http://en.wikipedia.org/wiki/T-84#Ukraine
FOR EXPORT PURPOSES ONLY, The Ukraine has produced at least 10 T-84s with 120 mm NATO Cannon.
I have NOT read the actual specifications behind the 120mm NATO or the 125 Russian Guns, but I do want to point out that when the Former Soviet Union developed their cannons the Soviets had a policy that they would never duplicated the same caliber UNLESS the weapons were interchangeable (Thus a 152mm Gun had to be able to use the same ammunition as a 152 Howitzer and a 152mm AA Gun). Thus the Soviet Union had a 120 Mortar and a 122mm Cannon and this 125mm Tank run, but all three may be 120mm barrel size. The different Caliber designations meant the ammunition were NOT interchangeable. This is to avoid situations like the US having at least two different 75mm cannons in WWII (One for Cavalry/Airborne troops and one for the French Model 1897 the US was still using as late as 1943, through the 75mm Tank Gun on the Sherman was a modified version of the Model 1897 and thus such Shermans could use French 75mm Artillery ammo).
Thus the conversion from 125mm Gun Tube to 120 mm Gun Tube may NOT be that difficult for I suspect the Germans stole the concept from the Russians in the first place (The Russians came out with their 125mm in the mid 1960s, the Germans with their 120mm in the early 1970s). Now, the Gun tubes are probably different enough NOT to be able to use each's other's Ammunition, but switching from one tube to the other is probably NOT that difficult (THese tubes also have very short tub life, I read one report about 2000 rounds before the tube has to be replaced, or more correctly the inner liners need to be replaced).
Example of such change in tube types was when Israel (and later Egypt) converted the T-54s/T-55s they had in stock from Russian 100 mm Ammunition to American/British 105mm Gun tubes (so that all of the tanks could fire the same ammunition as the US was giving M-60s tanks to both Israel and Egypt in the 1970s and 1980s).
As to the use of 125mm or 120mm gun tubes by the Ukrainian Army itself, the Ukrainian army has stayed with the 125mm Gun Tube for they can make that ammunition and tube themselves and the difference in performance between NATO 120mm and Russian 125mm is NOT that great to do a change UNLESS it was to reduce supply concerns. An Example of the later is the recent decision by Poland to switch to German Leopard IIs to replace their derivative of the T-72, th PT-91. Till Poland joined NATO, Poland saw no need to replace its tanks and stayed with the 125mm Cannon. After Joining NATO Poland started to replace these T-72 derivatives and their 125mm Tubes with Leopard IIs from Germany and their 120mm Tubes.
Thus if the US wants to Supply ammunition to the Ukraine, all the US has to do is buy it from Poland and replace those 125mm stocks with 120mm stocks.
Thus this is a 125mm tube, there is no reason for either side to be using 120mm NATO Gun Tubes.
http://en.wikipedia.org/wiki/List_of_equipment_of_the_Polish_Land_Forces
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