I haven't read everyone's comments, but I'll offer a couple of observations:
1. Chrono Bullet and Eradicating Bullet are about evenly matched. One has 11 more damage but 10 less R.Acc/R.Attk. than the other. But if you fire off a Coronach and get the afterglow every few minutes, then that adds another 10% to ranged attack.
The two bullets are not even closely matched. Its the Chrono bullet that makes R15 Annihilator even compete in terms of raw damage with Fomalhaut.
Ranger is
the most accurate job in the game. We don't need the extra racc on the bullet.
Ranger really only shines in high buff situations, be that magical for Trueflight/Wildfire moments or physical for Last Stand/Coronach/Jishnu's moments. As such, we are going to be at capped Ranged Attack more often than not. Sadly the +10% ratk aftermath from Annihilator isn't doing much for you outside of 6 man situations, and sometimes not even then.
2. You have to consider the job that the weapons support. Fomalhaut is specifically good for Last Stand given the R15, the TP bonus, and the ability to do Radiance. Additionally, it's a balanced gun for 2 jobs. Anni. is for RNG only, and you have to consider the traits and other gear RNG has.
Its very valuable to look at the use for both weapons, sure. But I find Radiance to be a complete waste more often than not on RNG. Let me get AM1 or AM2 up for making constant Light with a variety of 2 step options between many jobs vs getting AM3 up, meaning giving up some WSs along the way, blowing that Radiance for
maybe 15% more damage over a Light, then have to get that AM back up again and missing a possible skillchain.
Also the STP+5 from R15'ing an Annihilator REALLY helps close the gap between the two weapons.
Both guns are good to have, but for RNG, I'd say keep Fomalhaut handy for scenarios where you'll be spamming Last Stand. For Trueflight...run a test for yourself. But if you want a truly impressive weapon for RNG, get the Su5 R15 Crossbow with Scout Bolts. The bolts alone are expensive as hell, but the damage output...
I hope you weren't discusing Trueflight with Annihilator. That's the
absolute worst possible option for Trueflight. Fomalhaut has its TP bonus to boost TF numbers. Armageddon has its insane +70AGI for both WS mod and dSTAT. And Gastra....well, is Gastra.
The Sharanga might be a fun weapon to use, but when using a crossbow your only real option is Trueflight or Wildfire. Last Stands are paltry at best compared to ANY of the RMEA guns, and even compared to say a Holliday's Last Stand. And when using Trueflight, even with Scout's Bolts, you're looking at a Weapon Skill/TP mode damage ratio of at least 80/20- if using Gastraphetes its more like 90/10. It might look pretty in your chatlog with the added effects, but its still gonna lose when using Last Stand to any of the guns, and its gonna lose to Gastraphetes AND Fomalhaut (under 2250TP) for Trueflight. And I'm not talking just in the WS numbers- I'm talking parse.
I think you should go ahead and go back and read thru some of the responses- a lot of what you mentioned was covered and "shot down" per se.