Great thread, it should be a sticky.
Like every response in this thread I too have my SWAGS.
Such as the check holding the boolit to the rifling. SWAG . . . the check is the last part of the boolit to enter the rifling, if the boolit is not already turning at the twist rate of the barrel and requires the check to do this I would think you have far worse problems than a possible benefit or detriment of the check. I wouldn't want to have to try and clean that barrel . . . gas check or not.
I don’t believe for a second that being a “heat shield” is any part of the function of a check. Lead absorbs heat very slowly; much to slow for the micro second it’s in the bore to “melt” the alloy. If the heat of the burning gas could melt lead why doesn’t paper or plastic shotgun wads melt or burn? Or even discolor them? Copper, or worse yet aluminum checks absorbs heat far faster than does lead (and aluminum checks do work) so how could a check be a “heat . . . “shield”?
Let’s say you have an extremely high quality bore and your rifling starts off at .030” wide. Even the best made barrels cannot maintain that .030” perfectly for the length of the bore, at some point it’s going to narrow a bit to say .027” and then widen to say .031”. Once the boolit is engraved to .030” and then reaches the narrower part high pressure gas leaks by with the very same effect as an acetylene torch cutting steel. This is NOT melting, it is a high pressure abrasive gas cutting and stripping it. This is where a check, a properly functioning lube AND A PROPERLY FITTED BOOLIT of a SOFT ENOUGH ALLOY for the pressure can have a big effect at reducing or stopping leading. Too hard is in all probability the cause of more leading than using or not using a check. Too soft an alloy usually gets the blame but too hard is more often than not the cause but even this to a large degree can be minimized by a properly fitting boolit. A poor fit and too hard an alloy and there’s trouble in paradise, gas check or not. In addition to sizing the checks I am completely anal when casting, boolit bases for long range loads, i'ts a perfect base or it's no better than a sprue regardless of what the rest of the boolit may look like.
I am a firm believer that checks must be fitted flat against the base of a perfect/flat/square boolit base. The higher the velocity and the longer the range the more important this becomes. If a check is installed crooked, think of that last instant as the boolit leaves the bore (think freeze frame photo) and one side of the check is still in contact with “one side of the rifling” by only a few thousands of an inch but because of the crooked check the other side is free of the bore completely. There is still pressure pushing the boolit, not peak pressure but pressure none the less so where is the boolit going? Two things I can be reasonably sure of; 1> not straight and 2> not where you want it to go. I have made up punches to size checks flat (taking out the concave) and slightly flaring the sides of the cup. They slip on fully and easily, fit flat and crimp on tight.
I think the check does help scrape the bore and helps keep it cleaner by scraping powder, lube and any minor leading from the previous shot. I think the checks adds strength to the boolit base and helps minimize base deformation which would allow higher pressures to be used.
These are some of my SWAGS on checks and I’m sticking to them . . . right up until I change my mind.
Rick