Quoted:
If it's a Bushmaster Carbon-15, the lowers are standard and will accept any AR-15 upper. If it is a Pro Ordnance (or very early BM) Carbon-15, the lowers will not accept a standard AR upper.
Not sure what you mean by "not very secure." Unless there is something wrong with your upper, the rail should be 100% solid on the reciever, even without the retaining screw. If there is any movement in the rail then you should probably send it back to Bushmaster.
it's the new bushmaster superlight, it's the bushmaster representative who told me their aluminum upper may not be perfect fit to their carbon lower
the fit between rail and upper is not perfect so there are some play, in fact when i got my bushmaster brand new i can literally lift the end of the rail up by about 1mm by hand, it's sent back to bushmaster and they replaced the upper, the new one seems to be tight (no movement by hand), but will it hold under heavy recoil i don't know
actually the bigger problem is not the "lift force" but the "push force", since rail and upper are held by dovetail (the rail slides onto the upper), there's no force to hold them together along the horizonal direction (barrel direction) except that single screw. but the recoil force is primarily along the horizonal direction (the scope will be pushed forward on the rail after fire), all the high g kick will be on that single screw, it's really dumb to design it this way
this guy has the
same problem