The 20mm was only fitted to Mk II PBR,. It was actually a resurrection of a WWII design that was developed for Elco PTs, where 20 was mounted to the gunner's right. The WWII version could mount both the Hispano-Suiza or the Orlikon (the thinking being that the boat crews would use either the aircraft or th ship cannon based on local supplies were available at forward basing).
The VN edition only mounted the Orlikon on the right, and a 8" searchlight to the left.
In both cases, the weight of ammo was the problem
The other install that never quite panned out was using the 81mm/cal..50 combo mount. The problem there was not being able to see targets in the minimum range envelope of that 81 (and a PBR not being a good "repeatable" platform for registered fires). Add in the problems of storing the heavy 811 ammo, and the loss of flexibility on the .50 and it just was not worth the penalties.