I found this bug since FM+ 1.0.3 and when it updated to 1.0.4 I still found this bug.
When I fire a gun (Any guns in any content packs) the bullets sometime disappear in mid air and it disappear continuously.
I realized that it less buggy when I shoot the target that stand at my eye level but when I shoot from the high place to the target that stand lower from my position or I shoot from the low place to high place target, it bugged.
Anyway when I stand near the target(4-5 blocks) and fire a gun, it didn't bug.
This bug annoyed me and other players so much because guns can't be used in some situation that they are bugging.