The ultimate solution here would I think involve MV Sense.
However, right now MV Sense only supports people counting. If you are looking for weapon counting you should submit a feature request and make a wish.
Weapons come in all shapes and sizes, I think trying to identify these through software would be difficult and you could end up with a lot of false positives, Hockey sticks for instance resemble the length and rough shape of a rifle.
As mentioned though use the make a wish feature.
Not exactly what you're looking for but it would be cool to have metal detector events combined with automatic snapshot taking via the new snapshot api. Interesting use case.
A step further would then be to have those snapshots analysed by third party image recognition looking for weapons.
Today the MV analytics are focused on anonymized human object/motion detection. There are other wishes and feature requests coming in, such as vehicle detection, and of course the API will grow and get enhanced with additional capabilities as well. For now, although there is no feature for this today, the MV team is listening and collecting data. Please make wishes as discussed, and more importantly discuss your use-cases with your Cisco Meraki team and they can get this feedback directly to the MV PM team, and inserted into a specific feature request.