I need to mount some aftermarket picatinny rails to the fore-end of an Armalite M-15 upper which has their free floating fiberglass tube.  Armalite used to sell a unit that was already configured like this, but it is no longer available and I have to do the mods myself.

Any recommendations on how to best attach these rails to the fiberglass tube so that it is both secure and which will not screw up the fiberglass FF tube?

I'd go with Magpul rail sections with the threaded backer.  

Hopefully we have a fiberglass expert, but I'd suggest blue painters tape and a really sharp drill bit.

Depending on the hole diameter, see if you can find a Forestner (spelling) bit.  Will cut a hole like like a hole saw instead of like a twist bit.  Should make a cleaner cut.  If you cannot find one that small, start with a new high quality bit and drill a pilot hole that is 50% the size of the final hole.   DeWalt makes good bits with a pilot hole starter.

Also use the Blue tape like OleKenny said.  Since you are drilling on a round surface, your drill is going to want to walk, so drill ores if possible.  Tube in a "v" block for stability.

Take your time.

77

emulate PRI's set up

Use Magpul.  Fine for weapon light or other small objects.  Not optics.  Do not expect precision.  Might not bear weight well.

Tagged: I'd like to see the end result if you're inclined to post photos.

Also, A Forstner bit is definitely the way to go, just make sure the tube's dimpled/ marked so it won't walk... be aware that they dull faster than traditional bits as well.

the PRI rails have a concave/ rounded base for a 2" tube. works much better than a flat bottom style, and has a side benefit of helping you line them up.  here's PRI rails on a Clark carbon tube. I drilled the holes large so I had room to tweak the rails before I locked them down. was very solid.