2 possibly relevant technologies are coil guns and tubular linear motors.

Coil guns:

Pros: Simplicity, and seem to be quite popular with the maker community. I managed to find numerous example circuits etc.

Cons: very inefficient? down to 1%, but that might not be measuring what I think its measuring.

Tubular linear motor:

Cons: as can be seen by comparing the 2 Wikipedia pages above, TLMs have much less info available online. it isn't so bad though, TLMs are just a particular type of low speed linear motor, and is basically a flattened permanent magnet dc motor.

pros: like dc motors could reach high efficiencies (90%+). Less need for high voltage. easier to control?

Plan

my preference at the moment would be to go with a TLM. my main concern at the moment is stopping/starting the cylinder/cylinder head. coil guns and coil gun research seems focused on moving the projectile as fast as possible. Not so much about stopping it. So I'm not too sure how many hoops I would have to jump through to get that.

Only downside is I can't use ball bearings any more :(

clear tubing has been ordered, as have neodymium magnets.