Not a rules answer so much as a physics one. Much (most?) of the noise of a chemical slugthrower comes not from the "bang" of the propellant but from the bullet breaking the sound barrier as it goes supersonic. Silenced firearms either use subsonic rounds, or part of the job of the silencer is to slow the bullet to subsonic velocities.
The speed of the "Gauss" needles is much higher, and would generate a louder sonic boom. The only way I can think of to reduce this boom would be to play with the shape of the needle to make its aerodynamics such that it does not create as loud a boom. Whether it is possible to actually make one silent, I have no idea - I have no knowledge of transsonic aerodynamics.
Depending on how "hard" YTU is, you may want to allow special "silent" Gauss ammo, probably at a higher cost, and perhaps with some drawbacks (range, damage, accuracy, etc.), for game balance.
OTOH, the sonic boom is waste energy, so a "silent" needle might well have better speed, and hence range and damage.
I am not sure if this is really appropriate for this site, but hey, since there is not that much activity, I figured, what the heck...
GAUSS WEAPONS QUESTION... I am working on a Traveller recon scenario for some marine PCs. It will require the use of silent weapons. I am, therefore looking for some weapons to outfit the intrepid squad with. While I know that lasers are silent, I would like outfit the squad guass rifles, as they better fit the technology and situation.
Looking in Book 4: Mercenary, I see that the weapon utilized an electromagnetic field to project the needles. I am thinking that this means that the weapons are nearly silent...perhaps making a zipping noise as the needles leave the barrel of the rifle. Do you agree?
From Book 4: Mercenary, Pages 36 & 37:
"Gauss Rifle: The ultimate development of the slug thrower, the gauss rifle generates an electromagnetic field along the length of the barrel which accelerates a 4 mm, 4 gram needlebullet to velocities of 1500 meters per second. The round itself consists of a dense armor piercing core surrounded by a softer metal covering, ending in a hollow point, giving the round both high stopping power and a good armor piercing capability. Flight along the barrel is nearly frictionless, with spin stabilization imparted through magnetic bias."
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