The alleged "bubble" has zero to do with flight and "wind tunnels". A wind tunnel will allow assessing of the ballistics and flight. A cup point or flat point, will NOT fly efficiently, a round nose with the "cap" will fly marginally better. No one even claims a "bubble" will form for the bullet in flight. Why don't we see all these supersonic cup pointed aircraft otherwise. The unscientifically supported "bubble" is what is claimed happens in the flesh of an animal. To claim to explain why the claimed wider wound canal or channel is wider than the calibre size. A magical "bubble" of air forms in front of the projectile, wider than the calibre size and creates the damaged.

I believe some have tried ballistic gel type test material to shoot the hydros into and show the wider "wound channel". This is probably the right approach.

By scientifically proven, I mean an independent testing of a suitable sample size, the hydro with the same velocity or at least powder charge, against the same weight FMJ's and SPs of varying designs. Same velocity, same bullet weight, same calibre. But independent. And not people sent free bullets by the maker. For stories in magazines and the internet.

I would also like to see a physicist with a theoretical knowledge of ballistics and aerodynamics, explain how a "bubble" forms and works.

Sorry it sounds like a lot of hokum to me scientficially. The reason i have said I would like to see it proven.

As for the people that say it works, Good-O. Pay for them and use them. Most intelligent commenters I have seen say it is better penetrating than a expanding SP, but a lesser wound channel, and possibly penetrates less than a dedicated FMJ. Or maybe penetrates similar to a FMJ. Claiming it is better than a SP AND a FMJ again, wishful thinking IMO. It is an expensive compromise bullet.

One advantage is the driving bands, and less frictional wear and tear on barrels and rifling, pressure etc. Possibly a big advantage in think walled barrels and older vintage rifles.

If hunting elephant, I would use a dedicated FMJ every time before a hydro. If hunting buffalo, maybe I would try it, if I was confident it would cause a greater than FMJ wound channel. Not hard to use complementary SPs and FMJs though. And have the best of both types of bullet.

In some markets and jurisdictions, one has to use a monometal projectile and pay for the nose for them.