MP400 is a pistol caliber AR-15 receiver, no stock or that tube section that would be within the stock, and a barrel of a Tec -9 with less holes.
The Empire-9 is a Brügger & Thomet MP-9.
The Tolen is an MP5k or SP89 without a foregrip and smaller barrel, along with a rail system covering most of the top of it.
The Ahnuld-12 is a stockless, short barreled Winchester Model 1887 with a rail system, named after famous actor Arnold Schwarzenegger in Terminator 2 as it was his signature weapon in it, famous for some scenes where he flip cocks it while driving q motorcycle.
K-121 is a Heckler & Koch HK21.
M4A1 is an M4A1.
mk46 is an mk46.
M4-10 BR is an m14.