My game has a RT who is well on his way to becoming an Arch-Heretek.  He has high Int, Tech Use, and Trade (Armorer).  One of his goals is to develop weapons and tactics to help the Imperium survive.  One of the first things he designed/built for the ship's Arch Militant was a .50Cal sniper rifle.  Since .50Cal is the round used by heavy machineguns, the .50Cal damage profile is based on the Heavy Stubber.  While it is still a basic weapon, it is large enough to require bracing to fire without penalty (-30 unless braced).  Bulging Biceps allows firing without bracing, similar to a heavy weapon.  The weapon is superior to the long las or sniper rifle, but much heavier and more limited in deployment.  The ammunition counts as heavy ammunition (it can share ammunition with heavy stubbers, although snipers tend to pick and choose rounds from ammo boxes to ensure they get the better rounds).  At this point he has custom built a few, but is working on the manufacturing schematics that would allow for mass production.  The weapon is usually used with Explosive (+3 Dam, counts as heavy ammunition) or Man Stoppers (+3 Pen).  The weapon is well within the technology level that hive world regularly manufacture, so I don't see a problem with it. 

Class              Range      ROF     Damage     Pen      Clip      Rld      Special                                                   kg     Rarity

Basic SP        150m       S/-/-      1d10+5 I         3          5         Full      Accurate, Needs Braced (-20)          15      Rare (based on complexity)

If I were the GM I'd say an STC pattern already exists for such a weapon. Essentially it's just a bolt-action rifle, scaled to .50 caliber and manufactured to fine tolerances. It should come with a scope as standard too.

I tend to require the players to aquire STCs and manufacturing patterns through aquisition or roleplay.  There may be such an STC, but it isn't common (not published, and there are other sniper rifles published).  I use the Inquisitor's handbook crafting rules for the most part, with components for an item being 2 degrees less rare than the final product if you have the appropriate Trade skill.  Components are 1 degree less rare if you only have Tech Use.  I also allow player to craft anything up to Rare without the STC/Manufacturing pattern.  Of course the players have a best quality Laboratorium on their ship, so crafting is quite a bit easier for them.

I forgot to add that it has integral silencer and top rail to mount desired scope.  The party Arch Militant has SP weapons as his specialty and uses either Explosive Tipped or Blessed ammo.  With the explosive tipped it does 1d10+10 (Pen 3) damage in his hands.

WilliamAsher said:

That's not how the Arch-Militant ability is supposed to work. Appropriate choices include Basic, Flame, Heavy, Pistol, Melee, and Thrown.

WilliamAsher said:

In the Real World (tm), silencers are useless on any projectile weapon with a supersonic velocity - for obvious reasons.

Cpt. Harkonnen said:

In a setting where caseless ammo gun that can easily swap between magazines (something that is hard to do today even), with all kinds of exotic ammo. They have laserguns, powered armor, giant mecha, starships, FTL, amazing efficiency and so much more.

Archeotech is simply put, AMAZING, it is efficient, durable and powerful. It outstrips our modern tech overall, the only area where modern day humans have better tech would be in computers, and in the 40k setting they outstripped us there too, even getting fully sentient. So they regressed in that area because of the risk of robotic uprisings and chaos infesting and corrupting their tech.

Besides, they have heavy stubbers (possibly around .50 cal) and autocannons (repeating powerful cannons that fire explosive ammo)

If a player wants a .50 Cal Sniper Rifle (for this example, let's take a heavy stubber, make it S/-/- and accurate with an appropiate sized magazine), what is the issue, it won't even be as good as the Angelus Bolt Carbine and the Sniper Bolter (both of which have great damage, more pen and tearing WITH accurate), so it hits weaker, will be big and heavy, obvious to see (but will have cheaper ammo). So why does it matter? It won't be a game breaker, just a more powerful hunting rifle and even harder to conceal

Cymbel said:

The heavy stubber is typically described and operated more like a .30 cal/7.62mm GPMG rather than a .50 cal HMG. As for autocannons, the fluff is that they have lost the technology to make explosive ammuntion for them and these are instead firing large solid projectiles which may range from .50 cal (but likely larger) up to 40mm depending on sources. I do find it a bit odd that there is no difference between the man-portable autocannon and the main gun of a Predator tank since it should be easy to have light autocannons and heavy autocannons (no, not battle cannons - those are considerably larger) differentiated in the rules.

In 40k, the realm of REEDICULOUS calibers, I don't think it is crazy to assume that a pintle mounted heavy machine gun on a tank that has ridiciously huge weapons (even the humble chimera) would be .50 cal. Even, for some reason, it isn't, so what? 40k has no problem with guns, let alone a simple bolt action 50 cal weapon.

Second, on the autocannon, I remember seeing somewhere that autocannons are not AS good as they used to be (they used to have even better explosives), but ONLY solid? You would have to bring some proof and citations, thankfully I have. From the Inquisitor's Handbook on Autocannons:

Autocannons use cased, explosive shells fired at a high rate to destroy their target.

