I am looking at adding a SureFire light to my AR15 M4 type rifle from Bushmaster (16" barrel) with M33 handguards with built in upper and lower rails.

I am considering the M962SU with tape switch:

<http://www.surefire.com/cgi-bin/main.pl?pgm=co_disp&func=displ&carfnbr=134&prrfnbr=513&sesent=0,0>

If I mount this light on the lower rail.  Will I still be able to use my swift sling with out interference between the front sling swivel and the light?

Also, would it just be better to go with the Millennium Rail Vertical Foregrip System for my AR?

Any suggestions would be appreciated on the two.

Think about mounting it on the starboard side rail of the weapon.
Also consider the push button version, and a vert grip.
Or, go with the M900.

Well, coupla things. First of which is your mounting platform. If you are going to use a forward grip, I'd reconsider the Fobus handgaurds/rails. Those Fobus rails are plastic and I'm not really very comfortable mounting much to them. Especially not a forward grip. You crank down on that forward grip pretty hard when you use it, and it worries me to have so much pressure on some plastic rails.

If you are not planning to use a forward grip, and just want the M962SU (great light by the way), you could mount it just fine. You could also mount your Fobus grips with the rails on the left and right side instead of on the top and bottom. Again, if you are not planning to use a forward grip, you could reposition your rails and mount the light to the side if you so choose.

In the end, I think one of the best AR15 lighting systems Surefire has is the M900. It has a powerful 3 cell light and a front grip built into one. Because the batteries are stored in the grip, it saves a little space. The M900 also has a constant on/off feature in addition to the pressure pad giving you two power options instead of one. On top of that, the M900 has the two little LED navigational lights which are not only pretty cool, but VERY useful. And while I REALLY like the M900, I can not recommend using it on the plastic rails of the Fobus handguards.