Holy shit! That thing, but portable would be so awesome! What kind of rounds does this thing fire? Looks big and bad!

That type of rifles appeared during WWI to destroy the first tanks. The first weapon of this kind was the Tankgewehr M1918,also known as Mauser T-Gewehr with a 13,2 mm rounds. At the same time the tanks were improving so did the rifles. There was 2 kinds of evolutions for this weapon. The first kind wanted to use the regular ammunition but at high speeds (1100 o 1200 m/s) but this eroded the barrels very fast. The second kind opted for heavy ammunition. The average was a 20 mm caliber. This size allowed it to use various types of ammunition: incendiary, explosive, drilling ... Both types were heavy (the second kind were heavier, around 40 kg, plus a HUGE recoil), needed bipod and a sistem of buffer springs for the recoil, and with an average size of 2 m. At the start of the WW2 they proved very efective but the improvements of the tanks reduced it's use. They were portable, but usually needed more than one soldier to carry and they were anti-tank rifle specialized soldiers (or even units). The "smaller" version of ammo was, for example a 7.92 mm (see picture) It could penetrate 15 mm of armor, sloped at 30° at 300 m distance, or 33 mm of armor at 100 m. [spoiler]https://upload.wikimedia.org/wikipedia/commons/thumb/7/7d/MWP_DS7_92.JPG/799px-MWP_DS7_92.JPG[/spoiler] For the bigger version, see the picture (the 3 on the right). [spoiler]https://upload.wikimedia.org/wikipedia/commons/thumb/6/6b/Three_20x138B_shells_and_37_mm_shell.JPG/450px-Three_20x138B_shells_and_37_mm_shell.JPG[/spoiler] They were 20×138mmB (here you have some info if interested: https://en.wikipedia.org/wiki/20%C3%97138mmB )

That's some amazing stuff. Thanks for the info, I appreciate it.