Must be solid bar or as damn near it to stop it folding under load.
I remember being intrigued by one of my university lecturers, who had a passion for motorsport, trying to devise a process for boring out the centre of a forged steel driveshaft.
Imagining he was trading strength for lightness I assumed it would be a fairly linear relationship until he pointed out that the material at the centre of the shaft was more or less only along for the ride and it was the material near the circumference that actually did all the work. On that basis he was able to shed a disproportionate amount of weight without significant detriment to the strength. One of the few lectures that sticks in my mind...
Another I remember involved the lecturer pouring liquid nitrogen down the barrels of a side by side replica shotgun he'd made from a noggin of wood and some cold-drawn, seamless boiler pipe, then stuffing corks into the ends and aiming it randomly at his students as the gas expanded until the corks exploded out with considerable violence.
We spent most of that lecture under the desks until coaxed out by his intriguing assurance that by using liquid nitrogen you could deaden pain.
Having pulled on a rubber glove he then proceeded to stick his thumb into a flask of the stuff, grimaced for a minute or so, then pulled it out, put it on the bench and hit it with a hammer. Needless to say, it shattered into a thousand pieces causing panic amongst the med students who also had to study engineering in case they ever had to knock up a wooden leg or something.
Assuming that the bloke had pulled this stunt before without reducing his quota of thumbs I wasn't taken in, but it looked impressive. It later transpired that the thumb of the glove was stuffed with mince...
I bet he'd not be allowed to do the thumb thing these days (the shotgun trick was bloody dangerous too! But great fun.) because some do-gooder would want counselling in place for those fools who fell for it and so the world loses a little more of its flavour and becomes a little more bland.
I'm only a plumber from Cannock...
This is not put on as a public entertainment... it's put on to reach a certain goal.