If your careful I don't see why not, just don't use one thats really coarse, the metal is not all the tough, don't want to really gouge it out. When I filed mine I was using one of my fine needle files used by locksmiths to file keys by hand.

Update!! I filed a slot for it this afternoon. it seems perfect, I won't know until I get my new BUIS... the current one is just too fat to fit, but I think I did a pretty good job on it.
btw.. for those who were wondering. the CA SIR can fit up to 8.4v 1400mah... I got a feeling the 9.6v 1400mah might work, but I don't have one around to try it out! My gun is setup to be CQB only, so around 330-340 fps, so battery is not an issue for me.
In one of the pictures, u might see some black paint, but that's just because of the flash. You don't see anything. :D

Pictures!!
http://www.stealthcore.com/pic/IMG_0321.jpg
http://www.stealthcore.com/pic/IMG_0322.jpg
http://www.stealthcore.com/pic/IMG_0320.jpg 13 GROOVES WOOHOO
http://www.stealthcore.com/pic/IMG_0315.jpg
http://www.stealthcore.com/pic/IMG_0317.jpg

THANKS GUYS!!!

it wont hold a holosight? Fuck that noise.