This method slaps the check onto the bullet when it is supported only by friction in the sizer die
Well, the check is installed by hand. Then you place the bullet on the seating stem. At this point you have gravity holding the check on the bullet. Or more accurately, gravity is holding the bullet against the check. Then you put the oversized bullet through the sizer. At this point you're mashing the gas check against the bullet with a pretty good amount of force to get the bullet through the sizer. If you do not believe this is so, try pushing the bullet through the sizing die with a pencil, instead of your press. So there's a heck of a lot more than friction holding the gas check on while things are sized and crimped.
not seats the bullet centered into the check- if that makes any sense.
Doesn't make any sense to me. When you forcefully squeeze an oversized bullet wearing an oversized gas check through a narrower diameter sizing die, how in the world can the gas check NOT be centered?
Perfectly pre-seating the bullet into the gas-check by hand on a hard surface helps- but its not 100% either.
The seating stem is a hard, flat surface, more or less exactly perpendicular to the axis of the sizing tube. When you push the bullet through, it gets things pretty close to perfect via this geometry + the pressure of pushing the bullet through the sizer. I mean, when the bullet is pushed through the sizer, it's not just a tiny carbide ring. It's a long, tightly fitting tube with a very slight constriction. It centers and straightens the bullet as it goes through. Then the pressure of sizing pushes the gas check hard against the seating stem; and even if the base of the boolit is not straight, the pressure of the die on the oversize gas check, itself, would press the check flat against the seating stem as it goes through. So it's the angle of the seating stem surface that determines how straight the check goes on. No matter how half ass you seat it, or how perfect, the check will end up following the shape of the seating stem. I have never seen a check go on crooked, despite crooked bases/sprue lines. I GC and shoot most of my "blems" with base defects, and if I didn't label the bag, I wouldn't be able to tell them apart once the check is on. The OAL of the blems probably has more variation, but the checks go on straight.
Now, don't get me wrong. I have read plenty of posts where people complain of this problem. There's even a guy on Cast Boolits that invented a slow and complicated gas check seating device to fix this problem (and the mechanical principles it uses seem to be more or less the same ones in effect using the Lee sizing dies, except I suppose you're taking that tiny bit of ram slop out of the equation - and then likely losing this marginally more perfect alignment when you size the bullet, anyway). I am not sure if it's the level of straightness that I'm overlooking, because I don't measure my gas checks with a micrometer. Or if the inventor maybe never used a Lee sizer, properly. I'm really curious why his invention needed inventing. If it's the level of straightness down to the fraction of a mic that accuracy hounds are after, I wonder that the chamber pressures don't alter the gas check shape/angle just a little when the round lights off, anyway. I'd think the check would be fire-formed to the base of the bullet. Or maybe this problem of crooked checks is much worse - obvious, even to the naked eye. But it only occurs when the gas check shank is too big for the check or when the bullet is really short and stubby, so I've never seen it?
I'll add that pre-seating the dang things is another time step you can get rid of by having a better sizer.
Are you saying you can just drop the bullet and gas check into a "better sizer" and it automatically seats the check for you? I assumed you have to at least have the gas check on the shank before the sizing, using any method. I'd be interesting in hearing how this works.
I'm not trying to be argumentative. I'm genuinely curious what these better sizers do better. Apart from the Star lubesizer, which has obvious advantages and receives universal praise.... despite pushing the bullet and check through the sizing die nose first, just like the Lee.