The MBUS Pro Offset sights are designed to accommodate M1913 spec rails, to include those that are oversized or undersized compared to M1913 tolerances to some degree. The limits on minimum closed dimensions were engineered to prevent overtorquing and damage to rails that fall within specifications, however, a grossly undersized rail dimension may fall outside of the minimum size adjustment and not allow the sight to be tightened on rails significantly smaller than min gauge size.
Since the MBUS sight program has kicked off, we have seen a significant number of rails introduced that fall well outside minimum specifications. We have initiated an inline change that will allow fitment on a broader range of rails that fall further below minimum specifications, but all MBUS sights currently fit with all rails made to M1913 specifications, and we have verified this with gauging of any sight returned for inability to clamp on a rail.
100% of sights returned for this issue have closed and held properly on a min spec M1913 gauge. Likewise, we have many rail/handguard samples from various manufacturers that fit loosely into a min spec female gauge, and some that have significantly undersized dimensions, even compared to these min gauges.
It is important that you not over tighten the attachment screws as this can damage the mounting hardware. If your sights cannot tighten down within approx. 20 inch lbs or less, please contact Magpul Tech Support or your hand guard manufacturer. We are glad to take a look at any issues, but the problem will likely be with an undersized rail for issues of this type.