We have nothing to go on beyond what you have quoted. There is no game definition of “sacrificed attacks.” But the only thing that makes sense to me is that sacrificed attacks are attacks you do not make. The bolts stay where they are, loaded in the crossbows. If you were supposed to make the attacks, but they automatically miss or something, that would have a lot of additional ramifications that would need to be spelled out and the ability doesn’t.
So I think it works, and it’s a neat trick. But it’s also still a bad trade—for attacks you could have made, anyway. With thanks to @minnmass for doing the math, you are more likely to get at least one critical hit without sniper than with it so long as you use a weapon with a 19-20 threat range or better. And that’s at least one—actually making the attacks gives you a chance to make more than one, while that isn’t possible if you’re sacrificing all of your attacks to expand your threat range as much as possible.
The key thing here, to me, is that the ability never specifies which attacks you sacrifice. Sacrificing an attack with a good chance to hit is a bad idea. Sacrificing your third iterative that has a −15 penalty? That’s a much better-sounding trade. Still, that’s at high levels, and at high levels this has to compete with Improved Critical or keen, since sniper does not stack with those. Sniper is generally going to lose that competition.
There are some advantages to one big hit, rather than many, smaller hits, per @Prevarications’s comment, but since we are more likely to see a critical hit by attacking more, at best we should sacrifice just one attack to achieve 19-20 threat range if our weapon has a usual threat range of just 20.
The more important thing that @Prevarications points out is that there is no requirement that the sacrificed attacks be ranged attacks.¹ If you have natural weapons as a ranged attacker, those are melee-only attacks that you might not be within reach to make, so those can be “freely” sacrificed. In other words, you pulled the wrong thing from that web enhancement—what you really want is the kobold and its claw/claw/bite natural weapons. Converting a 20/×4 weapon into a 17-20/×4 weapon isn’t terrible for the cost of a race that’s quite decent in other respects, and a level of fighter (or the feat you might otherwise have gotten with that level). It’s not great, either, but natural weapons are relatively easy to come by,² so you can easily expand it past 17-20; you could even do better than the 15-20 range available by just taking Improved Critical on an 18-20 weapon.
Unfortunately, there is only one ranged weapon that gets a ×4 multiplier, the exotic war sling from Races of the Wild. The war sling is an interesting weapon—if you have proficiency with both it and skiprocks, you can launch a skiprock with the war sling and ricochet it off of your first target and into the next. Doubling your attacks per turn is a great thing to do when crit-fishing, and if sniper lets you get a solid critical hit range, that’s pretty awesome. And the targeteer gets proficiency in any two ranged exotic weapons—like the war sling and skiprock!
From there, the deepwood sniper from Masters of the Wild is probably the way to go. Its keen arrows effect arguably does stack with sniper (though this is because it’s a 3.0e prestige class so your DM may well amend this—ultimately it’s fairly minor either way), and then its projectile improved critical ability increases the ×4 multiplier to ×5 at 2nd and ×6 at 7th.
Other than those, you definitely want Dead Eye from Dragon Compendium (note the errata changes the BAB +14 requirement to BAB +1!), and ranged attack mainstays like Point-Blank Shot, Precise Shot, and Rapid Shot. Targeteer can help with any of those. So can psychic warrior, which has the added bonus of also having access to a bunch of abilities to get more natural weapons to sacrifice for sniper. You’ll also probably want to dip warblade or swordsage (Tome of Battle) for the blood in the water Tiger Claw stance.
In all, a kobold-playing-to-their-strengths targeteer/deepwood sniper with a war sling and skiprocks is probably winds up being one of the most effective ways to play a pure damage-dealing ranged-attack character, shockingly enough.
There’s no requirement that the attacks you actually make be ranged, either, for that matter.
As examples, the kobold qualifies for the Dragon Tail feat from Races of the Dragon, 1000 gp can buy the mighty arms graft from Faiths of Eberron (though this might not stack with claws), Bind Vestige + Practiced Binder from Tome of Magic allows a ram attack from Amon. Illithid Grapple from Complete Psionic can be taken up to 4 times for up to 4 tentacle attacks, but it requires Illithid Heritage and another illithid feat, so that’s pricey. If you’re looking to invest that much, you’d do much better just taking 2 levels of totemist from Magic of Incarnum. With that, you can easily shape chaos roc’s span (Dragon vol. 350) and dragon tail (Dragon Magic), and bind girallon arms (Magic of Incarnum), for 7 natural weapons (2 wing buffets, tail slap, and 4 claws) and at that point you don’t even need to be a kobold (though it would add a bite).
