SAS use of M4
I believe that SAS uses the M4 and other M16 derivatives is that they work closely with US Tier 1 units that use the M4 (Delta, Seals) and Tier 1.5 (Rangers and Marine SO). Curly Curleson, the Australian Colour Sergeant from 1st Sabre Squadron that was our instructor for 3 straight weeks at Recondo Class 03 in November 1966 had been very happy with the SLR but had been issued an M16 when they started working with SF and liked it but preferred the SLR. He said his first two shots with the SLR had killed two VC near Baria, RVN. He also did not particularly like the RVN shrub since he had been patrolling the "proper (3-tier) jungle" of Borneo where they had been working for several years.