Take great care with making a note of the exact distance it is in, and also use Loctite 243 when re-fitting if this is the grub that protudes into the air passage transfer as per RB2 Airsporter.
I re-built a RB2 (which had never been apart since new) a few years ago, and removed the grubscrew and neglected to measure exactly how much it stuck out into the airstream, I then had several times to strip the gun as it would not make more than 8 ft/lb despite all my efforts.
I left the screw in the position that seemed to make the most power, which was just over 9 I think, but was at a loss, all new inside etc.
I spoke to Jim Tyler about this and he said "air leak"......I said "what....down past a grub screw????"
I had not Loctited the screw.......I used number and letter drill shanks to measure the screw height into the transfer, then removed it, re-fitted it to the correct position with Loctite and left 48 hours........It went straight to 11.3 foot pounds
So if you are thinking it will alter the power, you may be right, but you may also be giving yourself hours of grief.
ATB, Ed