"The MG 42 (shortened from German: Maschinengewehr 42, or "machine gun 42") is a 7.92×57mm Mauser general purpose machine gun designed in Nazi Germany and used extensively by the Wehrmacht and the Waffen-SS during the second half of World War II. It was intended to replace the earlier MG 34, which was more expensive and took much longer to produce, but both weapons were produced until the end of the war."
1. I think it would be really stupid to retire and scrap all the MG34's before all units had been equipped with the MG42.
2. The big advantage with the MG42 was lower cost and ease of production.
3. It also had a higher rate of fire and of course a higher rate of ammo consumption. Note that the cyclic rate of fire isn't the really important factor. The effective rate of fire is more important, the MG34 was at 150 rounds/minute and the MG42 was at 154 rounds/minute so there wasn't a huge tactical advantage with the MG42.
4. The MG34 was still being produced up until the end of the war.
"Effective rates of fire:
The cyclic rate reflects how fast a self-loading firearm can fire, extract, reload, and recock its mechanism to prepare for follow-up shots. This is not, however, the effective rate of fire. The effective rate of fire is the rate at which shots can be accurately (effectively) fired at a specific point target. Many firearms have been developed with high rates of fire which have proven to have extremely low rates of effective fire. An easy example of this is the MAC-10  submachine gun. The MAC-10 in 9×19mm Parabellum originally fired at 1,200 rounds per minute. At this rate individual shots can no longer be heard so its fire sounds like a giant zipper being ripped apart. When a MAC-10 fires, the probability of hitting a target with more than 1 or 2 rounds is quite low. So it has a very poor effective rate of fire.
The effective rate of fire of the MG 34 is listed at 150 rounds per minute while the MG42 achieves a slightly higher 154 rounds per minute. No air-cooled machine gun can fire sustained for very long before overheating and malfunctioning. The effective rates of fire of the MG 34/42 were higher than allied weapons, but the cyclic firing rate is not a huge factor in this, the ability to change barrels is. The cyclic rate of the MG 42 can be altered by installing different bolts and recoil springs A heavier bolt uses more recoil energy to overcome inertia, thus slowing the action. It must be noted that those heavy bolts also were used along with stiffer return springs. The standard MG 42 bolt weight for a normal rate of fire is 550 g (19.40 oz). One of the roles of the MG 34 and MG 42 was to provide low level anti-aircraft coverage. A high cyclic firing rate is advantageous for use against targets that are exposed to a general-purpose machine gun for a limited time span, like aircraft. For targets that can be fired on by a general-purpose machine gun for longer periods than just a few seconds the cyclic firing rate becomes less important.
For a relevant current example, the FN MAG/M240 general-purpose machine gun series used by many (NATO) militaries cyclic rate is listed as 650 to 1,000 rounds per minute. But its effective rate of fire is approximately 100 rounds per minute."
I would say: go right ahead and use that MG34. Blame me and Wikipedia if anyone complains