The OFP Field Manual's description for the BMP-1 includes the following:
"With aggressive anti-tank and relatively high speed capability, this combat zone transport also provides visual systems and gunports for each troop member aboard, thus allowing them to add to the BMP's own killpower with their personal firearms."
While the manual is stating facts about real BMPs, the OFP gaming engine does not support firing personal weapons from onboard a vehicle. This cannot be implemented in OFP because of the complex code modifications that would be involved.
(Source: Ondrej Spanel, BIS Lead OFP Programmer).