Rather than an explosion the burst disc will fail when overpressurised. This is scary enough and whenever it has happened at a fill station in the past the queue have scattered pretty quickly.Does it just overpressurise the steelie if you use a 300bar tank? Hence cause an explosion?
It is possible to CAREFULLY fill a 3000psi cylinder from a greater source, but it is risky and should only be undertaken through a regulated fill station at the appropriate pressure for the cylinder.
Check out the HPA1 in my earlier post.