[DonVoigt]

I do not see a 1916 DWM P 08 picture, but a non-dated luger with a commercial proof.

Given your experience with the snap cap, it is likely .30 cal.

I have no Idea where you are from your profile, so I hesitate to say the normal test is to see if a "standard" wooden pencel will enter the bore! But that is a clue.

Of course you could shine a light into the chamber and see if you see the shoulder of the .30 luger chamber or simply a broken shell left in the chamber, or some other problem.