My personal view is the killer dumped the weapon on the evening. The location where it was dumped is interesting. The killer wanted to get to the A96 (and in my view then headed off probably in the direction of Inverness) - and the map gives some insight.
The killer (again my view) left Crescent Road, then onto Marine Road and turned left. To get to the A96 they had several options. There are three streets closer to the scene they could have used, and one street slightly further up. There was also the option of 'taking the back road" (altonburn Road) - they did not do this, as it becomes single track and although rural, it would be easy to be (potentially) stopped or noticed. Although Seabank Rd is not the furthest road (in town) from the scene allowing access to the A96, it is - if you look at the map, the "most direct" (Marine Rd, Seafield Rd, Seabank Rd) - to go to Manse Rd (next down) would waste a little more time, while any of the nearer streets are just a bit too close (also to the Police Station). The drain is just before the traffic lights - where cars regularly stop - conveniently just before getting to the main road.
Keeping in mind the killer could have dumped the gun elsewhere (ie drains on the way) - not gardens - I think this was a planned location and the killer had the 'escape route' planned. While there's pictures of the Police searching the beach (a seemingly obvious place the killer might throw the weapon - perhaps into the sea) I don't know how extensively they searched the drains/gardens in Nairn. Possibly they did to a point - and Seabank Rd is "close" but not "too close" to the scene.