I wonder how it compares to the British L7 105mm.
The M29 deserves its own thread.
From what I've read about it the T5, when using the same ammo as the subsequent T140 gun would be quite close to early L7 APDS performance using plain AP rounds, except AP can be superior against sloped armor. Quite logical given that it's a huge L65 while the L7 is a quite compact L52 gun. It would certainly have been a very powerful gun once the ammo was fully developped, closer to Cold War 120mm guns than to the L7.
Regarding the 105mm gun option it's worth noting that the T29 could have used the T140 later in the 1950s. The T140 was a lighter gun and further development with 160,000 psi steel would result in even lighter barrels, 4800lbs. Combine with an improved muzzle brake and new mounts and recoil absorbers and you could have a far lighter and possibly more compact gun with less recoil, thus giving more internal space in the tank. Eventually APDS ammo could reduce the weight of the round, making it easier to handle.
The T30 had a 15.5cm cannon
The T34 had 120mm almost though I am not sure same as M103 heavy tank.
Not the same 120mm. The M103 used the T123 while the T34 used the T53. Of course the T34 could eventually get this gun as well.
Now regarding the idea of having any of the T3X tanks in service, the first aspect to note is that the US Army didn't want them not necessarily because they had serious flaws, but rather because they at first didn't see the need for such tanks and because they wanted an even better heavy tank. Indeed the requirements were perhaps impossible to meet even in the 50s, especially regarding armor penetration. And of course by the 50s ammunition development and other improvements meant that the heavy tank concept became progressively obsolete. Even the M103 was outright refused by the Army and produced in low numbers for the USMC.
Considering how low the postwar US military budget was, a timeline where any of those tanks is procured likely implies that the US is far more conscious about the need to have a decent conventionnal capability, and therefore a larger budget. Given how low it was it could easily be doubled or tripled and still be about the CW average in military expenditure, so there's some room but Truman and his tax cuts are likely butterflied.
The main butterfly I could see is that the US has a far more comfortable level of R&D and will likely have less issues with technological parity with the USSR in tank design, as was the case OTL in the 50s because of a rush to get new tanks in production ASAP. This might even allow for an earlier M60A1-like MBT and butterfly the M103.
When it comes to how the T3X would compare with the Soviet JS-Series, the JS-3 would be outmatched because it was very cramped and would have awful long range accuracy, very slow ROF , only marginally better tactical mobility (except for autonomy), and the D25 gun would be too weak to penetrate the US tanks frontally. It's armor would still be tough to anything but the latest special US ammo however. The T10 is more comfortable and somewhat more mobile and better armored, but you really have to wait for the T-10M to have a jump in firepower and mobility. Alas, US MBTs and useful HEAT shells were around the corner at that point.
