For a long time, the LRMAME (MAME Current) core has a bug whereby the right mouse button cannot be used. You can map or configure the left button, but not the right. It doesn’t have to be complicated, because only that fails, the right and center button.
This affects all MAME gun games, that you can only set the shot, but not the reload.
I leave it here also to record and see if someone can solve it.
All this tested with the core updated to 0.235 and the latest nightly, although it has been present for a few years.