What you said might be true, but it requires of you to know some intimate details about the ZTZ-99 of the size and arrangements of the turret.1 Also you cannot say exactly how long the penetrator actually is judging by pictures of it 2. There is no way to know exactly how the tank works 3, but from some sources I've read and they did not give great details also, that the gun out penetrate the 120mm gun on Abrams. Plus beside the penetrator size there is also the velocity of which the projectile is fired4.
Let me adress your points in order
1- As a former US Army tanker and some one who has been inside multiple t-series and T type tanks tanks and who has a working knowledge of the auto-loader used by the ZTZ-99 (China admits it is a 2A46 autloader clone) YES I CAN, make valid comparisons
2- Copy of the Russian system implies the same leangth restrictions, this is reflected in the ammuntion we see being loaded which apears to be 560mm long. lack of a bustle rack system implies limited interior turret room (confirme with pictures and first hadn accounts) taking togeher we can indeed confirm that the ZTZ-99 is using a short rod penetrator.
3- As a former tanker, I know tanks they all basically function the same. The exit to feed their gun, and ge tthat gun into possition to kill things. Snce I served on M1's upto the A2 and on older M-60A3TTs I have a good graps of the technologies involved as well.
4- Assuming China has not only duplicated the technological wizardry that went into the M829A3 depsite the Americans head start of decades in designing APFSDS for thier 120mm main gun, and some how managed to get a 560mm penetrator to perform like an 800mm+ Long rod penetrator.
.5M*v^=KE/j
PRC (sing weight of the Russian BM42 wich is simialr in size and design) 3.25*1760*2= 11,440j
US 5*1555*2= 15550j
ie the US round has nearly 25% more muzzel energy from the start.
Then we get into actual round design, here I am out of my depth but, what I do know is longer= better when it comes to penetrators.
Even the new Russian rounds 9bieng fired form the new bustlke rack autoloader) are in excess of 700mm long.
The US round is also a 3rd genheration improvement, While China's is still a first generation round.
The US spends more on R&D than China spends on its entire defense budget.
if China's gun had anypower at range they wouldn't bulky and slow tube launched ATGNM's for long range engagments
The mere idea that the Chinese gun come sanywhere near the performace of the NATO gun is absurd.