Back in the 80’s, the Honky-Tonk Man used a guitar gimmick, where he would whack his opponent on the back with a guitar and it would explode dramatically. This gimmick was used for many years after that by different people. I’m also pretty sure the guitar was playable as well.
It occurs to me that the guitar must have been specially constructed to explode in such a way. I would imagine that a real guitar is quite hard to break by striking it against something, and I’m pretty sure the back would collapse, not the sides. Not to mention the shrapnel would have presented a real danger to the wrestlers.
Just watching a few rounds on youtube real quick, it looks like they’re made out of balsa wood. It’s the same wood that these are made out of if you made one in grade school, it’ll break if you look at it cross eyed.
Is one - not a whole lot of detail, but looks like balsa wood is common in the wooden like objects. I think for glass and stuff - sometimes it is made of sugar.
It wouldn’t surprise me if the scored the wood on the inside so it would break more dramatically - some bombers do this with pipe bombs - by basically giving the wood (or metal for pipe bomb) areas where it is weaker - it allows you to control where it will (most likely) break.
As for how they exploded, the sides didn’t break only because the sides are ‘standing up’ and got hit on, what’s left of, the end grain. If you made a guitar out of heavy cardboard, the same thing would happen. It also looks like they filled it with some kind of powder so smoke would come out. As for shrapnel. It’s probably just a combination of ‘not much’ and those guys are 350 pounds of muscles and steroids, they can take a few splinters. Pro wrestling might be fake, but they still get tossed around and hit with chairs, this is just one more bump or bruise.
