Furet and others
3 months ago I too was looking to build an SPR without some of the bells and whistles that others are using.
Heres what i did.
I ordered an Armalite M15T model upper direct from Armalite, however instaed of the 24" barrel I had them install the 20" CMP barrel in pre-ban configuration, this way I could install the break of my choice and do the float tube work first myself, then permanently attach the break if I chose to, (or buy a pre-ban lower)
Now I had it set up like the M15T with its free float tube. I marked the tube where I wanted vent holes and rail attachments while on rifle, then removed the flash surpressor and gas check and unscrewed the Free float tube,
I then drilled heat vent holes in hand guard at pre marked locations. I didnt want to weeken the structure to much so I didnt add to many approx, 1 every 2 inches at2oclock and 10 o'clock then a few on the bottom near the front. then I purchesed some GGG rails for around 30 bucks each and a surefire rail and used flush fit stainless nuts on inside of the barrel. This had a unplanned result of stiffening the tube even more after drilling the holes, all in all I ended up only saveing about $100.00 over a PRI tube, but it was fun!!
To make a long story short If you dont need all the rails go with and stay with the Armalite tube it is very sturdy and rugged, ive had no problems and I can be somewhat abusive sometimes during training. Thats why I use ACOGs also hahaha
good luck
chuck
BTW I get 1/2MOA or better on a good day with a TAO1 ACOG and this rifle and Blackhills 68BTHP Molys.