is jab axe a true combo? if not how easy is it to escape
So I ran the numbers here using the Kurogane calculator.
Jab 1 is frame 3 - FAF 22 - Base Knockback 10 and Knockback Growth 80.
Axe is frame 6.
I'm going to use Corrin's weight of 98 as reference because it's a difficult MU and this might be useful.
With both characters at 0%, there are 10 frames of hitstun. Definitely not enough.
With both at 50%, Corrin gets 17 frames of hitstun.
The wiki says the axe kills at ~125%. It depends on the opponent of course, but at that percentage, Corrin gets 23 frames of hitstun. Axe hits in frame 28, and Corrin's fastest move is frame 5. Both moves would hit at the same time, but I expect the Axe to have a priority over her jab/dtilt.
In short: it's tricky, but Jab 1 alone could combo into axe at high enough percents if the opponent has poor frame data.
However,
this becomes a lot easier with Jab 2.
Jab 2 has 20 BKB and 120KBG and is also a frame 3 move. This means Villager hits the second jab during frame 25. The only downside is that its FAF is 24, but this is compensated by its considerably bigger knockback.
At 125%, Corrin gets 23 frames of hitstun. Then with 128% jab 2 does
37 frames of hitstun.*
I actually tested this against all heavyweight characters, and only characters that can hit you with a jab
before or at the same time you hit jab 2 are:
Ryu (not to mention Focus), R.O.B, Samus, Yoshi, Captain Falcon.
It is worth mentioning that heavier characters would probably need to be at more than 125% to die to the axe anyway. At 130% a (frame perfect/buffered) Jab 1>Jab 2>Axe combo is guaranteed to work against any character in the game. The heaviest character with a frame 3 jab is Samus, and she could only hit you in frame 26, 1 frame too late.
I'm assuming frame perfect reactions from your opponent. It's very likely that you'd be able to hit them with the second jab without much difficulty, specially if they're not pros.
Note: in the scenarios above most opponent would still have at least a 2-3 frame window to powershield your second jab.
A character like Corrin would have to be at 141% to get 25 frames of hitstun and make jab 2 a guaranteed hit.
I'm not completely sure this would work against every character because very light ones might be knocked far enough by the second jab that the axe hitbox won't reach them, so this needs some testing.
When the target is below 100% there seems to be plenty of time to escape it, so this should be used strictly as a kill setup.
*The interesting thing about this setup is that it also works for Villager's back throw setup. 37 frames of hitstun are enough for Villager to grab most characters right after Jab 2 (frame 39).