I was thinking about putting on some of the replacement handgaurds that have the rails on them on my AR-15 standard length rifle. I would like to put my Trijicon Reflex II on it, but I was wondering if they are tight enough to keep it accurate, or if they are just for adding on other extras, not intended for sighting?

From what I've read concerning this:

The KAC RAS isn't stable enough for optics, but the RAS II (if you have a flattop), and the SIR (there IS a model for A2s out) by ARMS are more than steady enough. I believe some of the others are also, such as the C-more rail and the BMAS rail handguard. Again, this is based on what I've read here, but the people who posted the info seem to be the "go-to" guys about this kind of thing (new-arguy, 3rdtk, and a few others).

I can tell you that the M5 RAS top half is very stable.  It locks into the barrel nut and front handguard retainer.  It is very solid.  I wouldn't put a scope out there, but it would be fine for a red dot.

I concur.  I used to have an EOTech mounted on a RAS, & it worked great there.  I thought it quite the ergonomic setup.  
I've since upgraded to an ARMS SIR (recommended) as I got tired of rail panels.  I don't miss them.

Are you talking about a $300 RAS system or the $100 Fobus handguards with rails molded into the plastic? Also, what level of accuracy are you looking for?

Not trying to be a thread hijacker, but is there a thread concerning RAS II versus SIR?  I would like to know how much strength one would be giving up going to a RAS II over a SIR system.  I know that the RAS II is weighs about half of what a SIR system weighs.

Thanks!

I want to put a Trijicon reflex 2 on it, & be able to shoot like it was mounted on a flat top receiver out to 400 yards or so, & be able to put them into a human sized target.

I'd get the free floating rail systems.  If you want to go less expensive than a KAC RAS, Bushmaster makes a free floating rail tube now.  If I recall correctly, they are 200 as opposed to 400.  Bandit or Excaliber Arms had an upper for sale a while back and it did look sweet.

You really want a free-float tube then, though in my experience it is possible to get that level of performance out of a regular RAS. Haven't seen it at 400, so I can't comment but I know people who are happy enough with an Aimpoint mounted on a regular RAS for 300yd E-silhouette shooting.

I would still recommend free-float though. The only non-free float rail system that stands a chance of meeting your criteria is RAS and it costs $280-300. Free-floated RAS costs only $320 and I think RAS-II is cheaper still. You can also find a lot of free-float tubes with rails in the $170-200 range if you aren't looking for a hard use item.