I've always seen those in easy ps, but I've never really learned what they do. Does it useful if you have a chaos scroll? Can you use it with item maker? Matt Edit: Thread moved to bug reporting in regards to the non-dropping of Gun and Knuckle Stimulator's.
It randomises the stats of the weapon you make. So if you were making a Dragonfire Revolver, instead of being fixed at 78att, it could range from between (73-88, it would be 73-83 but we have godly stats), just like if it was dropped from a monster. Unfortunately, either gun stimulators do not exist or there is only 1 monster in the entire game that drops them and it does so at a very low rate.
If I remember correctly, using a stimulator guarantees that the outcome will not be below average when it comes to attack. So for a Dragonfire Revolver it would be 78-88. Using a stimulator has a 10% risk of booming though, unless that was removed in Royals. (I have a vague memory of it being removed, but I can't say for sure)
I just checked and both gun and knuckle stimulators don't currently drop... So I'll add them in the next planned SC.
The 10% chance to boom is still there, but the stimulator works as David said it does. A non simulator maker gives it a stable average attack while the stimulator allows it to go under average and above average.