It's perfectly acceptable to use various unapproved technologies as long as you don't get caught. But once exposed—and if you happen to "violate" a major prohibition—and Mars Headquarters decides to "pay attention," they will make sure you understand exactly what the Adeptus Mechanicus is all about. They will teach you that for them, "heretics are more detestable than xenos!"
Of course, if you have the power to contend with them, like the Forge World known as "Acheron Eight" that likes to research Tyranid technology, Mars might just turn a blind eye. This hierarchy is mirrored at all levels: Forge Worlds treat Tech-Priests the same way Mars treats them.
This is why "death of person, disappearance of tech" has become the norm within the Adeptus Mechanicus. "Grave-digging" research is an accepted practice, and the "grave" isn't necessarily an ancient one from the Golden Age, but could be the new "grave" of a recently deceased Tech-Priest.
"Improved?" Father Louis asked, somewhat incredulously, as Gamma was about to show off.
A junior apprentice, acting as a hype man, triumphantly disassembled the lasgun. He pointed to the parts Gamma had modified.
"Father Louis, look at this!"
"Oh, that's a really good idea; it must have borrowed design concepts from the hellgun."
"Exactly! Now look at this!"
"Hmm!? Adding two circuits to make the energy more concentrated, increasing power and reducing energy consumption."
"Right! Now look at this!"
"Tsk! This is...?"
"Alloy 96, with better electrical and thermal conductivity."
"That's right! Why didn't I think of that? Tech-Priest Gamma, I must say, your talent is truly impressive!"
Father Louis looked at the little guy in front of him and sighed inwardly. He knew how long it had taken him, a vat-grown individual, to break free from simply doing what he was told, to understand what "I" was, what "thoughts" were, and what "joy, anger, sorrow, and happiness" were.
This child, however, seemed to act like a "person," with thoughts and emotions, from about a month after he arrived at the Temple! Has technology advanced so much? Or is there some other reason? I really want to study him!
Gamma suddenly shivered, but being in the midst of his pride, he didn't pay it any mind.
"Hahaha, you flatter me, but unfortunately, Alloy 98 is expensive, so this improvement has no practical significance, Father Louis."
"..."
"Father Louis?" Gamma looked up at Louis, who was staring at him, and asked in confusion.
Louis snapped back to reality and quickly said, "The simple structure of the lasgun is why there are so many models; it has a lot to do with the materials available at the place of manufacture. The important thing is the concept."
"I see. I was wondering why I couldn't find the material for the power cell on this Lucius pattern."
"Right, this element is relatively scarce on our Forge World, and using it on a lasgun would be too wasteful. The cost would increase too much with trade, which doesn't fit the lasgun's purpose."
Gamma nodded. "Exactly. Even the worst hellgun is better than a lasgun!"
"Haha, that's the point."
After the gun test, Gamma had to behave himself for a while. Without money, he had no choice. He considered finding Anduin to help him sell the lasgun, then buy several old, broken ones, repair them, and sell them again, repeating the cycle. But he couldn't bear to part with his finely crafted lasgun. Even if he could, he didn't want to constantly "dance on the red line" of lawlessness. What if one day, for some trivial matter, the higher-ups started to take it seriously? How would an underage individual on a Forge World be judged for manufacturing and selling armaments?
He figured he would have money later anyway. For now, he was living a "normal" life without any special advantages. Before going to sleep, he would even violate the ancestral precepts of the Adeptus Mechanicus by reciting the "Lectitio Divinatus" he got from the black market. He was so loyal, but the Emperor just wouldn't acknowledge him.
Unfortunately, things didn't go as planned, and Gamma's normal life suddenly ended. After class that day, he had just bought dinner from a terrifying-looking vending machine-servitor.
"Ah Fan, one can of Grox and one serving of dried vegetables, and one energy starch bar."
"Received. Preparing your order, esteemed Tech-Priest Gamma."
"Ah Fan, you know, the first time I came here, I had nightmares that very night. Every time I come, I have to chat with you a bit to confirm you truly have no consciousness before I can eat."
The servitor's monotone voice replied, "Unable to comprehend. Please clarify."
"Ah Fan, I heard that servitors like you not only have your prefrontal lobes removed to lose self-awareness, but also have hallucinogens injected into your brains so you'll always have beautiful dreams."
"Unable to comprehend. Please clarify."
"Ah Fan, I have a great laser gun. If you still have consciousness, say something else, and I'll give you a quick end, even if I get punished."
"Unable to comprehend. Please clarify."
