By "dart rifle" I meant "jet harpoon", I apologise.
With Jet Harpoon and Dart Autogun it also works correctly.
The weapons just have very similar handobs, so it's hard to see the difference.
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Maybe you used some pistols?
If yes, then that's a change in OpenXcom.
OpenXcom supports drawing dual-wielded pistols (two 1-handed weapons)... the original game could not do that, it showed one or the other.
However when holding both a 1-handed weapon and a 2-handed weapon, the 2-handed weapon wins (=is always drawn) in OpenXcom.
Btw. the reactions are unaffected by this drawing mechanism, i.e. the last selected weapon will be used for reactions (if possible), even if not drawn on the unit sprite. (If not possible, the other weapon will be used (if possible), same as in the original).