Pulse rifles don't use energy cells as ammunition. They have solid ammo, which the gun uses a secondary energy cell to help activate and fire.
1) I never that the Pulse Rifle energy cell was used for ammunition.
2) You contradicted yourself in saying they have a secondary energy cell (where's the primary?).
3) It's the energy cell that starts the ball rolling that's being put in the lasgun.
I think what he's trying to say, is that the Pulse Rifle's Energy Cell is a "Secondary" cell, because it's used to assist the gun (by loading and charging rounds, etc) rather than actually being used to fire. IIRC, the lasgun has no "ball" to roll - its simply converting the energy from the Power Cell to an Energy Projectile or some sort. Pulse rifle cells may not be used for ammunition, but Lasgun packs are depleted very similarly to ammunition. Hence a simple conversion by simply using a different energy cell won't work, even when reverse engineered - because it's not what the Tau Energy Cells were designed to do.
The Tau also don't entrust Gue'vesa with the manufacturing knowledge for any of the Tau wargear they may have. So even aside from the fact that such a feat of reverse engineering would be far beyond anyone except Admech who'd never countenance the idea, it wouldn't be available to do so.
Why not? Especially if the Humans are the most common form of labor available in this cluster and have been properly trained for a couple generations into the Greater Good? The AdMech in this area is gone, and all their technical dogma. These Humans are being taught by Fios to do the job they don't have the numbers for.
The Tau would not share the secrets to how they manage their weapons "in the name of the greater good." The advancement in Tau's Technology is so vast that their manufacturing is most likely done completely by AI-controlled Engineer Drones in specifically designed factories and workshops. The Tau would no sooner give the Imperial's guns in order to attempt to ally with them then the Imperium would give the Orks some tanks for them to loot, in order to gain their allegiance. It just wouldn't happen.
To be honest, if the Guard wanted some stronger lasgun varients, they can simply overcharge the cells, or build new guns to use different sized cells. IIRC, the Lasgun, Long-las Sniper Rifle, Hellgun, Multilaser and Lascannon all use the same type of energy cell. Personally it would be wiser resource wise to delve into new varients using the current selection of cells available to the IG then to attempt to reverse engineer foreign energy cells and duplicate them before making them standard issue across certain units of the IG regiments.
I understand you put the query up to get some input on the Name of the rifle, but discussions about plausibility will almost always come with the package free of charge
We're all hobbyists and fans here - and even though it feels like you're changing what you've spent time and effort creating, a second (and 200th) opinion can still help bring out the best potential for the weapon. Don't take it as though people are deliberately shutting down your ideas, we're just trying to make it fit in with the rest of the fluff. If Mat Ward used this forum, we'd offer him the same critique we offer everyone else (Except he'd probably get LOTS of C&C