I use an 11.5" Bushmaster for duty use and currently have the Knight RIS and Knight 1" light holder with a 6P installed at the 6 o'clock position, but I'm considering the M500A to replace it or just go to a M900A.  The problem is that I use the Trijicon Reflex on the top rail of the RIS and by going to the M500A I'd be unable to use it.  

Is the 500A a good light and is it quite durable?  I've only seen one other 900A in the field and I think it'd seem pretty awkward on an 11.5".  Is it worth looking at these lights or should I just stay with the 6" in the 1" mount?  I'm worried about the balance and the short barrel hurting the light as well.

There is no9 reason not to use the M900.  The muzzle will be well ahead of the M900 and if the flash hider is an A2 style (A2, KAC, Gemtech) and closed on the bottom you REALLY have nothing to worry about there.  The M500 willplace more of the light's weight forweard and upset balance more, plus you wont have a forward grip with the M500.

We are putting M500s on Patrol carbines and slowly upgrading SWAT carbines to M900s and rails (M4 RAS). The M500 is a good choice for general patrol use; the pressure switches and related gear are all out of the way where Murphy has a tougher time breaking them. We are going with the M900 on SWAT weapons because there is not other real equivalent to it in terms of output and with the LEDs, and because most of our folks are converting to the vertical foregrip school of thought. We are running rails to accomdate other gear, particularly the IR lasers that are critical if you are goinf to run sucessful NVGs ops.

I use my SWAT carbine for patrol use, and the M900 works fine. On the 11.5" we run (most are standard 14.5" M4s) the M900 works fine. One potential problem is that if you are running the M900 and rails, you are going to have a difficult time getting a working rack to keep it inside the patrol car. We are cobbling together racks, but they are having to lock around the receiver extension with the stock fully extended, and I am not sure that is not going to be too much stress over the long run. You can get off-the-shelf racks for an M500-equipped carbine, though you can only use a 20 round magazine while the weapon is racked.

Either systme would work great, but if you want to keep the rails and the flexibility that offers, then go with the M900.

I think the M500A might do the trick.  The Knight RAS + Trijicon Reflex has worked well, especially at night, but it's pretty heavy.  I've learned that I just need a really good light and I'm fine.  My concern with the M900A was that we don't have the rifles in racks, they're just sitting on the seat next to us (some guys use a rack if the car has one) and I think that vertical foregrip might get caught on something as I pull it out of the car.  The M500A seems so self-contained; however, one of the guys was saying that it was fairly fragile.  

Thanks for your imput.