[ArchonShiva]

Why does it work in 40k?

Are effective weapon ranges incredibly low, or maybe weapons are incredibly inaccurate, jam all the time or hold very low ammo and reload or cycle very slowly?

Then again, maybe it doesn't work in 40k either.

How close to the source (which I barely know) do you want to stick?

Misunderstood, badly maintained technology is easy to make cumbersome to use. Maybe firing a ranged weapon requires being stationary, drilling spikes into the ground, getting a lock-on (which registers on your target's sensors) and so on, making you melee bait.

Give weapons a huge Bulk, and impose Bulk on unaimed shots (sort of like a better version of Third Edition Snapshot). Cap Move and Attack at 9 with ranged weapons. There's a ton of stuff you can do here.

As an aside, GURPS rounds are 1 second. If you take the duration of a 40k round (6 seconds, 1 minute, I have no idea) and determine that this is the actual rate of fire, you can possibly both follow the lore and make ranged weapons stupidly slow.