"Ah Fan, I both hope you can give me some feedback so your torment can end, and I also hope you don't, so you truly have no consciousness."
"Food prepared. Please retrieve. Enjoy your meal."
Gamma took the can. "Something is missing."
"Re-verifying: Grox can, dried vegetables, energy starch bar. Verification complete. Nothing missing."
"Ah Fan, I'm going to eat now."
"Order complete."
Carrying his dinner, Gamma hadn't gone far when he was blocked by someone, caught red-handed by Magos Lauster. Seeing the Magos's expression, which seemed ready to retaliate against society at any moment, Gamma wanted to say hello and quickly slip away. "Annoying" people should be discerning.
"Tech-Priest Gamma, come with me," Magos Lauster said.
His escape thwarted, Gamma could only reply, "Yes, Magos."
Gamma trotted along behind Magos Lauster to his private warehouse, passing through a series of security measures including biometric scans, iris recognition, and password input. Gamma's eyes darted around, sizing up Magos Lauster's treasure trove. Could it be that Magos Lauster doesn't have long to live? Is he planning to take me as his successor and give me his "inheritance" early? What a beast!
A power sword! A bolter! That jar must contain Tyranid! And it's moving! Three Castellan robots!
Wait a minute! That green glowing thing is a Necron phase weapon, isn't it?! Magos Lauster, that old geezer, isn't planning to silence me, is he?!
Eyes down, "I am an idiot," Tech-Priest Gamma stood obediently like a mere grunt at the doorway.
Magos Lauster, pushing a box taller than himself, saw Gamma's respectful demeanor and decided to scare this precocious brat. He had never seen a Priest so uninterested in his collection. He deliberately drew out his words, saying, "Gamma..."
"Oh, oh, yes, Magos."
"You've been at the Temple for a while now. How have I treated you?"
Trembling, "I'm done for," Gamma could only reply, "Very well, I suppose..."
"Suppose?"
"The Magos's grace is higher than Mars's Mount Olympus!"
"Hmph, now it's time for you to repay me, the Magos!"
"Ah!?"
"Tomorrow, you will take this box to the Manufacturing Great Temple. It is my gift to the Archmagos!" Magos Lauster said ominously.
What 42K religious terrorist, making children human bombs!
Gamma plopped to his knees, wailing, "Magos! I'm still a child! This heavy burden shouldn't fall to me!"
"Everyone else has their own things to do, no time!"
"I can too! I can also work for the Temple!"
"You?" Magos Lauster said dismissively.
"I can! I've mastered everything I need to! I'm already a qualified Tech-Priest!"
"That, I 'really' didn't know!"
You're lying! Who said that nothing that happens in the Mechanical Temple can escape your notice!
However, Gamma, who was usually quite intelligent, now realized he had been tricked. He stood up sheepishly, hung his head, and asked:
"Magos, what exactly do you want me to do?"
"Heh, quick on the uptake. Since you said you're a qualified Tech-Priest, then it's time to serve the Omnissiah. Let's start tomorrow."
Old geezer! Be careful, I'll report your misdeeds to the Emperor!
"Uh, hmm, alright, Magos, but my body..."
Gamma gestured to his height, which only reached a little above Magos Lauster's waist, trying to awaken the conscience of the inhuman Magos. But a Tech-Priest's moral bottom line is directly proportional to the organic content on their bodies. Magos Lauster patted the box and said:
"I've already prepared it for you."
"Magos, what is this?"
As the box was unlocked, an exoskeleton power armor, over two meters tall even when squatting, appeared, with a seat in the middle for the operator.
Cool! If you had this benefit, why didn't you say so earlier! I could have started working much sooner!
"I'm lending it to you for now. This is an engineering power armor I replicated, based on the Battle Sister Paragon Warsuit."
Still lending it! What a cheapskate!
Gamma looked at Magos Lauster, who had a hint of pride, and quickly flattered him, "Wow, Magos, your technology is truly beyond my imagination!"
"It's just a casual work from my younger days, heh heh. With this, your body won't be a problem."
"Yes, Magos."
After explaining to Gamma what he needed to do, Magos Lauster summoned a servitor to push the box and leave with Gamma. Gamma's future job would be to assist Priests who were too busy, going when called, and staying put to continue training apprentices when not needed. Don't be fooled into thinking it sounds easy. You should know that Priests are currently so busy that they are using various supplements and potions to maintain their physical functions and compress their sleep time, so Gamma's future workload will be substantial.