There are various conflicting reports on the internet about the gun statistics from Wing Commander: Privateer. I was curious about it and did a bit of digging, and the best source that I found for how the guns actually behave in-game was the data at K's Cluttered Loft, which incidentally also includes information about how to read the values directly from the Privateer / Righteous Fire data files (which I've since verified on my own copies).
Interestingly, the values on the gun chart at wcnews.com happen to be very much wrong, for the most part. It's certainly possible that some of the gun values got tweaked in different versions of the game, so perhaps not all copies are the same. My copy matches the data given at K's Cluttered Loft, though, not the wcnews.com version.
If you're just looking for a quick answer to the question, "What's the best gun in Privateer / Righteous Fire?" then here you go. The Tachyon Cannon is by far (IMO) the best gun you can buy in the original Privateer. It offers the most damage per second, with a very reasonable energy cost, and compared to the extremely overpriced Ionic Pulse Cannon and Plasma Gun, is still quite affordable (it wouldn't be too hard to buy two of them with the proceeds of one 3-mission run).
In Righteous Fire, the Plasma Gun gets a significant boost which would almost make it worthwhile, though personally the slower speed and increased energy usage (compared to the Tachyons) keeps me from using it. If you ignore cost considerations, the Fusion Cannon is a better gun than the Tachyons, delivering higher damage per second without requiring much extra energy. The gains are pretty incremental, though, so you'll have to judge for yourself whether or not the 0.7cm/sec advantage is worth the 80,000Cr price differential. Personally I find a Centurion with four Fusion Cannons to be delightfully nasty, so I'd recommend upgrading as you get the funds.
If you like to pick your shots carefully, of course, the per-shot damage may be more important to you than the Damage-per-second column.
From the Manual:
Speed (m/s) Damage (cm) Energy (GJ) Refire Delay (sec) Price (Cr) Damage/sec Energy/sec Laser 1400 1.8 4 0.30 1,000 6.00 13.33 Mass Driver 1100 2.6 6 0.60 1,500 4.33 10.00 Meson Blaster 1300 3.2 8 0.40 2,500 8.00 20.00 Neutron Gun 900 6.2 18 0.65 5,000 9.54 27.69 Particle Cannon 1000 4.3 11 0.60 10,000 7.17 18.33 Tachyon Cannon 1250 5.0 8 0.40 20,000 12.50 20.00 Ionic Pulse Cannon 1200 5.4 15 0.70 40,000 7.71 21.43 Plasma Gun 840 7.2 22 0.80 80,000 9.00 27.50 From the Game Data Itself (Privateer):
Speed (m/s) Damage (cm) Energy (GJ) Refire Delay (sec) Price (Cr) Damage/sec Energy/sec Laser 1400 2.0 4 0.30 1,000 6.67 13.33 Mass Driver 1100 2.6 5 0.35 1,500 7.43 14.29 Meson Blaster 1300 3.2 8 0.40 2,500 8.00 20.00 Neutron Gun 960 6.2 16 0.65 5,000 9.54 24.62 Particle Cannon 1000 4.3 11 0.50 10,000 8.60 22.00 Tachyon Cannon 1250 5.0 8 0.40 20,000 12.50 20.00 Ionic Pulse Cannon 1200 5.4 15 0.60 40,000 9.00 25.00 Plasma Gun 940 7.2 19 0.72 80,000 10.00 26.39 Steltek Gun 1175 10.0 19 0.45 0 22.22 42.22 Steltek Gun Mk2 1250 19.0 17 0.37 0 51.35 45.95 From the Game Data Itself (Righteous Fire - Steltek Gun stats given only for reference; the player can't have those without hacking the game data files):
Speed (m/s) Damage (cm) Energy (GJ) Refire Delay (sec) Price (Cr) Damage/sec Energy/sec Laser 1400 2.0 4 0.30 1,000 6.67 13.33 Mass Driver 1100 2.6 5 0.35 1,500 7.43 14.29 Meson Blaster 1300 3.2 8 0.40 2,500 8.00 20.00 Neutron Gun 960 6.2 16 0.65 5,000 9.54 24.62 Particle Cannon 1000 4.3 11 0.50 10,000 8.60 22.00 Tachyon Cannon 1250 5.0 8 0.40 20,000 12.50 20.00 Ionic Pulse Cannon 1200 5.4 15 0.60 40,000 9.00 25.00 Plasma Gun 940 8.5 17 0.65 80,000 13.08 26.15 Fusion Cannon 1350 6.6 11 0.50 100,000 13.20 22.00 Steltek Gun 1175 6.0 19 0.45 0 13.33 42.22 Steltek Gun Mk2 1250 1.0 17 0.37 0 2.70 45.95