Does anyone have any experience with these Martini Greener GP Guns that have been converted from shotgun to rifle? There is one for sale in 38-55 that caught my attention but if it is not a quality gun I will direct my attention elsewhere.

First  Greener warns that the  Shotgun Actions should not be converted to center fire.

That being said there any number of these converted actions out there in centerfire working with out a problem.   

There one variation out there that perhaps should be passed over for conversion.  The take down models have a split receiver and supposed to be weak. I would recommend against building a high intensity cartridge on such action. Greener warns agains it.

Now the negatives.   

These have to be the ugliest of the various type Martini's made. They have a "face" only a mother could love so to speak. 

They can be difficult to assemble.  Greener made the seating knuckle extra deep and installed a very strong striker spring.  It takes a great deal of force to seat the block in the knuckle. 

I have seen a bunch of different work arounds for assembly. None simple, some work better than others. 

My 577 3" NE is built on one of these actions. Beast of a gun.