An Imperial Guardsman has absolutely no clue whatsoever as to how his weapons even work. Starting to pick them apart is practically tech-heresy.

 It doesn't take much understanding to assemble a gun from a kit of parts. We could even suggest that the parts use future science tech to bond together without tools.

And we can also introduce problems - parts can be misaligned, mounted backwards or just useless or massively inappropriate (so everyone carries a heavy module on the end of their rifle that no one remembers was a sonar pinger that was useful three hundred years ago when the regiment was fighting on a pitch dark world. It was supposed to interface with a special pair of goggles, all of which have long since been returned to stores).

Plus we can stick accessory rails on everything, from helmets to chairs to underwear to books so everything can be accessorised. For instance, they make this mug fitted with rails to allow the attachment of handles:-

You could say that, but I doubt you could make it stick. Human technology is a thing of nuts and bolts, not parts that auto bond to each other.

And that mug is beyond ridiculous :P

So I would suggest not going all out at once with the customization if you're going for a realistic approach to your campaigns.

 Why say realistic?

You pretty much won't see a first world soldier these days without at least an optical sight attached to their rifle, and probably a vertical foregrip and some kinda illumination device.

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Even we Brits are getting in on the action - the latest incarnation of the SA80 comes with a rail-equipped front end for mounting accessories.

You keep on, however, assuming that the Imperial Guard is operated and trained as a modern day 1st world fighting force, when they are closer to WW1/2 than anything else.

Do you see modern day infantry being thrown in their thousands against enemy fortifications and trenches under hails of artillery and machine gun fire?

Yeah, frankly. You probably would see that if you had nations throwing million strong armies around like they did in WW2.

The methodologies of warfare in 40K aren't so outdated, they're just based on a form of warfare that has fallen out of style (again, not outdated). That of the "Total War", as in the total economic and industrial mobilization of a nation-state toward a military footing. The main reason this has fallen out of favour is in part due to the powers that be not liking that much unconsolidated military power floating around (hence the so-called Military Industrial Complex), and the fact that half of Europe got blown up the last time the nations of the world switched over to a Total War footing. But were you to see tanks rolling out in the thousands, where the opposing sides don't have a 30-40+ year technological gap between them, then yes you could certainly see great big giant battles taking place, where men charge fortified positions and are cut down by the hundreds.

Another factor is that modern warfare relies heavily in modern infrastructure. "Why don't they just bomb it?" isn't such a cute response when you realise that US air power is so punishing and far-reaching because they have air bases practically everywhere in the world. Whereas when dealing with, say, Catachan, only someone holding orbital superiority (and leaving those ships on station to act as a base) would be able to launch air sorties against the enemy. Suddenly that whole war becomes a horrible, attrition filled meatgrinder because you're lacking in an essential element in the way war is conducted in modern times.

The Imperium is fighting a thousand wars at once all accross the galaxy, they can't afford to strike with overwhelming force everywhere (again, a common trend of the US at war, who pretty much sets the example for how modern warfare is waged). So yeah, a theatre commander may find himself with just enough men and equipment to attain victory if things don't go entirely tits up too often on the battlefield. They can't always use an entire Army Corps to roll over a single division, or reduce an enemy's army to slag and ash with non-stop airstrikes. It's easy to turn up your nose and sniff, and pooh-pooh the Imperium and the way it wages war, when you look at things in their most simple terms. Sometimes they just don't have the necessary resources alloted and available for use on the battlefield. Sometimes they're fighting on a planet that's only been rediscovered in the last 1,000 years, and spent half of that as a fuedal society? So if you don't have an air assets and have to assault a fortified position, just what do you do? Well, you get the men to lace up their boots and get ready for an assault, just as soon as the artillery are done softening them up.

But cutting right back to the chase, the way the Imperium wages war in 40K isn't so much outdated compared to modern warfare (which is rather low intensity, the bloodiness of recent wars aside), just that it uses methods that aren't fashionable anymore.

(And for an example relevant to how things could get WW2 bloody in modern times.. years back a friend, and retired US Army NCO, once said of the possibility of war with N. Korea, that it would never happen. Because the 38th Parrallel is so heavily fortified, in pretty mountainous terrain, with hidden artillery emplacements aimed at Seoul, that it'd be such a "Stemwinding bitch" to fight through the American public would never tolerate the casualty numbers coming out of there. Take that anecdote for what you will.)

Edit: getting more on topic a bit, while I don't think the Imperium fights war as primitively as many believe, I doubt the Imperial Guard gives much leeway for personal customization of a soldier's personal kit. They go for mass production, because they half to. When you have armies so big that you don't even have the slightest clue how many are operating on even the Sector level, let alone Segmentum and Galactic, standardization is going to be a very very big deal for the logistics side of things (consider that the Lasgun is standard issue probably mainly because it's incredibly easy to keep stocked with ammo).

 Everyone in the Guard gets body armour, so it isn't like they don't get ANYTHING.

 

 The thing to remember is that the setting is devised almost entirely for the products to sell. And the setting is thus designed to enable the various unique armies available. If you like mecha and more anime designs you have the Tau, if you like huge armour and crazy powerful weaponry you have the Marines, if you like elves in space you have Eldar, if you like dwarves in space you had(!) Squats, if you like gribbly aliens you have Tyranids, if you like robots you have Necrons and if you just like footslogging humans you have the Guard.

As such the Guard will never get given stuff that really steps on the toes of any other army as it makes it harder to sell product that way. Guard don't have lots of mecha (they have just the sentinel), or elves or dwarves or whatever, but hey - they get abhumans, one of the largest variety of man portable weapons and armoured units, a massive array of fully fleshed out regiments and worlds to draw on when making your army and above all else pathos - the ability to feel some emotional link with your little model soldiers because they are human, just like you. And that link is played upon by throwing those little humans up against all manner of monstrous, overpowered and downright scary threats. Those little humans get a basic rifle, armour and survival gear, basic training and are then thrown into the galaxy spanning war of the 41st millenium in their tens of millions.

