“Why aren’t they picking up the fallen AK’s and ammo?”
Because it is often booby trapped. Militaries around the world learned this the hard way.
“In Malaya there is evidence that this practice was actively pursued. In Operation Purvey, some 10,000 rounds of .303 were doctored with high explosive. On firing, either from a rifle or a Bren light machine-gun, such a cartridge would detonate, burst the barrel, and kill or severely injure the firer. There was some concern about the legality of such actions, but General Templer, the GOC, dismissed these.
A further 50,000 rounds were ordered which were filled with a thermite incendiary composition. These when fired would simply melt and seal the breaches of the enemy weapons. The advantage of these was that if the enemy discovered the doctored rounds and managed to get them back into the British supply system they would not kill the British troops. The SAS also doctored weapons such as the Lee Enfield rifles, which were left for the communist terrorists (CT) to find.
It wasn’t only the SAS that were involved in such activities. Roy Follows joined the Malaya Police and spent much time fighting the CT. In one incident he recorded that he was summoned to the Officer Commanding Special Branch and given a bandolier of .303 ammunition. He looked at it and on being asked what it was stated that it was the normal ball ammunition. He was then told that in fact the rounds had been doctored and the propellant, a low explosive, had been replaced with high explosive. Follows was then told to leave this ammunition where it was probable that the CT were likely to find it. He recorded that, several weeks after he had left the ammunition, there was an ambush by the terrorists in which it was reported that, as they opened fire, there were a number of breach explosions, which resulted in the ambush being abandoned…”
(Source: “Malice Aforethought – A History of Bobby Traps from World War One to Vietnam”, Ian Jones, p 225, 226)
Damn !