It's nice how they keep squeezing real world parallels in there, a 9mm version of the Steyr Aug does exist, and its magazines come in 25 and 32 round variants, which is what you get in the game for the Steyr-Auch, in its basic and refurbished version respectively.
The Steyr AUG 9mm conversion fires 9x19mm Parabellum. Therefore, the Steyr-Auch fires 9x19mm Parallellum.
In all seriousness though, 9x19mm Parabellum is a pistol cartridge, and the Steyr AUG has what is called a "mid-length" 20-inch (50 cm) barrel, only a few inches more than the average carbine. It's generally referred to as a carbine.
I can't get an exact reading right now because I don't have the materials, but I'm going to put together a Super String q160+ Tornado at some point, and I absolutely assure you the damage per bolt will be no more than 25-30% higher than a single bullet fired from the Steyr-Auch, and with an additional 125% mech resistance penalty, as well as only 3/4 the optimal. Crossbows truly were designed to cause suffering in this game, by someone who has likely never laid hands on a crossbow.
Yes, the range even of 9mm Parabellum fired from a carbine is much greater than that of a broadhead bolt, but if ranges were actually realistic in this game, a novice would be able to fire a sniper rifle accurately from one corner of a screen in Foundry to the other. An expert would be able to fire a precision rifle accurately from one side of all of Foundry to the other.
Meanwhile, I challenge anyone to hunt a deer with potshots from a Steyr AUG 9mm conversion.
I just can't let it go. I don't want crossbows to be shit, it really annoys me.