I just got a Bushmaster M4 upper. I tried to install a mil-spec M4 side-sling adapter (from Pete-in-NH) and it won't fit under the front site post. The thin metal legs won't fit between the bottom of the front sight base and top of barrel. I also tried with a Bushmaster side sling adapter, and an M16A2 side sling adapter, same results. Are the front-sight bases that Bushmaster selling now non-milspec?