Hello, ARFCOMers (I guess I'm one now as well).  I've been avidly reading the posts on the forum for the past few months, and have managed to be hit with some form of the black rifle addiction without actually owning one.  I'm aiming for a Bushmaster Superlite 16", pinned telestock, A2.  Most of my questions actually pertain to the "Optics, Mounts, and Sights" forum, but this most popular forum seemed to be a good place for a first post.

First question, what does BBL stand for?

You could try to talk me into an A3, but at the moment, I'd really like to keep the carry handle: to make it easier to carry (duh), because it has the look of an AR, I won't have to buy an ARMS #40, and I don't think I'll be interested in a >1x scope for a long while.  Thinking of an ARMS #51 (comes with a side sling attachement :D) to mount a Surefire, and once I excel at using the iron sights, maybe an EoTech.  The recent ARMS #51 discussion said something about not being able to cowitness with the EoTech.  Is it possible to just mount the EoTech on the forward rail and still be able to see the irons through it, even if the dot and the sight post don't match (or is that the definition of "cowitness)? Ok, enough for now.  thanks guys! (for slogging through a stream of consciousness)

I think bbl stands for barrel. I would highly recommend the A3 with a flattop. If you put the carry handle on and never take it off your only out a few bucks but if you ever want the advantages of the flattop you out of luck. The flattop will give you lots of options in mounting scopes that you won't have with the built in carry handle. Lots of people wish they would have gone with the A3 later. You won't have to buy a Arms#40 unless your going to be in a situation like shtf, it's for emergencies when the scope goes down. You could allways attach the carry handle for the iron sites also. The Eotech will cowitness with the iron sites with a Arms#39 (I think thats the one), it attaches to the carry handle and gives you a rail above the hand guards. Yes you can see through the Eotech if the sight post and dot don't match (cowitness is when they do).

Thanks for the clarification about cowitnessing.
Persuasive arguement for the A3.  My primary concern is about the different models of the ARMS SIR.  I'd like a RIS of some sort, and so far would prefer the SIR (freefloat, secured to upper receiver, either A2 or A3 options, attractive/impressive appearence).  Switching between flattop and carry handle A3 would require switching handguards, be it the standard and a #45, or a #51 and #45 (ouch, price! and do #51s even work with an A3 carry handle?).
The #39 would be good except that I can't think of a place for the flashlight.  Perhaps I'll just pose this question: for those of you who use carry handles, why did you opt for the A2, or if you chose the A3 why do you leave the handle on?