A NOTE ABOUT "BUMP-FIRING" and "HELLSTORM"-type TRIGGER MANIPULATORS
I'll be the first to admit I'm basing my assessment of bump-firing by watching others do it and by examining their weapons' fire control groups afterwards. I have never really been able to bump fire successfully. Furthermore, if I understand bump-firing correctly, you are basically using your finger in the same way you would use a trigger manipulator like the "Hellstorm" system. Both bump-firing and the Hellstorm-type system give the appearance of full-auto fire in a semi-auto firearm by rapid manipulation of the trigger so as to limit the amount of time the hammer is held by the trigger and disconnector.
I could be wrong, but I lean toward bump-firing being unsafe for the same reason that it is unsafe to convert your gun to full-auto by filing down the disconnector. Since the AK action is not "timed", as stated above, by tampering with the disconnector or trigger's "grabbing" of the hammer during bump-firing, you are potentially toying with a timing problem that could eventually fire the gun out of battery. Since a semi-auto AK does not have the extra auto-sear lever that also prevents the hammer from falling until the bolt is closed and locked, by bump-firing you are potentially causing the hammer to release prematurely, detonating the round before the action cycles completely.
When the gun fires semi-auto, there is no way that your finger can release the trigger faster than the action can cycle and a new round can chamber and the bolt lock. Even when employing double and triple taps, you still cannot manipulate the FCG faster than the action can cycle completely. This is how the rifle is SUPPOSED TO function if all parts are interacting correctly. When a semi-auto fire control group is functioning properly, there are a number of natural "safeties" in operation: the gun cannot fire another round without releasing the trigger (gives time for the bolt to close and lock), and the trigger holds the cocked hammer until released (prevents the hammer dropping during the cocking sequence which would make a gun fire on full-auto until empty).
When you bump-fire, however, you are to a certain extent "cheating" the cycling of the action by not allowing the FCG to do its job normally. In essence, you are defeating the purpose of the disconnector and trigger hooks by keeping the trigger in motion.
I have made the following observations:
Just from my experience with a handful of rifles and different shooters, it appears that NOT ALL AK's can be bump-fired. The most successful bump firing I have seen has been in Romanian SAR's that retain their Century FCG's.
Century FCG's are notorious for questionable engagement of the trigger and disconnector surfaces that grab the hammer. The rifles I have examined that seem to bump-fire the best are those in which the trigger and disconnector BARELY grab and hold the hammer during the recoil stroke (disconnector) and trigger release (trigger hook).
In my experience with AK FCG's, the trigger and disconnector should both grab the hammer at their respective engagement points with approximately 1/16" of engagement surface between them. In guns that have successfully bump-fired, the engagement surfaces are often as little as one-third of that. IN OTHER WORDS, THE RIFLES THAT BUMP-FIRE THE BEST ARE THE ONES WITH FIRE CONTROL GROUPS THAT HAVE A MARGINAL RELATIONSHIP BETWEEN THEIR ENGAGEMENT SURFACES TO BEGIN WITH. Indeed, on one rifle, when I let off the trigger slowly and the disconnector released the hammer, the hammer was not grabbed by the trigger hook because the two were not "lined up"! The hammer fell and hit the back of the bolt just because I let off the disconnector/trigger engagement slowly enough that the trigger hook was not positioned properly to grab the hammer! On this rifle, only when I let off the trigger rapidly was the trigger hook positioned over the hammer fast enough to "grab" it!
It seems that bump-firing depends upon a poor relationship between fire control parts!
I have unsuccessfully tried to bump-fire a Bulgarian AK with a quality fire control group. I have gotten others to try bump-firing AK's with quality FCG's. I'm defining a quality FCG as having very positive and definite interaction between the hammer and the trigger and disconnector, i.e., right around the 1/16" interaction between engagement surfaces. A rifle in which the trigger and disconnector barely grab the hammer (as on a Romanian AK with a Century FCG) is basically being bump-fired by the shooter "jarring" the hammer loose from where the trigger and/or disconnector just barely grab the hammer in the first place. To cite an analogy: it is easier to knock a glass off the EDGE of a shelf than if the glass is in the middle. When you bump fire, you are repeatedly knocking the hammer off the "edge" of the trigger and disconnector, because if the parts were positioned in the more proper "middle", bump-firing would probably not be possible, or would at least require a mechanical trigger manipulator that is capable of more force than your finger.
I cite as evidence one of my fellow shooters: when his SAR-1 had the Century FCG, he was able to bump-fire at will; when he switched to a Red Star adjustable FCG, the rifle would no longer bump-fire! Clearly, the difference in interaction between the parts of the two FCG's played a critical role!
In a similar way, being that a trigger manipulator such as the Hellstorm is a mechanical device generally made of metal, it will necessarily exercise more force in "jarring" the hammer loose from the trigger/disconnector engaging surfaces than you could do with just your finger. Most trigger manipulators I have seen in action appear to work in nearly all weapons. As most manufacturers of these trigger manipulators claim that their product will work on most weapons, I can only conclude that these products are simply better at defeating the function of your semi-auto fire control parts than your mere finger would be. Your finger might only have the strength to bump-fire in a rifle with poorly-fitted FCG parts, whereas a metal mechanical device is capable of exerting the extra force necessary to defeat even properly fitted FCG's.
I am not a firearms engineer, but I understand a few things about firearms in general, and AK's in particular. In my opinion, when you bump fire you are defeating the natural "safeties" built into a semi-auto fire control group, and so are potentially creating a dangerous situation in which the timing of the gun's firing sequence can be just out of whack enough to fire the gun out of battery. Similarly, I am assessing trigger manipulators such as the Hellstorm system to be mechanical aids to bump-firing, but they are accomplishing the same dangerous tampering with the fire control sequence as bump-firing with your finger.
If any manufacturer of a mechanical trigger manipulator can cite good evidence as to why his product does not create an unsafe defeating of an FCG's function, then I invite that person to post his evidence here...
This forum is all about learning, and we are all interested in the facts...
In my final assessment, I am willing to concede that the jury may still be out on bump-firing and trigger manipulators, but I lean toward both being as unsafe as any other attempt to gain full-auto fire in a semi-auto AK...