You don't need any strings to bump fire an AK...
It just takes a little practice (alone)...
What seems to work good for me is...
I'll hold the rifle at waist level,supporting the forward handguard with my left hand,very loosely but still with a good grip.
I'll rack the handle to load a round, and I use my middle finger on my right hand to fire the gun.
I keep my thumb tucked tight in my palm so the handle doesn't rip off my thumbnail when it cycles (know from past experience)
I put my middle finger through the trigger guard and push firmly against the side of my hip.
I give the gun a little jerk forward and then hold it loosely to keep it going...
Sometimes it won't fire 'em off right away but after a few trys it'll go no problem...
Ironically, I was out today shooting with my wife and a couple buddies, and I figured I'd try to bump off my 100 round drum for the first time...
HOLY SH*T was that COOL !!!
It stumbled for the first 5 rounds or so BUT THEN, It took off like a friggin machine gun!!!
Once it started, I just couldn't stop!!!
I figured I just let it go till it jammed, but it never stopped 'till the drum was empty!!!
SOOOO COOOOL!!!
We were shooting at "stuff" about 100 yards away...
I was able to keep all the shots in about an 8ft X 8ft area...
Nothing but a cloud of dirt and debris everywhere...
The non-stop bump fire lasted about 15 seconds or so (wasn't counting)
It seemed to go on forever and I kept waiting for it to finish...
After I finished, I stood the gun upright and propped open the bolt so the barrel could cool off...
It melted the black synthetic handguards a little, and was smoking for about 10 minutes.
I guess it's not too good for the gun, but MAN was it worth it!!!
I definately recommend trying the bump fire technique with a 100 round drum.....WAY COOL !!! [:)]