"They had a special load for the purpose, buckshot with the nooks and crannies filled in with smaller shot. I guess the idea being that even a No 6 puncture might fester in the jungle and put a Red out of action. "
This is known as "Rhodesian Jungle", the theory being the large buckshot will clear away the foliage whilst the smaller shot did the damage. (Those guys weren't convinced of the merits of buckshot as an anti personal round).
Believed by some to have been developed by the Rhodesian SAS on deployment in Malaya.