Final answer:
It is crucial to get an old firearm with no data stamp examined by a gunsmith to ensure safety and functionality before any use.
Step-by-step explanation:
If you have a very old firearm with no data stamp, the safest and most responsible action to take before using it is to get it examined by a gunsmith. Gun forums may offer some insights, but they cannot replace the expertise of a professional gunsmith who can physically inspect the firearm for safety and functionality.
Attempting to figure out which ammunition fits by trial and error can be incredibly dangerous and is not recommended. A certified gunsmith will ensure that the weapon is safe to operate and can advise on the correct ammunition and care.