Mr. Guffey is on the right track...
From a Physics & Metallurgy prospective,
The bullet nose in contact with the 'Throat' (Forcing Cone Funnel) *SHOULD*, in THEORY,
Put a softer metal against a hard steel, AS PRESSURE RISES...
In this situation, with bullet in contact with the Throat,
The PRESSURE will have to spike to get that bullet moving.
There is no inertia or momentum from the bullet (Projectile) to help force it along into the barrel.
Now, remember!,
That bullet isn't just getting forced into rifling,
It's going to become LONGER, since there is mass being displaced by the rifling,
And the bullet is actually bigger in diameter than the 'Grooves' of the bore...
When you compress the sides of the bullet, it MUST grow longer, the only direction left for it to move.
Not only is the MAXIMUM diameter of the barrel SMALLER than the diameter of the bullet,
But you have Rifling crushing in on the bullet,
And that mass has to go somewhere since two solids can't occupy the same space at the same time...
When the bullet is a 'Plug' in the bore, the pressure blows out the cartridge case neck, you get pressure on the back of the bullet,
AND all around the outside of the bullet at the same time...
The bullet is 'Die Sized' by the barrel.
Completely an act of pressure, so the pressure has to BUILD & SPIKE until the bullet starts to move down the barrel.
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With LONG FREEBORE, the bullet gets a free pass, room to get up some momentum before it's forced into that funnel.
Momentum helps you out with getting the bullet elongated enough to fit into the rifling.
In a GLANCING blow between Copper/Lead & Steel,
Steel wins.
The steel is still going to have to deal with IMPACT compression, but the 'Gas Jetting' effect shouldn't be as serious...
So it's 6 one way, half dozen the other,
The throat compacts from impact, or it's gas jetted from pressure buildup/live burning powder escaping around the rifling grooves before the bullet starts to move...
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This is PERSONAL OBSERVATION,
Not exactly 'Scientific Proof'...
But observation of the after effects.
I recommended seating a bullet LONG in the case, crimping it into place, then inserting it into the chamber.
Turn the bullet about 1/8 turn back and forth.
This will show you the erosion in the throat without a high resolution, magnified bore scope.
The 'High Spots' will rub the bullet nose, the low spots won't.
This will give you an idea of the erosion pattern,
Even, or UN-Even.
My observations from a magnified bore scope,
Impact compression of the metal will usually be EVEN.
The free bore will simply get LONGER, but concentric and even.
If there is gas jetting, the impact compression of the layers of steel is doing a good job of keeping uneven erosion to a minimum.
My bolt .308s that I set long and let the bullet rest on the throat erode more unevenly, and they show 'Rivulets' that I *ASSUME* are gas jetting patterns...
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Now, this is a hobby,
I can't have a chamber/barrel stub made of pressure/temperature sensors,
No could I afford the super fast computer to make sense of the test data and model what's actually happening.
This is OBSERVATION DATA POINTS, not a clear picture of what's happening...
I shot .300 Weatherby for many years, the freebore is excessive by anyone's standards,
And the throat eroded very evenly in good quality barrels.
I've shot .30 cal rifles a ton, mostly all my life, and most of the barrels are still around here in one form or another...
When I got interested in this same question, I started having a look with the bore scope...
The tight chambered turn bolts all had uneven erosion,
And were exceptionally eroded in the rifling grooves.
While the long freebore semi-autos usually had a pretty even erosion, the entire throat got pounded forward.
This left me with a lot of head scratching,
And asking a TON of questions, reading everything I could find,
One old steel engineer gave me a clue about the excessive pressure built up in the chamber before the bullet starts to move,
And it's a basic principal of physics I overlooked for YEARS,
As pressure increases, so does the HEAT.
It's not the pressure that forms the bullet into the barrel,
It's the HEAT the pressure/compaction process produces.
Compress ANYTHING and it will heat up, raise that compression/heat enough, and ANY SUBSTANCE will first become plastic, then liquid, then vaporize...
VERY TINY amounts of copper introduced into a super heated plasma stream will blast through ANY steel...
Made me wonder if the heat/pressure was carrying copper/copper ions off the bullet jacket, or out of the cartridge case into the rifling and jetting the rifling away.
The bullet jacket would be protecting the rifling ends, and that would explain why the rifling wasn't getting 'Jetting' while the lands were...
When a bullet gets a run at the rifling,
The pressure doesn't spike as much, less pressure,
The copper jacket hits the entire rifling and wicks the heat away, plus acts as a physical shield to the jetting effect.
It's one THEORY, based on observation, and may be all wet from something I'm missing...
But the FACTS fit.
