I've heard about the theory that a rifle with active spring plunger type ejector will push the cartridge against one side of the chamber.

Now if I remove the plunger ejector from a bolt action, then use a long bullet seated to touch the lands, backed by a minimum load, will this be enough to fireform perfectly concentric fit cases?

This is not a answer to your ? about the ejector but it is related to firefroming brass.
http://www.bellmtcs.com/blowing_shoulders_forward1.htm

If the plunger makes you feel better,regarding concentricity isssues,it is easy to remove(and replace).

I like to engage the rifling while forming cases. The exact propellant charge weight,is dependant upon the amount of brass I need to move. For an Ackley Improved case or similar amount of brass movement,I use a pretty "juiced" load.

For less extreme forming,less intensity will do nicely...............

A trick which seems kind of Mickey Mouse but which I've seen used to fire form brass concentrically was to put scotch tape at the base of the case to center it up in the chamber. Regards, Bill.

The plunger ejector is junk, I only use Mauser's.

Are you bitchin' or braggin',when you say you only use Mausers?.................

Why bother? Your chamber itself probably isn't truly concentric. If you make a cerrosafe cast you will find out for sure. Frankly, I've had the best luck making concentric cartridges by first fireforming a case in my gun then checking for concentricity with a run-out guage. I then square the case and neck turn. I then neck resize with Wilson Dies and check again. Anything with a run out greater than .001 gets junked for matches. I've done some testing with 6ppc heavy bench guns and there is a noticable loss of precision with rounds over .002 run out. Now this is true on a match chamber which I cut myself and designed
and required to be as perfect as possible.
For an off the shelf hunting rifle, my tests indicate that with run-outs as great as .005 you won't generally be able to see a difference in precision and I'm talking about rifles that still shoot <1 inch groups. With Run outs >.005 the groups will noticeably open up.-Rob