I think you are being given bad advice by even suggesting that sticking to UL non-SIB is something you might want to do. You should buy an Omegaton A103 amp. It is not expensive (you are hunting atrox, after all), and it will help you with any weapon you use (except ones with max damage less than 24).
The problem with sticking to UL non-sib is that all the cheap one totally suck dps wise and also utterly suck eco wise at your levels.
If I were you, I would progress rifles like this :
- CCA/Chikara LR-3 Colonial (L) until you get to level 6/6
- Kallous-1(L), hunt with until you get 8/8 (gives ~ 9 dps). You could boost it with XR55(L) Hotfoot 10(L) to 12.7 / 13.6 dps.
- Buy A103, switch to Kallous-2 with A103, giving you 18 dps, hunt until you are 10/10
- Switch to kallous-3 + A103, giving you 19.8 dps, hunt until you are 13/13
- Switch to Kallous-4 + A103, giving you 21.9 dps, hunt until you are 16/16
- Switch to Kallous-5 + A103, giving you 29.6 dps (but consider buying A104)
With regen mobs, your dps matters a lot. See for example
http://eldslott.org/entropia for approximate costs of killing a mob with various weapons. For Atrox Young, the costs are :
- Your A-3 Justifier Mk.II+A103 - 5.5 PED
- Kallous-2(L)+A103 - 4.6
- Kallous-3(L)+A103 - 4.48
- Kallous-4(L)+A103 - 4.38
- Maxed A-3 Justifier Mk.II+A103 - 4.21 PED
- Jester D-5(L)+A103 - 4.13
- Kallous-5(L)+A103 - 4.03 PED
Spending 1.5 PED less on every Atrox killed only takes 12 extra levels, the "maxed A-3 Justifier Mk.II" happens at level 100/100, and pretty much every gun after Jester d-5 (included) is better than it.
May I suggest hunting Atrax instead of Atrox until you max Kallous-5(L) ?
Level N / N means that your lower level of hit / damage is at least N.