Think about it from the point of view of your average guardsman. As a green recruit, you are rubbish and only as good as your commanding officer and the men at your sides. Its only after you gain experience and show competency with special tactics and gear that you are likely to be promoted to something above basic footslogger, and even then you are still at the whims of your commanders and the archaic supply chains the Imperium uses. So what if John Doe has survived 3 months against the orks and got over 100 kills to his name, it'll be several more campaigns untill he is recognized as a veteran and moved into a unit where he can either beg, scavenge or steal something more than his basic gear. In game terms this is entirely covered by the fact that as you gain XP and your campaign progresses, you get access to better gear and training options. I really don't think the issue needs adressing for starting characters. And you can always persuade your group of players to roll up Well Provisioned characters from a High Born world, this will give them enough resources to upgrade the craftsmanship of their weapons and get more weapon upgrades and gear, but these should be the exception through their back story and not the norm for the Guard in general.

AluminiumWolf said:

Oh I don't know. This is (apparently) a US National Guardsman in Afghanistan with a <cough>  '10.5" Adcor B.E.A.R upper on a M4 Carbine lower'. This is not official gear.

This is apparently 'Official Policy'

 I don't think its possible to say whether the Imperial Guard allows for weapon customization. While the peaceful members of a regiment raised from an agri-world are not very likely to have much more than the standard kit, and more archaic looking troops like the Headhunters, Attilan regiments, and Praetorians are even less likely, there are many others. 

Athonian regiments, coming from a industrial hive world with near constant corporate warfare would likely have many attachments, particularly those which help in close quarters.

Most warlike or tech-savvy regiments are likely to have some sort of improvements to their weapons, I would think that the cadians, who live and breath military life would have a great deal of fancified guns, and would probably carry camo mugs with rails as a fashion item. Necromundans, or ganger regiments from pretty much any other hive world are not going to have standard issue gear for long. It would be hard to imagine a regiment founded from Gunmetal City to not have vastly personalized weaponry.

All of the regiments above are offcial, so with the great open ended-ness of 40k its very likely that somewhere there is the most tacticooled regiment. There is even a fully power armored regiment in the 5th ed rulebook.

There really is never a rule of thumb for the guard, and with a couple of changes to the rules it would be pretty trivial to make customized weapons. If the gunsight rules were more like: A gun may have as many sights as can be reasonably fitted to it, however a gun may only make use of any one gunsight at a time. This keeps out the silliness of sight-stacking which would happen really quickly otherwise. Maybe making bipods and tripods more useful for basic weapons (are they even in only war?) Those are really the only changes I can think of, besides adding some more attachments.

The unit my players play in just took Exterminator Cartridges and Red Dot Sights as standard gear for their lasguns with Regiment Equipment points. Meaning the weapons have an underbarrel rail for the Aux GL or Exterminator and likely a top rail for any attached Sight system already. Really, if you want a fancy Lasgun, just do that. Then use the already-provided Weapon Customization rules. Simple, and you won't end up with tacticool abominations like that wall of photos just posted last page.

First world militaries are the equivalent of noble armies or stormtrooper regiments, while the vast majority of guard regiments are given the equivalent of an AK-47 with iron sights because it's reliable and allows them to have as large of a force as possible for the least amount of resources .

You do not see bolter regiments of guardsman because a whole clip of bolt gun ammo is almost as expensive as a lasgun.  One thing that you learn in even the 1st world military is that every piece of kit you are equipped with was made by the lowest bidder.

Better equipped regiments exist, but are not the norm by far.  Point is, your guardsman pc in the game is not another piece of fodder like your commrade. You have fate points, and as such the emperor has a plan for you… and maybe it involves a red dot sight that you used logistics to get, or maybe it's standard kit for your special flower squad ^_^

 I've got a couple of buds in the Army and it doesn't seem like gun customization is all that common or even practical. It's mostly something done as a hobby or downtime. Sure men will sometimes go out of their way to buy their own grip or a non-standard scope but it's important to note that's on their own dime and if you go overboard people start to ask questions. Just like with the Guard the rifle they give you is something they may want back someday so it's frowned upon if you mess with it too much.

The problem, as most people are driving at, is the fluff of the Imperium itself. We're talking about a place who actually forgot a large chunk of science and technology and has been searching for clues and blueprints on how some of their tech works. Engineering is an actual religion in this setting for pete's sake. Pair that with the average education of an Imperial citizen and you'll end up with a Guardsman who knows how to use and maintain their rifle but not enough to crack open the housing and start dicking around with the laser emitters or various circuitry. Pair that with a very strict Creed that says "If you don't follow the rules a man in a black jacket is going to shoot you in your face" and people are going to be rather aprehensive to live out their Call of Duty fantasy on their gun (TF2 Hats is a whole different bag).

That said, from what I've read with the weapon customization options presented in the beta, it seems like everything has to do with how the Guardsman adjusts various things on his rifle. So nothing really requires dramatic additions or breaking down the entire guts of the rifle, and that seems absolutely fine. Things like a scope or red-dot wouldn't be that groundbreaking, but might be more of a matter that the gun was never designed to receive such an upgrade (no power outlet for addons, for instance) so an appropriate Armorer check would be needed to jury-rig something together. Mainly my hard point is that if it's something that requires the actual shell of the rifle to come apart, such as creating the Bullpup variant of the Lasgun, it's likely something only a Forge Master or some other wise Techpriest would know how to do effectively.

But like with any game like this do what you want. You want your Guardsman to ride gingerbread ponies into battle I can't stop you.