Sights with an open elevation drum

Open elevation drum

— this feature means that the adjustment barrel or barrels provided in the scope are not covered by protective caps, which means that you do not need to unscrew or remove such a cap each time to turn such a regulator.

This feature is relevant primarily for optical sights (see "Type"), and it means that this model is intended mainly for high-precision shooting (not necessarily for long distances, but with maximum accuracy). In this application, open drums are simply irreplaceable: the sniper constantly has to deal with various amendments, which often have to be made literally between individual shots and as quickly as possible. The disadvantage of the open design, some say, is that accidental contact with a foreign object can knock down the sight settings; however, manufacturers usually take this possibility into account and provide various design features that practically eliminate this drawback.

The opposite option — optics with closed drums — is designed for scenarios in which the sight is adjusted once (when zeroing in) and then used with constant settings. An example of such an application is hunting at relatively short distances. And for collimators, the difference between the open and closed design of the drums as a whole is not fundamental.