The problem with high tech crossbows is that the basic technology has certain limits. There is only so much tension any material can have.
If you have a material that is so elastic as to be able to launch a bolt supersonic, then you have the same noise problems that a rifle has. On the point of sound produced, crossbows aren't silent, they are a lot quieter then a supersonic firearm, but some subsonic weapons may be quieter then many crossbows.
I couldn't find a reference for how loud a crossbow is. From experience with heavy poundage bows the noise is quite considerable. I'd estimate 60-70 decibel (loud conversation), slightly less with a good damping rig. If you want to up the velocity of the projectile that noise is likely to get a lot louder, easily getting to the 90db of silenced rifle fire. (Silencer info garnered from
this finnish site)
Enough negatives, what advantages does a TL15 crossbow have.
Large projectile. TL15 explosives, or other tech such a rod penetrator rounds could make it reasonably effective. Somewhat similar to a grenade launcher.
Wide firing array. The arms of the crossbow make good locations for guidance sensors.
Otherwise pretty similar to a rifle.
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TL 15 Grav Crossbow
The grav crossbow propels its bolts using two small gravitic devices held perpendicular to the barrel. These devices couple with the bolt and allow it to be accelerated to moderate velocities.
The advantage over a more conventional rifle is mostly one of stealth. The gravitic signature on firing is equivalent to a grav belt failing to start up correctly, and considering the rarity of the weapon may not even be looked for. The heavy and large subsonic projectiles tend not to set off auditory sensors, the weapon itself is near silent. There is no recoil to speak of, though the breif gravitic pulse felt on firing can be unnerving at first.
The pods at the ends of the arms also contain the guidance mechanism for the bolts, the guided ones at least. This takes advantage of additional triangulation of the bolt in flight, allowing quite exeptional accuracy at range.
The arms can be folded against the shaft in some models. This is slightly less reliable, but far easier to carry. Technically there is no need for a barrel, though there is a need to hold the gravitic propellers well forward of the starting position of the projectile. This allows the unusual fully collapsible model, with a telescoping shaft and folding arms.
The fead tends to be a basic hopper fed from above the weapon holding 5 to 10 rounds. Some designs have a magazine below or need to be reloaded for each round.
The basic ammunition for the weapon is a solid bolt with basic inflight guidance. Often these can be recovered after use, with rechargable batteries this can be a worthwhile logistic option for longterm lone wolf operations. The battery pack is normally built into the stock and shaft of the weapon, though some can also be powered from a backpack power unit.
Other ammunition is available in both dumb and smart munitions.
How does that sound?