Chamber size is probly marked on the barrel.
disassemble forestock and check to see if the printed paper diagram is still there on the inside of the channel that shows the proper position of the washers and spring for both heavy and lighter loads. If not, then let us know and someone will shoot a jpeg of one or send you a copy etc.
Doesn't happen to say "sweet sixteen" on it??? Made in Belgium???
my favorite of the auto 5s. all of which were well made and reliable.