Monty,
According to Surefire the M900a will add 18oz to your carbine. The M962 series adds 11.8 oz if you are like me and like vertical grips, a M962 with a quality vertical grip like the dieter CQD or Tango Down will be very close in weight to the M900a.
The M900a will add weight but how it feels will be subjsctive to the user. I shot my carbine with a m900a for several hours which included double taps and hammers from the low ready and while moving and did not have any problems with the weight.
I'm using it on a SIR 50 with an EOTech 552 forward mounted and it still feels fast and handy to me.
I also have no problems using it for one handed shooting and mounted mag changes.
[img]http://a7.cpimg.com/image/91/99/23963537-b666-01800200-.jpg[/img]
[img]http://a8.cpimg.com/image/92/99/23963538-9a07-02000180-.jpg[/img]
[img]http://a4.cpimg.com/image/CA/9B/23963594-55f4-02000180-.jpg[/img]
With that being said, I truly feel the weight added by the M900a is definitely worth the capability it adds to your carbine. I must have looked at and tried all the different combinations to setup the M962 series and no matter how I set it up there were certain positons, grips that made it awkward to use. Not to mention the M962 was about 7 inches long and really added bulk to the sides of my SIR.
The M900a puts everything together in a smaller more efficient package. It puts the light 6 o'clock which I liked better than 3 or 9 o'clock, you get LED navigation lights and along with a great vertical grip that uses the ARMS throw lever. More importantly the M900a's controls work great bi-laterally, I have no problems switching from strong to weak side and using the light.
Just my 2 cents, as always YMMV. [:D]