So...Let me see if I have this right. The Japanese made the receivers of the type 99s out of strong, high quality steel but the barrels out of junk steel? Since the type 99 is probably ( almost certainly ) the strongest action here I assume the receiver wasn't made from low quality steel. And then they used junk steel for the barrels? But wait! They had plenty of high quality steel for machine gun barrels!
But let's say everything Aberdeen says is true. And I am quite sure that it is. The Japanese really did use a lower quality steel for the barrels, and chrome plated them for a longer barrel life. That's a stroke of genius, if you ask me. It was a less expensive way to manufacture the gun. And that cheap steel in no way lessens the ability of the rifle to do its job. And it turns out that the chrome plating had the added, unexpected benefit of protecting the bore from rust and corrosion, at least to a limited degree. It does make sense that machine gun barrels be of higher quality, look at the abuse they take.
First off, the alloying elements that make a good hard wearing barrel are not the same as those than make a strong receiver.
Second, metallurgical analysis of Japanese rifles show that they are made from plain carbon steel, with a tiny bit of manganese and a healthy dose of tungsten and/or chromium. This is more than adequate to make a very strong receiver. But, crap as a barrel steel.
The element that make a good barrel alloy are nickel, manganese, molybdenum, and vanadium. In WW2 nobody used vanadium, but the US did make some good barrels out of 4150, the Japanese did not have large amounts of nickel, manganese or molybdenum, they did have access to chromium and tungsten, neither of these two alone make for a good barrel.
Machine gun barrels take a far greater pounding than a bolt action rifle barrel, due to the heat from rapid fire*, so which is smarter, use the small amounts of manganese or molybdenum you do have to make a smaller amount of machine gun barrels, and use a work-around to make serviceable rifle barrels in the millions, or to try and spread your limited supply of manganese or molybdenum in to all barrel by reducing the percent in the alloy? (Just remember, if the alloy percent is too low, it may as well not be there.)
And yes, chromium plating was viewed by the Army as a genius way to make, if not a silk purse out of a sow's ear, a perfectly serviceable wallet out of a sow's ear. Prior to this, the Army had only considered chromium plating the bore of large guns (artillery) worthwhile. Afterwards, they seriously investigated chromium plating first machine gun barrels, and then all small arms barrels.
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* So, why didn't they chromium plate the machine gun barrel? It would have been a waste of chromium. The thin chromium layer would not have stood up to the heat. Modern chromium plating is ten times thicker that what the Japanese were able to deposit.