Sorry, forgot to answer - yes those are the empties from the spotting rifle which as ferrefixer says is a short 0.50" cal round (I thought they had zirconium tips though?).
The 0.50" short round gave the spotting rifle a lower muzzle velocity and was intended to match the trajectory to the Main armament. Even so the trajectories were not identical with the spotting rifle being flatter. There was a sighting board with 4 aiming points to align the Main Armament, spotting rifle, optical sight and CSWS. A bore sight was put in the Main Armament and aligned to the board, then without moving the gun, it was moved to the spotting rifle which was adjusted to its own aiming point, then the optical sight and CSWS likewise.
Digging a Wombat in was b****y hard work. First there was the weapon pit, then the shelter bay for the crew which was about 18+ feet long and 4 + feet deep, lastly the ammo bay both of which were covered by KIP sheets and given a couple of feet of earth for overhead protection.
The gun pit was covered by a "Cambrella", a frame work over which the cam net was draped. A sack was placed over the muzzle (to hide that 120mm black hole). Photo below has the cam net lifted up - otherwise you wouldn't see much!