Are there any disadvantages to using a non m4 upper with a m4 ramped barrel?
Yes you can.
Not optimal for very fast cycling ARs and a slightly steeper feed angle, but not generally a big deal in properly functioning carbines.
Are there any disadvantages to using a non m4 upper with a m4 ramped barrel?
Yes you can.
Not optimal for very fast cycling ARs and a slightly steeper feed angle, but not generally a big deal in properly functioning carbines.