Developments on T-90 variants are good ,but still there are questions about the firepower compared to abrams especially U.S developing the A4(APFSDS ),russia doesn't took steps to equalize U.S firpower....
The autoloader still limits the ratio L/D "diameter/length " of the ammunition,because of the round turret..
Russian 125mm BM-46 "Svinets" DU 650mm at 2km (1991) (22:1 L/D)
Russian 125mm BM-42 "Mango" tungsten alloy 520mm at 2km (1986) (16:1 L/D)
Russian 125mm BM-32 "Vant" DU 560mm at 2km (1985) (13:1 L/D)
Russian 125mm BM-29 DU 470mm at 2km (1982) (12:1 L/D)
Russian 125mm BM-26 "Hope" (1983) tungsten alloy 450mm at 2km (extended BM-22 13:1 L/D)
U.S M829A3 has aratio of 35:1(D=23 ,L=800 mm ) ,,A4 seems will be arevolution..
and
what about the problems of floor storing ammunation?
First of all you ask the wrong question regarding guns.
It matters little what the performance of the 125mm rounds are compared with the performance of 120mm rounds.
What matters is the performance of the 125mm rounds against western armour, and the 120mm rounds against Russian armour.
Having the best penetration and range figures is otherwise amateur fan boy crap.
There are no problems of storing ammo in the armoured underfloor autoloader. That has been proven in combat time after time after time.
As Ironsightsniper said here the most hit part of a tank is the turret front... the bottom of the tank is very rarely hit if every by enemy fire. Tank turrets have been blown off tanks long before the invention of the underfloor autoloader... explosives are basically pressure contained in a space. That pressure will press on all sides of that container to find the weakest spot, and when that weakest spot fails the pressure is directed through that spot. It is how rifle cartridges work... the bolt in the rear, the chamber on all sides represent strong structures that will contain the pressure of a rifle round.
The weak spot is down the barrel but there is a small blockage in the way... the bullet.
The resistance of the bullet blocking the explosion is much less than the resistance of the chamber of the gun and the bolt behind the round so the bullet "fails" first and is pushed down the barrel and out the muzzle.
Once the bullet is out and on its way down range the pressure is greatly reduced so unless there is an obstruction down the barrel that stops the bullet the chamber and bolt wont fail.
A blockage down the barrel... like dipping the barrel into water (water does not compress so when the bullet hits it down the barrel it can't push it aside because there is no room... it basically has to push the water that has come up the barrel back down the barrel, which is a lot of work... the bullet will slow down while the powder is still burning behind it creating more and more pressure. If the barrel behind the bullet fails or the chamber or the bolt then the bullet will stop moving in the barrel and all the gas pressure will exit where the gun failed.
Most tanks are pretty much sealed and the gas generated by a large amount of tank ammo exploding will create very rapid pressure increases (explosives explode, propellents burn much slower) and if that pressure is enough a common weak point for all tanks is the turret that like a bullet is just sitting on the cartridge case (hull).
The thing is however, to get the ammo to blow up you have to ignite it, which means you have to hit it with your penetration and an underfloor loader is only about 50 cm thick horizontally and is very low in the tank which makes it very hard to hit.
In the past the T-72 has a bad reputation for exploding because of all the loose ammo in the crew compartment which means any penetration will shower the ammo with sparks leading to a fire and explosion. Ammo under the floor is safe, as is ammo in the turret bustle because it is quite simple to design both so that the weakest part covering the ammo directs the explosion away from the crew compartment so even if it is hit the crew will be safe.
BTW length to diameter ratio for APFSDS rounds is no everything. A piece of fencing wire is very thin so a length of 400mm that is 2mm thick would result in a LD ratio of 200, but it would be pretty useless at penetrating hard targets... it would just coil up.
Also the ammo info you give for Russian ammo is from 1991 at the latest... Perhaps a direct comparison with the inservice US weapon in 1991 would be more appropriate?
That would be the M829A1 I believe... called a magic bullet except that NATO tests showed it would not have penetrated Soviet ERA at the time.