A justification is a compelling reason, a handwaving explanation is merely an apologist account.

Lasguns aren't particularly heavy or prohibitive as secondary gear at 4 kg (I believe there are lighter patterns besides), and Laspistols less so.

4 kg is 1/10 of the carrying capacity of a S/T 7 character. Armour usually comes in a 10+ kg, a second weapon at another 4, a sidearm at 2 and a melee weapon at another 3. That's 23 kg of those 45 already spent.
And I didn't think we were talking about laspistols.

You are missing the point. The weapon lacks properties it should have for no good reason. There is no mechanical reason to withhold properties that the Lasgun should have according to the IIUP, namely Full-Auto capabilities and adjustable power settings. None. If there was, maybe the deviation between the RAW and the IIUP would be warranted, but there isn't. That's the point I'm trying to make and construe; the Lasgun is so completely inferior according to the RAW, that retaining these IIUP granted properties would not upset the balance, thus their removal is without justification.

Ease of mechanics?

I'd say most sensible GMs would probably rule that basic essentials such as munitions, at bear minimum, most certainly do not come out of the pocket of Inquisitorial acolytes.

So we're talking "most sensible GMs" now and not RAW? Because the rules merely say that an Inquisitor approving of his cell's actions may grant them an additional stipend or supplement them with ammo at his own discretion.

Again, the sub-topic as repeatedly stated is completely relevant because it seeks to determine whether or not there is any mechanical reason for the Lasgun's existing RAW implementation and deviance from the IIUP. I do not see why this is apparently so difficult to understand.

It's funny you should say that. I believe it was Ghaunt's Ghosts that has a sniper correct his aim with a longlas based on the distance to his target, but you said that las-weapons should be unaffected by distance. Why? It's canon, after all. The IIUP is one source among many and considering the completely imaginative nature of the universe, it's rather irrelevant whether FFG (or rather BL who made the core rules) keep to that piece of fluff instead of contrary ones.

"4 kg is 1/10 of the carrying capacity of a S/T 7 character. Armour usually comes in a 10+ kg, a second weapon at another 4, a sidearm at 2 and a melee weapon at another 3. That's 23 kg of those 45 already spent.
And I didn't think we were talking about laspistols."

First of all, 4 kg is not a lot, even at 10% of the Carry limitation. Second, you're obviously not going to be carrying your Las secondary everywhere with you, all of the time; it is a situational item to be used and carried on a strictly as needed basis, and most of the time it will not be necessary. Third, why not involve laspistols in the conversation? They're pertinent because they undermine the little remaining utility a proper Lasgun has left.

"Ease of mechanics?"

I wasn't aware giving Lasguns a full auto mode by entering a single digit on a chart was so incredibly complex. With regards to power selector, it's pretty easy to spit out a paragraph detailing a damage/range/reliability bonus or penalty for the high and low settings, or even incorporate it into its description!

"So we're talking "most sensible GMs" now and not RAW? Because the rules merely say that an Inquisitor approving of his cell's actions may grant them an additional stipend or supplement them with ammo at his own discretion."

We could go by the RAW where Acolytes are absurdly expected to foot the bill for things like munitions and Autoguns would still be better. The fact that most GMs choose to subsidize common expenditures like ammo _compounds_ their many advantages.

"It's funny you should say that. I believe it was Ghaunt's Ghosts that has a sniper correct his aim with a longlas based on the distance to his target, but you said that las-weapons should be unaffected by distance. Why? It's canon, after all. The IIUP is one source among many and considering the completely imaginative nature of the universe, it's rather irrelevant whether FFG (or rather BL who made the core rules) keep to that piece of fluff instead of contrary ones."

When/where did I say that las-weapons are "unaffected by distance"? Second, as you have yourself stated, the Lasgun featured in the core rulebook is the exact same model as in the IIUP. Furthermore, Full Auto capabilities and setting selectors are fairly constant properties near unanimous to the Lasgun fluff. The fact of the matter is that FFG/BL should at the very least exhibit some consistency with their own fluff when there is exactly zero reason not to be. The deviation is basically arbitrary.

Lasers said:

?

GW and BL have, as far as I am aware, never shown to much concern with consistency in fluff. The "Rule of Cool" was and is still the defining prerequisite for fluff of all types. The only things that are set in stone are major items in the fluff and even they subject to a writers whim and are retconed semi-regularly.

Beware of hand waving: Think of the lasguns in the IIUP as a pattern variation of the ones listed in the core book. Look at how many variations of the M-16 there are, it's not hard to say the differences are just a pattern variation and move on. Remember that Black Library wrote the lasgun rules in question and by definition they should know all canon and everything they publish is canon.

"GW and BL have, as far as I am aware, never shown to much concern with consistency in fluff. The "Rule of Cool" was and is still the defining prerequisite for fluff of all types. The only things that are set in stone are major items in the fluff and even they subject to a writers whim and are retconed semi-regularly.

Beware of hand waving: Think of the lasguns in the IIUP as a pattern variation of the ones listed in the core book. Look at how many variations of the M-16 there are, it's not hard to say the differences are just a pattern variation and move on. Remember that Black Library wrote the lasgun rules in question and by definition they should know all canon and everything they publish is canon."

Why change the parameters of the exact same weapon from the IIUP to the core rulebook? There isn't a mechanical reason, the rule of cool doesn't apply, and there's no fluff explaination that it's some burst limited variant.