Just thought I'd throw something in here - specifically bolt weapons would be easiest to do the "special bullets" thing to. Lorewise, they have an impressive array of specialized bolter rounds that do things such as airburst to help ignore cover, spray a mutagenic acid, punch through ceramite armor, deliver a chemical cocktail of flaming doom, steer towards a target, track a target, etc etc.  The biggest trick to ammo types though, is would it be a variable on the weapon or instead (and seemingly the better idea in my mind) would it be better as weapon skills that could be swapped out - such as replacing the targeted shot with a shot that cannot miss, or the frag burst of the bolter's #4 with an aoe burning barrage.

Even then, I still like the idea of taking Eternal Crusade ideas and applying them in - even if basic. instead of changing Rate of Fire or damage or any of that sort of things, I'd even be perfectly happy if each weapon had a number of "slots" for specific things, and those things then generated the stat bonuses on the weapon. So I put a purity seal on my sword/gun and it gives me one of the stats - such as +damage to skill tag or what-have-you. I feel it would be easiest to keep that sort of thing super simple  - even if they are all seals or "marks" or something ( to distinguish it from armor-slot purity seals). Having weapon-specific parts (engines, barrels, magazines) might get things a little too over-cluttered in the drop tables of loot.

Some very fair points :) 

Agree that there is some fair wiggle room, just don't envy the developers in trying to pull it off. It's a case of juggling the player's expectations, the lore's possibility and what makes for a good ARPG. Most of the stuff written for 40k was written for the sake of making a pretty story, not a pretty game :D so it's gotta be a bit more of a headache than just making a new IP from scratch, like they did with Van-helsing. 

Here's hoping tho!

Had a similar debate elsewhere. 

Was suggested to me that guns in 40k aren't very limited. Now while I agree that you can play with
1 - Rate of fire
2 - Range of fire 

3 - Type of damage (Dot/Standard/Type)

4 - Spread of damage (different shapes!)

That's about it...

Whereas my point was with "magic" - You can shoot an arrow that bursts into 3x magic ponies that run around the enemy to tie them into a cocoon, then the ponies drink the blood of the victim. While that's somewhat hyperbolic, it's kinda true. There's no limit to most ARPG's in their skills, they can summon, they can cast, they can buff, they can debuff. Nothing really need explanation. Just "magic". 40k Has some flexibility sure, but it's not the same. Furthermore the more varied you make a gun the less iconic you make it. Once you get to the point that you have a long barrel bolter gun that is more like a bolt action sniper, is still still a boltergun in the eyes of a 40k player? The IP is something to consider.

Look, it's not that it's a terrible idea, but the core concept would introduce a ton of complexity if they were going to do it properly--and I mean even as reality-light as the material in the 40k Dark Heresy RPG.

Seriously though, real world ballistics are terribly boring. Translating any of the levels of that to a tabletop or video game is difficult, and trying to fit anything like that in Warhammer is... well. If you search my posts here you can see other comments worded with less potentially inflammatory wording that underline that.

Personally, my ideal in a game would be designing something from the ground up for my character to use, picking and choosing components like on a real rifle build (without all the time spent fitting them, because that'd just be dead space during play) from the lowly hammer spring all the way up to receiver options; but that's just way too complex to incorporate in a game, especially one with some fairly spirited back and forth on currently existing issues.

A bolter is a bolter is a bolter, unless you take the eight-ish inch barrel and crank it up to the Stalker pattern's 24-36" (hard to scale given the inconsistency of lore for Astartes size), you're not going to increase damage noticably. This is more poignant when you consider that the terminal ballistics of a boltgun aren't from kinetic energy dump like a modern firearm, but from the variety of slug used either exploding inside the target or spraying acid/toxins/incendiary gel/fragments outside.

Real world modification isn't as clear-cut as "more barrel equals more accuracy/range/damage"--that has a lot more to do with the chosen cartridge and is very highly contingent on the cartridge chosen; eg: the 6.5mm Grendel vs. the 5.56mm NATO--the latter, while being of similar (relatively speaking) dimensions has a 600 yard engagement range and is light enough for the slug to be underwhelming terminally, while having an optimal barrel length that ranges from 8" for close (sub 200 yard) engagement to 22" for longer engagement. The former, meanwhile, has a great ballistic coefficient, very little drop at range, and is much heavier--meaning it loses that energy more slowly and is useful out to 1200 yards, with an average of 72" of drop at 800 yards (which is, point of fact, actually less drop than the 7.62mm NATO cartridge). All while being less sensitive to barrel length, allowing for 6-12" barrels that can still accurately deliver rounds at long (800+ yard) ranges.

Certainly scopes do nothing to affect the accuracy of the platform either--they aid the shooter, but do not magically improve ballistics. Neither do stocks, grips, bipods--those are all shooter aids that have zero effect on the mechanical accuracy and reliability of the weapon. Modern muzzle devices, similarly, have a very small ballistic impact on ammunition--the more precise machining techniques allow for suppressors that do not have a detrimental effect on point of impact or terminal ballistics, for example, some hold zero even when using supersonic ammunition.

Essentially, most of the idea of modifying a weapon with muzzle devices, stocks, barrels, etc. would end up requiring them to come up with penalties/bonuses that changed depending on your class--while a full size unmodified bolter would be taxing for a Psyker to blaze away with at full auto, they're fairly simple to use relatively accurately in full power armor, meaning that the muzzle brake and recoil reducing stocks would be dividing by zero for the Crusader.

TL;DR: please no, because it's physically painful as an armorer to see how badly people mangle the realities of this stuff.

Yeah i tried to make a lasgun/plasgun build but after some time i throw them away and got back to melters. I wish we had no useless stuff to skill dat hampers our build. Like look at the crit tree, we have one branch were we are forced to skill ranged crit and on branch were we are forced to skill melee crit. If you make a melee only build that's at least one point for nothin. Why dont they make such points optional? Like still there is the 2% ranged crit but optional you can skill 1% crit, two dashed squares besides each other and you can choose what way to go. There are a lot of skill trees that suck. On the other hand, if you never change up stuff like your build or weapons you'll never miss something ;).

You don't need to add more buttons per say just change up the variation in ability type, perhaps tying it to the make or as mentioned before the name of the weapon. This system would perhaps allow relics to stand out more. 

They have a mechanic in mind to change at least stats on the gear afaik. We'll see what we can change with this in the future. At the moment we have 2 weapon slots with a 4 button template + buttons 3 and 4. Thats a total of 10 skills and thats a pretty good number in my eyes. In my eyes we should wait till we see the big picture and what we can do with the things they offer before we innovate new stuff. Dont get me wrong i love brainstoming but i dont know if this helps in those early stages of the game.

Some good ideas here to be fair and the naming of a wepon is just 40k all over so im well keen on this idea, the combination of each wepon part im a bit ya and na on this as with so many wepons would it be worth it but also it could be an awsom aspect.