I should probably add something of my own. I like failed firearms designs that had some features that makes them objectively better if you overlook all the downsides. Or generally anything where one person made it because they thought it was cool and no one with better judgement stopped them.
Like the Gyrojet pistols, unlike traditional pistols they used rocket projectiles. This meant two major things one the guns didn’t need to have any pressure bearing parts and could be made incredibly light compared to normal pistols (.88lb/.4kg vs ~2.2lbs/~1kg), and the down side they reached maximum speed much slower than normal cartridges.
This meant that in practical terms a target close enough might not give the rocket sufficient time to accelerate to effective speed and wind would have a greater effect on the projectile. Hence they don’t make them anymore.