I just picked up a sport two optic ready for 450, screaming deal in my opinion.. I wanna free float, but I don't know what size hand guard I would need to keep my factory picatiny gas block.. I have a 15 inch from another build but I wanna go short and not have to change to a low profile gas block.. anyone know the correct size I would need? In ex. 7", 7.25", 10" etc...