That would be an interesting calibration point - how does a reduced size railgun perform vs. a similar penetration laser, in terms of damage per second per ton?
At CAP 9, a 40cm Railgun dose 12 dmg (1-3-4-3-1 pattern). A 4-shot can fire once every 20 sec and weights 550 tons (11 HS), and a 1-shot can fire every 5 secs and weighs 179 tons (3.575 HS).
HS wise, a 1-shot railgun matches up with a 12cm laser (if we round up) at 200 tons (4HS) that dose 4 dmg in a (1-3 pattern).
Damage wise if falls between a 20cm laser and a 25cm laser. 20cm weighs 300 tons (6 HS) and dose 10 dmg (3-5-2). A 25cm Laser weighs 800 tons (8HS) and dose 16 dmg (1-4-6-4-1).
At CAP 9, both the 20 & 25cm laser have a RoF of 10 (due to rounding).
So a 1-shot 40cm Rail gun trades 1 to 2 layer of pen for for half the weight and twice the RoF compared to 20 & 25cm lasers. HS wise the 40cm 1-shot is clearly superior to a 12cm laser in all regards (though that should kind of be expected because of the tech level difference).
Note that, after the proposed change (25% charge rate for SS railguns), the 1-shot 40cm railgun can only fire once per 20 second, same as the 4-shot version. And I was talking about the SS railguns after the nerf.
Let's compare a few options for fighters. Ideally a 10HS fighter uses 3HS weapon for speed. Let's say 4HS weapons will also do in this comparison. Below the max tech are at inertial fusion level (RP <= 150000), which is very high for normal play.
40cm SS railgun with C9 (after proposed nerf, with 25% recharge rate):
Damage Per Shot (1) 12 Rate of Fire 20 seconds Range Modifier 80,000
Max Range 960,000 km Railgun Size 3.575 HS (179 tons) Railgun HTK 1
Power Requirement 9 Recharge Rate 2.25
Cost 216 Crew 11
Development Cost 1039 RP
It has a DPT of 3, and a penetration of 3 layers.
12cm full-size laser with C4:
Damage Output 4 Rate of Fire 5 seconds Range Modifier 80,000
Max Range 320,000 km Laser Size 4 HS (200 tons) Laser HTK 2
Power Requirement 4 Recharge Rate 4
Cost 64 Crew 12
Development Cost 565 RP
It has a DPT of 4, and a penetration of 3 layers. It is less than 1/3 of the 40cm SS railgun in terms of cost. It also requires much less research (C4 compared to C10). It has much shorter range, but fighters should have much higher speed compared to their targets, and can should be able to close the gap relatively easily. And such high RoF, high penetration weapon is suitable for anti-fighter role.
25cm 0.5x sized laser with C10:
Damage Output 16 Rate of Fire 160 seconds Range Modifier 80,000
Max Range 1,280,000 km Laser Size 4 HS (200 tons) Laser HTK 2
Power Requirement 16 Recharge Rate 0.5
Cost 160.0 Crew 12
Development Cost 894 RP
It has a DPT of 0.5, and a penetration of 6 layers. It's about 66.7% the cost of the 40cm SS railgun. It requires the same recharge tech (C9 and C10 are available with the C10 research). Such high damage, high penetration but slow recharge weapon is suitable for the 'hit and run' tactics which looks for high alpha strike.
10cm 4-shot railgun with C3:
Damage Per Shot (4) 1 Rate of Fire 5 seconds Range Modifier 80,000
Max Range 80,000 km Railgun Size 3.0 HS (150 tons) Railgun HTK 1
Power Requirement 3 Recharge Rate 3
Cost 41.6 Crew 9
Development Cost 456 RP
It has a DPT of 4, and a penetration of 1 layer. It's less than 20% the cost of the 40cm SS railgun. It is also only 3HS which enables a faster 10HS fighter. Fighters using such a weapon are usually for PD.
And a surprising contender: 15cm plasma carrot with C6:
Damage Output 6 Rate of Fire 5 seconds
Max Range 60,000 km Carronade Size 4 HS (200 tons) Carronade HTK 2
Power Requirement 6 Recharge Rate 6
Cost 14.7 Crew 8
Development Cost 271 RP
It has a DPT of 6 and a penetration of 2 layers. It's less than 7% the cost of the 40cm SS railgun. It only requires C6 to design, and 15cm carrot tech is cheap to research. Such high DPT, low penetration weapon is suitable for reducing enemy shields.
Among all these 3~4HS beam weapons, the 40cm SS railgun requires the most research, cost the most to build. Note that high cost weapons generally require more MSP on the fighter to fix firing breakdown.
Yes it is an additional options for fighters, but why? I don't see a clear role for such a weapon.