Whenever you hear about the L7, it's like "And god created the L7 on the 8th day" you know... But, let's debate this, because, factually, the L7 is not nearly as good as said to be, or rather, the D-10T is not as lacking as said to be vs the "mighty L7".
Reminder: The D-10T (TG, T2S) is a soviet 100mm tank gun first issued on the T-54 (as early as 1945 on the first prototype), and basically arming the whole family including the Type 69 chinese tank and the latest T-55 variants such as the polish T-55AM2. In fact it was never really used on any other tank, because T-54/55 family was so prolific that no other tank was made to carry it. It was used in its early form during WW2 in the SU-100 spg.
The L7 is a british 105mm rifled tank gun first issued in 1959 on the Centurion Mk.5, then adopted worlwide to retrofit existing tanks, or used as the armament for new developements, it became de facto NATO standard for tanks. The L7 origins are very simple, it was a large bore version of the 20 pounder (83.4mm rifled gun), using the same case enlarged to 105mm.
Fact N°1: D-10T case is bigger and longer than that of the L7. You didn't know it, one tends to imagine the opposite. 105x607mm vs 100x695mm. Rim base diameter of both cartridges: 147mm for both! Thus, the D-10T has a bigger volume for propelant.
But british propelants were very good, and it equalizes in performance... The other russian 100mm rifled gun, the D-54, used this same 695mm long case, but with higher pressure load. Stats wise, the D-54 outclassed the L7.
To put these in perspective, the US 90mm cartridge is much shorter and slimmer than the 100mm, and roughly as long as the L7 cartridge. 90x600mm, with a 130mm rim base diameter.
The german WW2 88mm KwK 43 cartridge is MUCH bigger than all of them, at 88x822mm with a 146mm rim base. Other german cartidges: 75mm KwK 42, 75x640mm, 122mm rim. It's bigger than L7, slimmer. Hell, even the PaK 40 cartridge totally dwarfs the L7 (and D-10T) at a whoping 75x714mm, but its very slim with a 100mm rim. The German 105mm FlaK cartridge (probably be the one used in a potential AFV 105mm gun) was 105x769mm with 136mm rim.
Fact N°2: D-10T has a longer bore, about L54 vs L52 for the L7.
The L7 was designed with a bore evacuator, but the D-10T recieved one in 1953 when it became the D-10TG, adopted on the T-54A in 1954.
So, what's with the L7? Pretty much nothing in itself. The L7 was simply built to shoot only APDS, and HE shells. It did so from the first day, when the D-10T used exclusively full bore AP shots and some HEATs. When the D-10T2S got APDS in late 1960s, it matched performance of comparable era L7 APDS. The L7 had fired full bore AP shells, it would in fact have been inferior to the D-10T, considering the lower volume of propellant and heavier projectile. It would have been greatly inferior to the D-54 firing BR-412D APCBC (1000m/s reported in D-54 vs 890 for D-10T, said to be over 900 for D-10T2S, the gun of the T-54B (1957) and all successor tanks.)
I speculate the L7 would have fired full bore AP shots at 850-880m/s. Most conventional HE and HESH ammo for L7 are between 750 to 900m/s, as well as early HEAT, but many modern designs are the so called multipurpose shell, lightweight, and even subcalibered, making HE/HEAT shells to 1100m/s+, but they carry far less explosive than conventional HE. HE designs for D-10T are roughly 900m/s shells, while HEAT are also 900m/s (and 1075m/s for modernised one). APDS wise, comparable types for both guns (first gen) are around 1400-1500 m/s.
These velocities prove that the L7 ain't more powerful, they are for all intent and purposes, equal.
D-10T2S also fired fantastic HEAT shells in the 1960s and 70s, and finally got APDS, then even APFSDS in the 1970s, comparatively, NATO HEAT and early APFSDS for L7 were roughly equal in performance to the same ammo of the D-10.
L7 outmatched the D-10 in the 1980s and after, because of new ammo developements, while the soviets considered the D-10T outdated and didn't care for newest ammo, focusing on 125mm.
Nowadays, many manufacturer propose modern ammo for 100mm that matches performances of modern 105mm ammo, both HEAT and APFSDS, in fact many 105mm APFSDS designs were simply adapted to 100mm gun.
Thus in effect, with comparable ammo, a L7 gun is not better than a D-10T2S.
L7 and D-10T are similar guns, contrary to popular belief, the L7 isn't "miles away better".
This myth comes from the israeli war of 1967 (and 1973 in a lesser manner), when arab armies using T-54 and 55 mostly had surplus AP ammo like BR-412B, not even BR-412D! and only a few HEAT for "critical moments", and simply NO APDS at all (maybe they had more HEAT and some APDS in 1973), and they faced highly trained israeli tank crews, most of them armed (freshly, like some rearming 2 weeks before the war in 1967!) with the so hyped L7, like centurions and retrofitted M48 (locally called shot and magach)....