I feel like xur isn't selling heavy ammo synths because bungie knows about the bug and that people are just constantly exploiting it instead of buying heavy ammo synths with glimmer. So because the community is using an exploit, they probably don't want to reward us with heavy ammo synths. They know that doing the glitch is more annoying than spending strange coins so until that bug is patched I bet xur will not sell heavy ammo synths, because they know we don't actually need it if everyone just uses the glitch.