It seems that bipods can not be equipped to the large rifles, the M320 and the GM6 rifles, these are probably the guns most in need of being stabilized for bipods, thanks for adding this great feature!
Edit: also seems you can't deploy with the riles as well. :)
start as a sniper with above guns, pick up a bipod, try to equip it to the gun.
That's because they already have permanent bipods attached to them. Though their deployment point is a bit bugged.
Hi, this is intended situation. As runekn already mentioned in his comment, these weapons already have permanently mounted bipods, so there is no need for another bipod. Marking as not a bug.