The MAS 36 was chambered for the 7.5 French round. The barrel in the pictures appears to me to be original with just the long upper handguard removed and the forend shortened or replaced. I didn't see any caliber marking in the pictures. 36's were imported again several years ago, and some were modified to fire 7.62 x 51 nato (.308 Win) by pushing a button into the front of the chamber for the shorter .308 round. This didn't work well. Quite a few MAS 49/56 semiauto rifles were also imported around the same time. They are arsenal marked on the left side of the receiver CAL 7.5 unless they were also reworked to fire the .308 Win round, then the 7.5 was overstamped 7.62 very neatly. The point of all this is that barring a caliber marking somewhere, this M36 is most likely a 7.5 French. I would get some cerrosafe from Brownells and cast the chamber, then compare it with the 7.5 French dimensions to verify or rule out that round. You don't have to have a 'smith for that. If you take it to a 'smith though, that's what he'll do.
Ammo is sometimes available, but fairly expensive. All that I have seen is full patch.
When I first saw a M36, I commented to the dealer who had them that they were very solid and strong looking. He told me that they had to be. They were designed to survive being thrown down a lot and occasionally run over.
Texas State Rifle Association Life Member
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A big fast bullet will beat a little fast bullet every time