K31 Swiss, not a 7.5mm, need to find out the caliber

[Teamklr2bar]

hello,  i just picked up a K31Swiss fairly chea.   it has been  sporterized. The work is decent.   It does not have the magazine.   I was under the impression that it was a 7.5x55swiss but when i attempted to test it the 7.5round will not even come close to chambering.  I am not aware of other calibers that this gun was originally chambered in.   maybe 6.5?

my question is do any of you know of any other semi successfull chamberings used in this gun?

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