the handguard itself.  

Secure the SIR body to your flat-top receiver using the 3 bar clamp(use blue loctite).

There  is a steel yoke that goes around the barrel nut and gets attached to the SIR body with 4 screws.

After that just slide the polymer handguard into place and lock it by pushing in and turning the two locking buttons 90 degrees.

OK, thats what I thought.  But since I didn't have the instructions, I wanted to make sure of the installation.