I haven't shot a million gas checked bullets, but I've surely shot many thousands. And all of them produced pretty much the same way: sit down with a bunch of cast bullets and a box of checks, get the check facing cup up and press in a bullet base. Then you run them all thru the lubrisizer, of whatever flavor you happen to use. Even if the base fits fairly well this makes for tired, sore fingers after a few hours of it (if you have that many bullets which I often do) and adds a time-consuming step to bullet production. If the bullet base doesn't fit so well (of which I have a few moulds that produce them a bit large in the toosh) I heat the checks (in the winter I use the top of our wood stove) to somewhere around 200 degrees I suppose and then the cool bullets generally will press into the heat-expanded checks fairly well. In my case I haven't had to go to the extreme of bumping up the size of the checks with some sort of expander gadget.
I made many many this way using an ancient Lyman lube-sizer to lube/size bullets with pre-seated gas checks. Then about 25 years ago I acquired a Star lube-sizer which made life easier, especially for plain-based bullets. But I kept on seating the checks the same old way, even for the ones for which I had a Star die. Then some time ago a fellow on another forum mentioned his technique of getting better cast bullet accuracy in a 25/20 by using 6.5mm gas checks instead of the normal 25 cal. So of course I must try this out. And dang -- 6.5mm checks don't stay on a 25 cal base, so you must use a different technique. Fortunately for me I have a .258 Star die I got from fellow Castbooliteer Lathesmith, so it was no big deal to start a bullet into the die, set a check on top the base, and finish the stroke for a perfectly applied gas check with no sweat required.
The lightbulb went on: if this works for oversized gas checks, how does it work for normal checks? And how about those tight-butted dang things that I'd been able to seat only by heating the checks? So I am here to report that it works great on all the bullets I have tried. Here's the technique in pictures: bullet ready to put into die -- bullet in die at about the right depth: gas check ready to slide over onto base: check on base ready to make the stroke: sizing stroke being applied: and the product:
I now have Star dies for all of the sizes for which I have big-butted moulds. I'll never slave over a hot stove again! And about 3 more dies and I'll have all my rifle sizes covered by Star (much thanks to Lathesmith who even makes 22 cal dies like in these pictures!) Oh yeah, I should mention that I give the press handle a sharp downward rap to use some velocity/inertia physics to seat the check fully before the begin sizing. This is probably only needed on the checks that wouldn't fit nicely otherwise, but I just have the habit now and do it every time.
If you have good-fitting checks this different technique is not greatly faster than the old "seat 'em first and then size 'em" technique, but it is maybe 50% faster or so. But it sure keeps you away from the old "sore finger" syndrome. And for the bad-fit boogers -- man, this is the only way to go!
And for you Lee push-through fellers, you can do the same thing there but it is probably slower than the old way. Unless you have a bad-fit bullet, in which case I think you'd be able to skip whatever step you have been using to get check on base and just use the Lee the same way I use the Star, with the bullet pressed a little ways up into the die and then the check popped up with a sharp rap before completing the size stroke.