This picture shows an MG 34 with bipod in the light machine gun role, without attached magazine. Notice the round barrel housing with the round cooling holes.

The same MG 34, now with attached round single drum magazine. The carrying handle is erect. Again, notice the round barrel housing with the many round cooling holes.

This picture shows a post-war gun of the MG 42-type (folded sight!) that has been fitted with typical MG 42 features such as the original muzzle trumpet and the wooden stock. You see it fitted to a tripod, Lafette 42, as a medium machine gun, being fired. Notice the single long cooling slit on the square barrel housing. Also note that the gun is fired from a remote trigger assembly when fitted to the heavy mount as a support weapon.

The same machine gun, now with the receiver's top cover folded forward as is done to insert a new ammunition belt. Notice the long, oval cooling openings on the top side of the sqaure barrel housing.
Nicely observable here also the four round cooling grooves just before the muzzle "trumpet".