obiwan, the Yugo's use a 1.6 because they are designed to launch grenades this is also why they use the RPK front trunnion, ya can't use a standard flat trunnion for grenade launching even with a 1.6 mm receiver because the rivets will shear under the much heavier recoil (when launching real grenades which are 1.3 lbs its advised to place the butt against the ground imagine firing a shotgun with 1 lb of shot instead of 1 oz of shot) The RPK trunnion provides opposing contact of sheet metal and trunnion as opposed to a lateral contact which only leaves the rivets to take the full force of recoil
The Chinese used 1.6 mm receivers originally because of heat treating issues the extra .5 mm provides more surface contact area if ya test a Norinco receiver ya will find its not heat treated in any area they relied on the heavier material same as with a milled receiver, This was what led B-West to not heat treat their receivers with their AKs built with Chinese parts ......... as many have learned and as I advised back then a 1 mm receiver NEEDS to be heat treated .5 doesn't seem like much but for an axis pin its alot....... ALL the Yugos built on Nodak spuds receivers are fully heat treated (Century uses NDS receivers too) the ones built on Vulcan receivers as well as a few others are not fully heat treated
