The bolt actions are the only guns it's possible to NOT waste a round reloading before empty, and then only if you have manual bolting selected.
Bolt gun scenario1: You fire, then press R before working the action. The animation shows you opening the bolt, ejecting the spent casing you just fired, and then loading 1 round to replace it.
Bolt gun scenario2: You fire, work the action, ejecting the spent casing and loading a new round. You then press R, and the animation shows you ejecting the round you just chambered, and loading two to replace the fired, and the unfired ejected round.
Semiauto: You fire then press R. In firing, you've spent one round, ejected, and then loaded a new round. moving the bolt to the rear to insert new round ejects the unspent round you didn't fire, and you load two more rounds.
This is all silly anyway because the Bolt Hold-Open on semiautos doesn't work with a non-empty (but seated) magazine. To reload a seated magazine that is not empty, you need to hold the boltcarrier to the rear with one hand and load rounds with the other. This requires some acrobatics, but is possible, just not in the way animated in-game.