As far as the community knows, internal ammo detonation is subject to RNG.
The datamined “det probability” does not refer to internal ammo detonations (I have tested this extensively, anyone stating otherwise is lying). It controls the probability of a tank experiencing a catastrophic explosion when killed, leaving only a crater and no wreck.
Internal detonation appears to be a server-side setting adjusted per vehicle at gaijin’s whims.
Some tanks just have low ammo detonation chances. Historically, we’ve had one outlier of ridiculously low ammo detonation chances that was changed by snail directly in update notes. The T29 on release had a bugged ammo rack that would not explode at all.
Why do many Russian tanks seem to eat ammo destroying shots and continue trucking? It’s a mystery as far as I know. We do not have access to the code that governs the probability a destroyed ammo rack kills a player instantly, and gaijin is tight-lipped on releasing any info on this.
Most we know is that: if you shoot an ammo rack on around 95% of tanks, there is a very small probability the ammo will simply disappear with no kill.
Some tanks (such as T29 on release) have a higher probability their ammo will just disappear.
The probability ammo explodes when destroyed seems to be above 90% of the time, for most tanks.
Players suspect some tanks have reduced chances, and we have concrete evidence that the chance ammo destruction leading to an explosion is a value that can be changed up or down. (See T29 on release).
Fragmentation or spall hitting ammo has no bearing on if it explodes or not. If you hit a piece of ammo with just fragments and do not destroy it, it will never explode.
Ammo must be destroyed (by anything) to have a chance of instantly killing an enemy tank.
I, for one, believe that any destroyed ammo outside of an uncompromised blow-out rack should immediately lead to a kill. It seems the designers at gaijin do not agree.