Even if all else were even, lasguns would be superior to auto weapons for one simple reason: logistics.
Don’t laugh. That’s not the Imperium being cheap; tactics win battles. Strategy wins campaigns. Logistics wins wars.
Sure, an infantryman could carry a bolter… but how practical is it? Those things are huge - even the ones designed for humans. And .75 Calibre? That’s kissing cousins to walking around with a twenty millimeter cannon in your arms! Leaving aside the issue of the weapon’s unreasonable size, how much ammo could the average infantryman really be expected to hump around?
Really, bolters are only practical if you’re a Space Marine. Or Ibram Gaunt, I guess.
So why not autorifles?
Logistics again.
Your average modern infantryman is told he should be sent out into the world with 210 rounds of ammunition: six magazines on his LBE, and one in his rifle. Now, this sounds like a lot right up until the moment when someone starts shooting at you. At that point, you start realizing just how quickly you can burn through seven magazines.
Soldiers know this too, so they’ll grab whatever extra ammunition they can carry. Even at the outside, though, this usually just means another four mags or so and maybe a belt for the machine gun.
The machine gun, which eats through ammo like you wouldn’t believe, and weighs upwards of fifteen pounds when empty. Why carry it? Because rifles are really not all that great at sustained automatic fire. And even if they weren’t, thirty rounds doesn’t last long when you’re going all out.
Enter the humble lasgun.
With its high-capacity power cells. Lightweight. Durable. Each one capable of holding a hundred and fifty shots, according to Graham McNeill. And you can recharge one by leaving it out in the sun. Or throwing it into a fire.
With its variable power settings, capable of blowing an arm off at the top, or just punching a hole in you toward the bottom of the scale. Not to mention letting you extend the life of your power cell and speed up your rate of fire.
With its select fire trigger group, capable of semi and fully automatic fire.
With its incredible durability and reliability. Ten thousand shots before needing maintenance!
Why doesn’t the Imperial Guard have “machine gun” version of lasguns? They don’t need them. Every single trooper has a fully automatic weapon of phenomenal endurance capable of firing half again as many shots as your average GPMG at its highest setting, and more as you step down the power throughput.
Everyone has a machine gun. One heavier than any modern infantryman could possibly carry. And in a package weighing more than two pounds less than an empty M-16.
Everyone can carry more than a thousand rounds of ammunition on their person, and wouldn’t even need to clean their weapon after shooting through their basic combat load.
Everyone can get more ammo just by starting a fire and having lunch if they’re cut off from resupply. Though that wears out the cell… eventually.
And, in a pinch, you can fix bayonets and stab a heretic with one.
You don’t need to issue machine guns. Your troops have lasguns. You don’t need to ship mass quantities of physical ammunition. Your troops have lasguns. Send ‘em a generator. They can make their own. And even if you feel like sending them enormous quantities of cells, your ammunition will still weigh less and take up less space on the transport. Why? Your troops have lasguns!
Honestly, bolters may get all the glory, but lasguns are miracle weapons in their own right.