Isn’t there a barrel design that intentionally squeezes the projectile to a smaller diameter?
There have been some taper bore designs for small arms, but the results gained do not match the huge change in price required to make them.
Then, there's the whole issue of really needing specifically tailored ammo to get the best results out of the taper barrel.
Which is what scuttled the German taper-bore anti-tank rifles. Which were about 50% lighter on the ground, along with being shorter overall. But, the need for solid tungsten shot cores pretty much doomed them just as they were fielded.
Mind, the Germans wanted everything out of the engineering, so there was no easy way to adapt the round to solid steel shot. They were squeezing 50mm projectiles down to near 40mm, which is a lot of "squish." And in a short barrel length, too.
This is unlike the Allies experience with the Little John adapters for 37mm rifles. The adapter squished a sheetmetal skirt down on the steel penetrator using a threaded-on barrel adapter. From (potetntially fickle) memory, these necked 37mm projectiles to 33-34mm. In the field, the adapter was discovered to be less-than necessary in actual practice.
The Brits advanced that a step with their "arrowhead" projectiles. Which was a solid shot from a 57mm round set inside a cast lightweight casing matching 76mm rounds. Give a 57mm round a 76mm "push" and it wants to go fast, very fast. Only real problem is that those rounds, when inaccurately cast would launch inaccurately as well. The 17 pounder tank cannon was good inside 300m, but was getting to minute of really big barn at 800-1000m.
