The barrel is straight. However, there are spiral grooves cut on the inside of the barrels. These spiral grooves, called rifling, make the bullet spin when it is fired. Just as a thrown football spins for an accurate throw, the spinning bullet makes for an accurate shot.
45 caliber Machine Gun and 50 caliber machine gun
Yes
a .45 caliber.
No. They will be too large to fit in the gun or be fired out of the barrel.
Varies according to the caliber.
50. caliber machine gun....period
Caliber "Caliber" is a word for the diameter of the bullet or the bore of the gun.
a .45 caliber.
Phosphating
9x19mm
The highest caliber machine gun in standard use is the .50 Cal. Gatling guns are used in 20mm caliber. XXXXXXXXX Russia and China use a heavy machine gun larger than the .50 cal BMG- it is the Vladmirov KDV (for the Russians), in caliber 14.5mm x 115.
See link below to the US Army Field Manual on the Caliber .50 M2 Machine Gun.