In real life,,,An Assault Rifle is a select fire (both semi and full auto) weapon that uses an intermediate powered cartridge. (between pistol /carbine and real rifle ammo)
The anti-gun news media and non-military folks who like to pose for photos wearing camo in their basement often incorrectly refer to semi-automatic sporting carbines as assault rifles.
While you could in therory convert an original BAR (1918A2) to fire 7mm Remington Magnum, (with enough money),,, there would be absolutly no advantage over the 30-06 cartridge for the intended application. In fact the belted magnum case would eventually cause a feeding problem from the magazine.
Firearm:......................... Assault Rifle?
M16,A1,A2,M4....................Yes
AR-15...............................No, semi auto sporting rifle /carbine
AK47 real...........................yes
AK sporter..........................NO
SKS carbine.........................NO, it is a semi automatic military carbine
AR-10 (real).........................NO,,,select fire RIFLE
AR10 semi..........................NO..semi-auto sporter
M14 (real)..........................NO select fire rifle
M1A..................................No, semi auto sporting rifle
Mini 14 select fire................yes
Mini 14 semi.........................NO