I ordered this to replace a 12 in freefloat hand guard without a top rail for back up sights. Install was a breeze just make sure you have the 1.25 in crows foot wrench which you can get from acme as well. For my rig with torquing to spec only needed one shim. Hardest part is getting the top rail alligned perfectly with the reciever. My existing mlok accessories all transfered over easy peazy. I will be ordering from Acme again.