An excellent photograph displayed in an excellent art site. Not being interested in being flamed, I will stop there. I feel that while art is not divorced from ethics or morality there are perhaps better places for them to be discussed.
I hadn't thought of using a bullpup frame... not sure why. As for the 30mm... That would make for a true armor piercing sniper rifle. I can't think of a better way to stop a tank and scare and/or kill the remaining occupants of said vehicle than to shoot the driver while he's driving.
The only way to make it a one-man hand carry weapon is a bullpup frame. That would limit the length to basically be the barrel and muzzle break. This got me thinking about something bigger though. Imagine the 30mm cannon off of the warthog made into a single barrel rifle. You're talking about a man portable tank killer that's a fraction of the cost to fire compared to a javelin or other rocket type weapons.