Now:
Double press "R" or one press(but sometimes it's too difficult) = unload one bullet
Long press "R" = change magazine if it in the player inventory. Change happens if next magazine is not empty

Now:

1. Player add pistol to one hot bar ceil(2)
2. Player open inventory
3. Move pistol magazine to inventory or ground
4. Move pistol magazine to hands(Swap)
5. Close inventory
6. Hold "R"
7. Press 2
8. Press and hold "R"

Quick way(without open inventory):

1. Player add pistol to one hot bar ceil(2)
2. And add pistol magazine to another hot bar ceil(3)
3. Press and hold 3
4. Press 3
5. Hold "R"
6. Press 2
7. Press and hold "R"

Future(without open inventory):

1. Player add pistol to one hot bar ceil(2)
2. Twice press "R"
3. Hold "R"
4. Press 2
5. Press and hold "R"

And add info to Controls
R (Hold) - change magazine if it in the player inventory. Change happens if next magazine is not empty
R (One press) - unload one bullet if pistol haven't magazine
R (Two press) - unload magazine and move it to player hands