I have a 20 gauge 500,I have noticed the various foreends for the 12 ga are plentiful but there is not much choice in the 20....has anyone here had success fitting a 12 ga part to a 20 ga shotgun?.I realise they will be a different size but as I am a machinist and have access to a lathe and mill etc I can easily make an adapter if that is feasible,just wanting to know if others have tried this route......

I too was wondering that . I seen a OD green synthetic one I like for a 20ga. The add said measure the forearm slide . Do I measure the whole forearm length

I installed a Maverick 88 12 gauge synthetic forend with pinned action bars on a 20 gauge Model 500, and it works just fine. Mossberg uses the same size receiver for their 12 and 20 gauge 500 series shotguns. The fit is a bit loose on the magazine tube, but it functions well. Of course this is replacing the whole assembly, tube and all. Fitting a 12 gauge forend to a 20 gauge forend tube would require sleeving.

This place has lots of Mossberg stocks for the 500C. https://www.havlinsales.com/
I've bought several synthetic 20 gauge stock and forend sets from them. As for your question it would seem that you could fabricate a cylindrical shim that you could insert into a 12 gauge forend to make it fit on a 20 gauge action slide.