So i'm not discounting your thoughts here as I am not as schooled in this, and generally I have appreciated your insight into most topics, but I'm a little confused on what you're saying.
It is my understanding that for traditional wax lubed lead bullets like my 180gr Montana Bulletworks, the pressure/heat can melt the back end of the bullet if ran hard enough regardless of the hardness, bullet to bore fit, or cylinder gap(no gap in rifles). Meaning you can only get lead alloys so hard before enough pressure and heat will start melting/flame cutting the back of the bullet. This is why every lead bullet website lists a max velocity in a given situation, and one reason Gas Checked bullets exist. Correct me if I'm wrong?
Now for coatings you're saying that the coating itself can now withstand that heat/pressure and therefore does not adhere to the same rules? so I can take my 125gr coated bullet and ramp it up to 2200fps in my .357 rifle and as long as bore to bullet fit is good, I should get no leading?
So, a couple points to clear up that might help:
- "Coating" has been used to universally describe any coated bullet, but really there are two main types of coatings, and they do perform differently: powder coating, which is a slick and flexible polymer layer applied by baking on a dry powder, and "Hi Tek" coating or variations on it under different brand names, which is some other type of coating applied wet (usually by tumbling) before baking. My personal experience (I'll point out that I was one of the first to experiment with powder coating bullets in the US, so it's been a while) has shown me that the Hi-Tek coated bullets don't stand up to the same level of abuse in terms of velocity, pressure, bullet sizing, etc. Unfortunately, most commercial coated bullets are coated with that stuff, rather than real powder coating. There are a few manufacturers that use powder coating, but it'll take some searching and careful reading of their descriptions to verify.
- Powder coating, when cured, will handle more heat than the lead underneath it. While it is a polymer, which most of us would think of as plastic, it's a "thermo-set" polymer, meaning once cured it doesn't melt - you can heat it up until it just burns away, but that generally takes more time and heat than a bullet is exposed to. Partly because of that heat resistance, but also because of its slickness, it works well as a boundary layer between the lead bullet and the steel barrel rifling, so as the bullet slides along the rifling that coating doesn't melt or wear away, IF the bullet is sized correctly*.
- Flame cutting requires flow of hot gasses past the thing being cut, just like oxy-acetylene torch cutting, or even like river water wearing away a sand bank. The point is that it doesn't happen just because of heat and pressure on the bullet base; there has to be a leak somewhere past that base for flame cutting to happen. That could be an undersized bullet, or gas escaping as the bullet base jumps a revolver forcing cone gap, or as the base passes a gas port in a semi-auto rifle or pistol. In those situations, powder coating is more forgiving than bare lead, but still has its limits and can be flame cut.
It's worth noting too, that in the case of an undersized bullet, a gas check doesn't seal any better than the bullet itself, but since the copper gas check can't be flame cut it protects the bullet base. I've recovered bullets that showed flame cutting in front of the gas check, along the driving bands, but because the gas check acts as a scraper in the bore there was no visible leading left in the barrel.
- In regards to getting a wax lubed bullet hard enough or fit well enough for high velocity - it is possible with extreme care, but most of us who've tried that experience leading, sometimes pretty severely. IMO, part of the reason for this is just friction between the barrel and bullet, to the point that it starts to deposit hot lead on the bore. Powder coating solves that though, by acting as that friction-reducing boundary layer I mentioned above, so the lead just never touches the bore in the first place. With that problem solved, it's a lot easier to achieve high velocity with no leading - all you need is a good gas seal, and that can be accomplished through bullet fit, hardness, and even a gas check if desired.
I'll also point out that while it's fairly easy to push powder coated lead to high velocity without any leading, it's more difficult to do so with good accuracy - that requires choosing the right alloy and hardness to handle the pressure, and the right bullet designs of course. As I mentioned earlier, eliminating lube grooves helps a lot with this, because they tend to collapse under pressure without something to fill them. Eliminating the lube grooves also makes bullets easier to cast, and provides more driving band surface and rifling grip, while shortening the bullet and increasing case capacity for a given bullet weight and style - seems like a win-win to me.
To answer your final question, yes IMO, if that 125gr bullet you mention is sized properly and has a good powder coating, you should be able to push it to 2200 fps in a 357 Mag rifle pretty easily. I wouldn't promise the same for Hi-Tek coated bullets, and keep in mind that sometimes commercial bullets vary a bit in size so you'll get maybe 8 or 9 out of 10 that fit right, and one or two that are undersized and lead the bore or fly off target wildly.
It's also been my experience that you can push a good coated bullet in something like your 357 faster than a comparable jacketed bullet before running into pressure limitations, because of the lower starting forces and lower bore friction compared to jacketed bullets. (Try driving a lead bullet through your barrel, then try it with a jacketed bullet, you'll see what I mean pretty quick.)
There's probably more to say, but this is already pretty long winded so I'll leave it there for now.