In all of my searches for an E Megatron, I've never come across one with a white tank variation. The closest thing to it is E Galvatron's predominately silver tank.
Now that I look at the picture, it does resemble that tank. Note the black barrel tip and what looks to be yellow/orange plastic near the tank's front section.
The normal tank for E Megatron looks like this as far as I know:
Also, due to the picture quality, it's hard to make out, but the tank looks like it is a slightly different hue than E Megatron's "white" plastic. I think the tank belongs to E Galvatron.