Israel has already been doing this in Gaza, but with airstrikes. Giving a robot the ability to pull the trigger is barely different to air-striking buildings based on statistical modelling the likelihood that terrorists are inside as your only form of "intelligence" — especially when you don't care about civilian casualties and have already proven to shoot first and not even bother asking questions later.
From a product development viewpoint, the gun is an uninteresting part. It's better to use something that already has a mature production line and has been thoroughly field tested. It's the vision and control systems they are interested in developing, the gun is just the chosen end effector for this application.
Even when they're ready to start deploying systems like this, there's a lot of value in using compenents that the military already has a lot of spare parts for and that personnel know how to maintain. I wouldn't expect a custom gun until units like this are commonplace.
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