here's some good way of making a great crossbow:
Wow, ten month necropost. Good job with that.
Signalum limb (it's like broken in how good it is)
Assuming you have ExtraTiC. You are right, that mod IS broken. Which is why I tend to not use it. And it didn't exist when the original post was made.
a stock (dosen't change much)
You can pick up an extra modifier by using a Thaumium Stock. It's useful if you aren't using ExtraTiC to stack redstone.
Fiery bowstring (and if you don't have it use normal, enchanted makes it do less damage do to the simple fact that it decreases the arrow speed)
Fiery is the string of choice, yes. I believe that was already amply documented.
unstable ingot binding (and if you don't have it use obsidian)
Wait, how are you getting unstable ingot bindings? Won't the ingot go boom when you try to put it into the smeltery? However, if your crossbow already HAS .5 draw speed because ExtraTiC is broken, obsidian binding isn't the worst idea for the extra durability. Granted, with something like Fluxed on there, it won't ever be an issue, but eh...
the modifiers you need is only reinforced (if you don't have iguana tinker tweak mod add moss then)
You'll likely want Fluxed as a superior option to Moss. Other than that, those of us that aren't using ExtraTiC need redstone.
bolts:
enderium coated (the most damage)
manyullium rod (I was using pokefennium about 18.5 but it was a bit too heavy, this great weight 12.38 (not too much nor too low))
slimy fletching (has 94% accruacy)
btw reinforced makes no diffrents on projetiles all it does is decreases the chance that it'll not break when it hits the ground(arrows, bolts and others with no durabillity bar)
and with this setup it does 40 heart by default (no modifiers)
Again, you are using ExtraTiC, which is not taken into consideration with the original experimentation. Many rods are a bad idea, just use Steel. More weight and easier to repair.
There is one argument for looting on a crossbow. It's Ghasts. They don't close in, they only drop 1-2 of their item. There aren't really spawners for them, unless you've also got Twilight Forest, which not every pack does. So looting is a good thing to have. Though I'd build a different crossbow for that than most tasks. Looting, light bolts with very high arrow speed out of the bow.
MFR Pokeball for the auto-spawner. Alternately, use EnderIO.
Also, there's no reason to put more than 1 Lapis on a looting/fortune item. It'll eventually accrue to Looting 3, so there's no reason to waste lapis.
And by the time it's gotten up there, you'll find that no one else has been playing 1.7.10 in years, there's been not just one but two presidential changes of office, and MC is announcing the release of 'Minecraft Pi' since it is version 3.14
Or you can use the otherwise completely worthless Lapis to give yourself Looting 3 when it will actually be relevant.