The receiver flats(under the chamber end of the barrel) is usually marked with the length in mms.
If this is not the case take an old credit card,or a new one if you've been spending too much, grin and cut it length wise to the same width as the chamber. Slide the cut piece in until you meet resistance and mark the remaining amount of the card. Measure the part that went into the chamber and that will tell you the length.
Jim

The 2nd Amendment IS an unalienable right.