There are a relative few semiauto designs without return springs in the stocks. Converting anything else to PGO means a major re-work of the action to get the bolt closed after firing.
The ones that come to mind are:
High Standard Supermatic, which was the basis for the High Standard Model 10A and 10B Police shotguns. See
http://en.wikipedia.org/wiki/High_Standard_Model_10 for pics and more details. The Supermatic has been out of production for a good number of years now but used ones are still available, a company called Mitchell Arms has copied some of the old High Standard designs but I don't know if they picked up the Supermatic.
Al Crouch used the 3-shot Remington Model 58 Sportsman gas operated semiauto in his prototype Model 10 design, since it didn't have an action return spring in the buttstock- those are still to be found used as well.
The J. C. Higgins Model 60 is also on that short list, for all I know it may be the Sears version of the High Standard Supermatic. Sears owned a big chunk of High Standard stock early along, and insisted that the company produce some of its designs for them.
Those are all I can think of from years gone by,
lpl