I would rather not check every iteration of W40k RPG for the flavor text, but that example AND the extremely powerful damage (more like an exploding high velocity shell) seem to fit with the commonly held view that 40k autocannons are similar to modern day ones

The Heavy Stubber does too little damage to be a .50 cal, and it does roughly the same damage as an Armageddon Autogun, which is along the lines of a BAR or an M14. So the Heavy Stubber seems to be based off of an M60 or a Browning M1919, though its weight is far greater than it should be for a weapon of that class. Heavy Stubber is 35kg/77 lbs, Browning M2 .50 cal is 38kg/83 lbs, not including the tripod. By comparison, the M60 is 10.5kg/23 lbs.

Also, If I recall, the Imperial Infantryman's Uplifting Primer says that the explosive autocannon shell has been lost and thus they are forced to rely on solid slugs. However, this is the same book that says Genestealer claws are blunt and puny and that a Guardsman should recite the "Litany of Stealth" while attempting to surprise-attack someone, so it should be taken with a grain of salt at best.

Even if they actually did lose the explosive autocannon shells (which I'm sure they haven't), they are still perfectly capable of making explosive shells for the assault cannon (see: Deathwatch, any given Space Marine codex) and a wide variety of explosive bolter shell.

Hmmm…

"Esteemed High Magos, I have, uh, 'discovered' an ancient STC of explosive autocannon shells. I suggest we put them into production at once, in accordance with the blueprints I've… also discovered. I'm sorry, Beneficent One?… Uh, Y-Yes, of course it looks like the assault cannon shell! This is, um, a testament to the genius of the STC and the Machine God's design! If it works, then logically it would be best to pattern after it, r-right?"

Cymbel said:

Take a look at either Black Crusade or Only War and you'll get this line:

An autocannon is a crew-served heavy weapon, a self-loading high calibre cannon that uses dense solid shells to punch through armour.

Note that these are both far more recent sources than the one you cited. Also note that ALL autocannons (including the one from the Inquisitor's Handbook) do Impact damage. If they were meant to be firing explosive shells, then the damage would be of the Explosive type.

Boss Gitsmasha said:

It's never wise to assume that Imperial technology can bridge even the tinniest gaps. To them the ammo for an assault cannon and the ammo for an autocannon are quite different.  The range of the Autocannon is also far greater than that of the assault cannon, so it likely fires its rounds at higher velocities - which is something that would benefit firing solid slugs far more than explosive rounds.

Incidentally, the assault cannon also inflicts Impact damage. There needs to be a change to either the damage type (to Explosive) or to the fluff that says it uses explosive rounds.

HappyDaze said:

So? The autocannon can fire solid shells and for armor piercing duties, those are great, but it doesn't say anywhere there about how they are unable to fire explosive shells, nor how the tech to them is lost beyond the grasp of man. On the damage type, it could be a typo, FFG may make great books, but there are definitely mistakes within them.

However, why don't we get an answer from FFG? Or at the very least show sources from other FFG works showing that explosive rounds in Autocannons never ever exist or are ever used, ever. Besides, this is 40k, the canon and tech are so schizo at times that some worlds use solid shell autocannons, others explosive, most a mix

Edit:

http://wh40k.lexicanum.com/wiki/Autocannon#.UOYxYqNOztQ

The best source for 40k canon says they fire both

Cymbel said:

Why don't you show me a single example of an Autocannon statted up to do Explosive (X) damage in an FFG book? I don't think it's a typo when all of them consistently do Impact (I) damage.

As for Lexicanum, I don't consider it the best source for anything related to the RPGs.

And the other ones use solid shells only, of course they do impact. I was just saying it COULD be a thing on this one, knowing that they use explosive ammo in them.

Lexicanum is a great source for 40k fluff and acknowledged as one of the best to be honest. And look, I showed you a link with sources that said Autocannons fire explosive ammo. Can you find me sources where there never can be an autocannon that fires explosive ammo ever? And that Autocannons can ONLY fire solid slugs, because the Imperium has no ability to put an explosive inside a large shell?

Cymbel said:

I would dispute that point. While 40K obviously has some pretty strict limits on how they use computers and computer equivalents, the things they can do with them are pretty damn awesome. Sure, they don't hook everything up in networks (because that would be just asking for scrapcode and such to do horrible, horrible things to them), but something like the MIU or cyber familiars or the various other augmentations and implants (or, hell, the semi-ai in some Imperial vehicles) are certainly not inferior to modern technology. I'd say that, as with most 40K things, they have stuff that is inferior to modern stuff, stuff that is at modern levels, and stuff that is so far beyond modern stuff it's flat out amazing, at varying levels of use in various places, with higher-end stuff mostly confined to the the Mechanicus. Although it's not necessarily developed along the same line as our technology seems to be developing, given their love of organic integration, servitors and the like..

Of course that's disregarding the question of whether Machine Spirits are real and if so, just what they are, which may place their technology either higher or lower on a theoretical technology scale.