I have recently bought a HBAR AR15 A3 Flat-top.
I want to put my Harris bi-pod on it.

It has a Floated barrel and a 4 sided weaver rail mount system on the gas block.

Do I put it on the 'floated' sleeve or the gas block. I would have thought the floated part as puting it on the gas block rail would effect the barrel harmonics.

Any clues??

You can put it on either one, won't affect anything. I assume the gas block is set up for a removeable front sight. Myself, mine is on the float tube but either will work.

I have a Bushmaster V-match with floated barrel and forward hand grip tube. I drilled and tapped into the tube and mounted a Harris bi-pod that has been there for 8 years.