I need to drill a hole in a DPMS aluminum free float tube to install a Harris bipod. Problem is there's a 4 rail gas block and there's not enough room to get my #5 adapter in there.

I'm thinking I'll have to mark the free float tube and remove it to drill the hole. Also, I'd like to index the tube so the hole is either lined up with or in between the flutes on the free float tube, for cosmetic reasons.

I assume the free float tube needs to be torqued like a barrel nut? I have installed a DCM free float tube before but that was easy using a standard barrel nut wrench. Any suggestions on how I go about this? Strap wrench? TIA

The easy way would be to just drill & tap the handguard for a #8 stud instead.  No disassembly required.  I have one on a V-match that way and have had no problems with it.  

If I were to do it over again I'd probably use a #5 or #2 with a through hole for added strength, but I don't have the access problem you do.

I've never seen the DPMS handguard, but on the V-match handguard it looks like you could use the studs on a barrel wrench in the small holes around the end of the tube to turn it.  

I just dealt with the same issue (gas block preventing access). I just went to the local gun shop, picked up a stud, drilled and tapped the correct size hole, used 1 spacer and loctited it into place.  So far, so good.

The DPMS tube is probably on with some loc-tite, but should come off with a good, hard hand twist.  You may want to forgo this tube all together and look into DPMS' new Picatinny rail ff tubes, which I did.  Less expensive than the RAS or SIR systems, but allows you to mount whatever whereever whenever.  Will run you about $160.