Good afternoon, again

As per basic rules all weapons have three firing modes - single shot/semi-auto and full auto. While single shot and full auto have obviously clear meaning, "semi-auto" is a bit confusing - what is it? IRL semi-auto weapons (like, for example, SKS carbine, Enfield L1A1 (British version of FN FAL) or M1 Garand) are capable of firing single-shots only (have no capability of firing in bursts or full auto), however the weapon uses recoil or gas engine to automatically self-reload until there are ammo left in the weapon's magazine.

And than we have the standard Imperial Guard's issue lasgun (M36 M-Galaxy short pattern) - that is said by canonical sources to be a select-fire weapon (like, say, an IRL AK-47) capable of shooting both semi-auto and full-auto ( http://wh40k.lexicanum.com/mediawiki/images/4/4b/Primer-19-lasgun.jpg - the proof is here), however, as per both Dark Heresy and Rogue Trader rulebooks lasgun have the capability of only "single shot" or "semi-auto". So the question is - what is the game's "semi-auto" actually - merely several single shots in quick succession by manually pressing the trigger each times, or it is a short "limited" burst of full auto fire, like the 3-rd burst mode on M16A2 or 2-rd burst mode on AN-94 Abakan?

It's like the 3 round burst. It's pretty much you doing a controlled burst, either manually or through the selector switch of the weapon. Obviously the full auto is your spraying and praying to hit something; however, with RAW you will most certainly hit something.

All weapons do not have the three firing modes. The specific weapon will indicate which modes they have and how much ammunition each mode uses. Exactly what that mode represents is probably different for each weapon.

Traditional Semi-automatic weapons fire one shot per trigger pull and have an automatic action cycling to bring the next round into place. They can fire multiple shots "per round" because they've got a quick action and reasonable recoil, allowing you to shoot quickly with some semblance of accuracy. Or perhaps they have a burst fire setting as Nameless mentioned. Or if they're energy weapons they've got a quick recharge time, but can't actually fire on fully automatic because that would overheat components, require expensive tech, or the designers decided that would waste too much ammunition.

A single shot weapon would be a bolt action rifle or pump action shotgun, where you need to work the action and manually cycle the next round into the chamber. Or perhaps an energy weapon with a longer warm up time before the next burst of plasma is up to temperature or the capacitors are recharged. Or perhaps they have a lot of recoil to compensate for and it takes some time to wrestle the gun back on target. Or a combination of all of the above.

As for the lasgun specifically, it's not worth worrying that the stuff in the books doesn't match the stuff in the RPG doesn't match the tabletop. They're all written by different people from different companies. Whether by accident or for the purposes of game balance that's the way it is. If you absolutely have to reconcile then say that the Calixis sector's standard lasgun pattern can only fire in 3 round bursts or single shot.

Whether by accident or for the purposes of game balance that's the way it is.

Most likely for the purpose of game balance, I suggest. Should the lasgun retain feature of firing full-auto in long-bursts, than the autoweapons like autogun and autopistol would be pointless for the player unless some other measures to balance things are introduced (like the costs pf weapon in question).

PS: Such things have already led to autogun being effectively cut (no longer listed as a option wargear in the Rulebook and Imperial Guard/Astra Militarum codex) from the tabletop itself (no difference from lasgun per game's mechanics).

If you absolutely have to reconcile then say that the Calixis sector's standard lasgun pattern can only fire in 3 round bursts or single shot.

Well, before Only War Rulebook was published, reconciling was pretty simple - since all previous rulebooks' "vanilla lasgun" could easily be not a fully fledged Imperial Guard-issue lasgun, but a cheaper PDF (Planetary Defense Forces)-class model, that don't feature autofire or power adjuster for the shot. However, since such assumption is no longer an option, and up to me semi-auto only lasgun for Imperial Guard is too inferior-class weapon and an absolute no-go for me, however while M16A2-like 3-rd burst is generally okay.

So - to prevent miscommunication it would be better option to stick to house rules to clarify the approach? Or are there any official commentaries from the rulebook's authors?

I'm pretty sure the auto mode on a lasgun fires weaker lasers. If a player asked me if they could do auto on the regular lasgun, I would say yes it is auto 6 but does 1d5 damage instead of 1d10.

If a player asked me if they could do auto on the regular lasgun, I would say yes it is auto 6 but does 1d5 damage instead of 1d10

Rather interesting home-rule, I suggest ))

PS: Well, up until Only War rulebook's publishing I suggested "vanilla lasgun" be PDF's model with an Imperial Guard-isuue M36 M-Galaxy short pattern lasgun have stats something like this

Basic; 100m; S/3/8; 1d10+3E; 0; 60; Full action; Reliable*; 4kg; 125; Scarce

* Imperial Guard lasgun could also be set to fire overcharged shots. When set as such, it burns through ammo cell faster three more times (so fully charged cell lasts 20 shots and 1 shot consume 3 "ammo") and loses "Reliable" trait, also it could now only shoot single shots. However, it gains damage of 1d10+5E and penetration of 2.

Most likely for the purpose of game balance, I suggest. Should the lasgun retain feature of firing full-auto in long-bursts, than the autoweapons like autogun and autopistol would be pointless for the player unless some other measures to balance things are introduced (like the costs pf weapon in question).

PS: Such things have already led to autogun being effectively cut (no longer listed as a option wargear in the Rulebook and Imperial Guard/Astra Militarum codex) from the tabletop itself (no difference from lasgun per game's mechanics).

There's plenty to differentiate them beyond firing modes in the RPG's.

Different range, magazine size, reload time, weight, and reliability. Autoguns can use speciality ammunition that las guns can't and vice versa, although las guns don't get nearly as many types of specialty rounds.

Like you mentioned, even the US army doesn't issue their standard infantrymen a full auto weapon, and those guys love dakka.

Well, technically even the M16A2 was capable of full auto, however bursts were limited to 3 rounds only, however AFAIK now the Americans are issuing their troops a full auto M4A1 assault rifle.

Why does it have to be full auto?

Well, I don't mean adding "long burst" full auto mode, only to clarify that "hey, lasgun is capable of firing bursts, e.g. it is not "three press of trigger to achieve three shots, it is one press of the trigger - three shots with high rate of fire". Per game mechanics we still stick to the rules of "semi-auto" fire (the stats of the weapon are not buffed up and remain the same) however it is specified that it's not merely semi-auto lore-wise ))

Stats buffed up to what?

What lore? The Rogue trader book makes no mention of burst fire. Nor does Only War.

It isn't specified because it doesn't matter. Per game mechanics there is no burst fire mode. There is single, semi-auto, and full auto. If you want there to be a burst fire mode you'll either have to create one, or simply say that one of the modes represents a burst for that type of weapon. Done.

simply say that one of the modes represents a burst for that type of weapon. Done.

And that would be the extent of proposed "house rule". Exactly the point.

Per game mechanics there is no burst fire mode. There is single, semi-auto, and full auto

Some addition. Per game mechanics there is "standard attack" (this is generally a single shot from most ranged weapons, however, not from all), and there are "semi-auto burst" and "full auto burst". Since "semi-auto burst" do offer an increase in accuracy.... well, it seems more legit that it is really a short burst mode, found on some IRL firearms.