I've been asked many times: what's the deal with two piece ammunition? Isn't it faster to load a gun when you only have to lift one item, and not the shell and propellant separately?
In an abstract scenario, maybe, but real life has a number of caveats. One is that ammunition is very heavy, especially once tank guns evolved from tiny 37 mm pea shooters that could be loaded with one hand to massive cannons where two piece ammunition could be required. Since odds are that you don't have an unlimited supply of formidable musclemen in your army, you might need to reduce the amount of weight someone would have to carry all at once, by splitting the shell into two parts, for instance. Recall that a loader would often have to flip the shell around, and that rotating an item that is very long requires more energy than rotating two shorter items the same amount.
The other problem with big ammunition is that as the caliber grows, the length of the shell grows as well. Since tank interiors are very cramped, this causes problems. The British make a note of the extreme length of Tiger ammunition in their first impressions and then comment on the extensive problems encountered when trying to load the Tiger II, whose shell was even longer. Note that the loader in this case has issues with both the weight and the length of the round.
Soviet trials of one piece ammunition discover the same thing: a 122 mm shell is too long and heavy to be able to load conveniently, limits the vertical range of the gun, and does not actually increase the rate of fire.
When tanks started using automatic carousel loaders, the issue of loader fatigue disappeared, but there was still a benefit to using two-piece ammunition. With long one-piece ammunition, it has to be stored in either a bulky turret bulge or surround the fighting compartment like a ring, making it very difficult to reach the driver's position without exiting the tank. Two piece ammunition can be stored in a much more compact way, without either of these drawbacks.
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In an abstract scenario, maybe, but real life has a number of caveats. One is that ammunition is very heavy, especially once tank guns evolved from tiny 37 mm pea shooters that could be loaded with one hand to massive cannons where two piece ammunition could be required. Since odds are that you don't have an unlimited supply of formidable musclemen in your army, you might need to reduce the amount of weight someone would have to carry all at once, by splitting the shell into two parts, for instance. Recall that a loader would often have to flip the shell around, and that rotating an item that is very long requires more energy than rotating two shorter items the same amount.
The other problem with big ammunition is that as the caliber grows, the length of the shell grows as well. Since tank interiors are very cramped, this causes problems. The British make a note of the extreme length of Tiger ammunition in their first impressions and then comment on the extensive problems encountered when trying to load the Tiger II, whose shell was even longer. Note that the loader in this case has issues with both the weight and the length of the round.
Soviet trials of one piece ammunition discover the same thing: a 122 mm shell is too long and heavy to be able to load conveniently, limits the vertical range of the gun, and does not actually increase the rate of fire.
When tanks started using automatic carousel loaders, the issue of loader fatigue disappeared, but there was still a benefit to using two-piece ammunition. With long one-piece ammunition, it has to be stored in either a bulky turret bulge or surround the fighting compartment like a ring, making it very difficult to reach the driver's position without exiting the tank. Two piece ammunition can be stored in a much more compact way, without either of these drawbacks.
As with many cases in engineering, there is no one answer that is automatically "better". It is important to think carefully about your constraints when making a decision.