As I indicated, I think that you misread "Medium Velocity" as medium-sized and then mentally substituted "Large" for "High Velocity." Maybe I missed something?
That appears to be part of the issue. So we have differing cannon velocities - from mortar/low-vel through med-vel, high-vel, and hypervel (in two flavours). Noted.
Note that the "jet" is actually solid (not molten) copper. There's a lot of nonsense out there about "molten copper" jets and even "plasma jets." Experiments firing sectioned shaped charge warheads into water tanks show conclusively that the jet is not molten. You can also do back of the envelope energy calculations that show that it just can't melt the liner. That's with ridiculously optimistic assumptions. All of this is why I vastly prefer the term "trumpet": it doesn't cause the conceptual confusion of "jet."
It sounds to me then like what it really describes the the plastic deformation of a solid into a new shape that is directed into a very particular shape (a penetrator) without anything being in a liquid state. Does that capture it?
The liner actually acts as a fluid under the extreme pressure caused by the explosion. Fluid dynamics defines its behavior. The trumpet is pretty irregular, but the same warhead type produces the same general outline.
I thought your comparison with the 76mm was against the prior 75mm gun the Americans used. So it wasn't quite a 'same bore' issue, but you get a bigger bore (ever so slightly) and a higher muzzle velocity and that leads to reduced explosive payload...
The 76mm gun had a much longer barrel and the rounds had more propellant, giving them a much higher velocity. The 1mm difference was irrelevant to performance.
So, the gun was a higher velocity and that changes the performance, but reduces the efficacy of the HEAT round then? (vs the lower velocity 75mm?) Or were both the 75 and 76 limited in about the same way for how much HE you could pack in and thus HEAT performance might have differed much between the two guns?
Or was the 76 capable of firing further but with a less effective HEAT warhead?
I mentioned this before, but the higher velocity of the 76mm required thicker walls in the HE shells, reducing their explosive content and reducing their effectiveness. You would get similar effects with HEAT rounds. However, they didn't bother developing a HEAT shell for the 76mm gun because it wouldn't have offered any advantages over the APC or APCBC rounds.
I was offering the two guns as an obvious comparison, but neither gun actually had a HEAT round. Given the small caliber, it just wasn't effective. Some guns the same size or even smaller did use HEAT rounds because they were lower velocity or something.
Kinda fuzzy, but the middle one has a much longer barrel.
To allow the higher muzzle velocity.
If the 75mm sherman had a non-HEAT anti-armour round and a HEAT one and so did the 76mm sherman, was the ratio of efficacy versus armour the same (HEAT vs. other round type) in both generations or variants of the Sherman?
Somewhat lower expected effectiveness for the 76mm, as noted. Vastly different ratio, though.
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