At last, it finally came after a month waiting. They were both from eBay and sold at a reasonable price plus free shipping too, quite cheap for my standard. SIXSHOT was about USD$139 while MEGATRON USD$76. Planning to keep them in box for a while. There will a day I definitely open them for display. But for the meantime, I just put them in the storage like the rest.
This was actually a reissue exclusively by TAKARATOMY.
As for MEGATRON, this was a ehobby exclusive collector's edition. The only different between this and the original is its colour scheme. The rest are all the same.