I can confirm this.

Drink All: animation / sound plays, then you get stuck with "Cancel current action" for eternity.

Drink Canteen: works fine

[M4 with T3 RDS sight]
M4 in lowered mode
click [RMB] once
brings up gun, same zoom as without gun, can zoom in with [NUM+] and by holding [RMB]

[M4 with any other sight]
M4 in lowered mode
click [RMB] once
brings up gun, zooms in to the max zoom level of the RDS automatically

[any other gun]
gun in lowered mode
brings up gun, zooms in to the max zoom, except in LRS, which can be zoomed further in, cause LRS is like that

So, yes it zooms in by holding the [RMB] or the [NUM+], but this isn't the way it supposed to be and zooming it manually each-and-every-time-you-want-to-look-at-something-you-saw-over-there is quite annoying. Wish I could post pictures when the issue is clear, but I'm currently at work.

I've added some screenshots where you can clearly see that the T3 RDS sight doesn't have the same amount of zoom as e.g. the revolver, and also that the M4 zooms better without a sight than with this sight.

Then why do all the other weapons, scopes and sights doesn't work that way? Including the M68 sight, which is basically the same red dot sight as this one.

I can confirm this, pressing [RMB] makes the gun rise, but the view (zoom) doesn't changes at all, making the scope all but useless. It is still possible to zoom in and out with the [NUM+] and [NUM-] keys (at least for me), but it should zoom in automatically like with every other gun and scope.