And that 4-6% will buy you another 50-60 meters of reliable fragmentation and tumbling, which is not insignificant.
Usually you get fragmentation or tumbling, not both.
So it applies to the only ammo that the Aussie mil will be using.
You're sure about that? The Aussies wouldn't ever use some m855 or Mk 262 when doing joint ops with the US or another nation?
Obviously, but if you've got 20" barrel in a 30" bullpup, the length becomes much less of an issue. If I can get a 20" barrel in a rifle the size of 14.5 M4 Carbine, in a light infantry role, I'd take that any day of the week if it gives me another ~50 meters of fudge factor for imperfect shot placement.
That doesn't answer the question as to why they offer different lengths. The gain in performance isn't huge and isn't something you should bank on. A less than ideal shot is remedied with more rounds on target.
I don't know, why doesn't everyone just walk around with 10" barrels?
Because 10" barrels are inferior to 11.5" ones, and offer near zero projectile performance when talking about M193 or M855 ammo
. The rampant belief that an ultra short rifle is needed or necessary is simply not true. They have their purpose and it isn't as a general purpose setup. The CQB/FISH/FIBUA/urban warfare work has been done with much longer guns with little difficulty. Having the ultra short rifle is great but it's a tradeoff and one that needs to be carefully considered. If concealment, weight, OAL suppressed or dedicated work in tight quarters are of primary concern then an ultra short rifle may just fit the bill. If the majority of a rifle's use does not involve any of the above, then a longer barrel is likely a better choice.. Just saying..