Relevant quote,

'CREWS; assault rifle,' Skaffen-Amtiskaw said. 'Seven fourteen tonne batteries; seven-element single shot to forty-four point eight kilorounds a second (minimum firing time eight point seven five seconds), maximum single burst; seven times two-fifty kilogrammes; frequency from mid-visible to high X-ray.'

• The Use of Weapons

Firstly we have to figure out what the numbers mean.

There are a lot of problems with humans and being near five gigaton lasers. So, by "kilogrammes" and "tonne" they are most likely referring to TNT equivolnets.

This interpretation is also inline with the pistol from "A Gift from the Culture" which was rated at 3.33 gigajoules

Anyway, that means the 250 kgTNT comes out to ~1 gigajoules.

Incidentally the recoil from a 1 Gj laser is nearly equivalent to a modern assault rifle.

Momentum for light is Energy = momentum times the speed of light.

p = 1046000000(kgm²/s²)/299792458(m/s)

p = 3.48 n-s

The L85 (British military assault rifle) fires bullets at 930 m/s which mass ~4 grams.

p = 930*.004

p = 3.72 n-s

If the Culture assault rifle in question was little bit lighter than the L85 (not an unreasonable assumption), the felt recoil of the each gun even more inline with each other.

When you think about it makes a whole lot of sense. Having some absurdly high energy output would make the gun really uncontrollable for a non-superhuman.

So, this lets them pack a LOT more power in the gun while still keeping it weldable by a "normal" human. It also keeps it simple because they do not need to cram tons of inertial compensation tech into it.

I have to wonder if that particular number was very intentional.