I was sitting in front of my computer reading the latest edition of Handloader, and decided to check my old post to see if any new replies had come in. Veral, I hadn't seen your last reply until now, so thanks for the information, which I will definitely try (filing the bevel base, that is). The bullet is Magma's 250gr. RNFP, which has a very SLIGHT bevel base (not much at all), which should be easy to file off. I have some Lyman 255 gr. SWC's with a flat base that I have loaded up and will go out and try. I haven't had much time for experimentation but I have had a little and have learned something each time.
Number one, I got the Magma bullet to stop leading in the cylinder throats when firing them with about 10 grains or so of Unique. I could not have imagined that it could take so much pressure to stop the leading, but it did. The barrel still leaded, but it was plainly visible that this was smeared on lead, caused by lube inadequacy. I want to get problem number one fixed (cylinder throat leading) before I even begin to worry about number 2 problem (barrel leading -- which I suspect will be easier to fix).
One thing I am convinced of, though, at this point, is that there are two culprits involved here with leading. One is friction, as you state in your literature, Veral. However, I am now more convinced than ever that lead melting/lead spray is involved. Veral, you call this gas cutting, but is this not one and the same thing? If gas cutting is not actual melting but the blowing off of microscopic pieces of solid lead, the effect is the same. Anyway, the two types of leading I described above have a totally different appearance, under a microscope (I use a good one which I have access to -- a Meiji EMZ stereo, which probably retails for around $2,000.00). The leading in the cylinder throats has a sprayed on, melted appearance, and is extremely difficult to remove. Under microscopic inspection, the LEADES (the short, angled, tapered portion between each chamber and each throat) has lead spray appearance. The barrel leading on the other hand appears flaky, smeared. That has to do with lube failure.
To further support my belief that actual melting is going on, I have examined recovered bullets around the base perimeter. There, I find unmistakable signs of melting/gas cutting. I'm not talking about the uneven, melted appearance of the base viewed with the naked eye. What I am talking about are microscopic rivulets visible only under the magnification. There are rivulets (not always, but occasionally) around the lee side (non driving side) of the rifling grooves where gas tends to cut, which run comparatively deep (5 to 10 times as deep as the rifling grooves), where no friction against metal could possibly take place, as they are too deep, and it appears like a ditch bank washed out by water. I don't think these would be visible under a magnifying glass, but they definitely are under a good microscope.
In further evidence of this, at low pressure, where the case doesn't seal up against the chamber walls very well, there are flat lead deposits around the top perimeter of the brass (along with black soot). This was discussed above. How else could the lead get there? Gas cutting is melting lead (or blowing microscopic pieces, whichever you will), and blowing it backwards. This lead is then smashed between the expanding case and the chamber wall. This cannot be cause by friction, however the case may be, as the bullet at no times comes in contact with that portion of the chamber.
The above lead me to the unmistakable conclusion that there is more involved in leading than friction alone.
I truly think Veral has stated the solution, however: a full, tight front driving band. I measured my cylinder throats since leaving the above posts (remember, they were firelapped) and they are now about .454 back towards the leade (right in front of the bullet), tapering to about .453 up towards the exit point. It appears that if a good seal is maintained up front, between the front portion of the bullet and the throat, this may benefit nearly as much as an obturating base in deterring leading. That is, if gas pressure passes up around the sides of the bullet but cannot blow by the front band, it cannot easily carry anything with it as it has nowhere to go, and pressure builds much more rapidly back inside the case, causing it to expand and further seal gasses. Obturation then takes place much more quickly. If gasses on the other hand are allowed to leak out past the front of the bullet, pressure cannot be built nearly as rapidly and lube and lead are carried away, and obturation is much more difficult to obtain. Case seal is more difficult to obtain. Veral has addressed this problem with a full diameter front driving band. Nobody else seems to have figured that one out. There are a lot of "fake" LBT style bullets out there that have an undersized front driving band, which gives up much of the performance of the real thing. All they have is the wide Meplat, but no front driving band to speak of.
I think that the fact that I have had a .454 space at the beginning of my throats, with a Magma bullet with a smallish front driving band, has caused much of my problem. I will, then, keep experimenting and trying different things, and posting my results.
I think we all owe untold credit to Veral Smith, however. In the area of bullet design, it would by no means be a stretch to call him today's Elmer Keith. Certainly his contributions in this area have been just as original, just as revolutionary (if not more so, which is probably closer to the truth), and just as timeless. His designs are still considered to be the last word in heavy handgun hunting. I think we are all fortunate to be able to communicate with this man, via this forum, while he is still around.