I know this is really late after the original post, but for anyone who has a similar question, follow Nicollo's answer above. To reiterate what he said: Chapter Trappings and Signature Wargear WILL get you permanent items (this is a Core Talent). At the end of this post there will be all three talents listed for acquiring permanent gear and taken from the official Core PDF. Also, it is stated in the Core Rulebook that the GM can award permanent gear as they see fit (an artifact that you find out in space might be one example), though usually this is coupled with forcing you to take the Signature Wargear talent if you're the only one getting the gear (or planned to receive the gear). It is worth noting immediately, that under Signature Wargear (Hero) it explicitely states that even if you find an artifact, you still need to have the talent to bypass the need to Requisition it.
My personal opinion and tips are below and should be taken as opinions only.
Regarding Fluff: I do know from reading several books by different authors that crafting beyond proper maintenance and machine spirit appeasal by non-Adeptus Mechanicus is frowned upon by tech priests, even within their own Chapter. Some authors have made it so strict that to innovate is borderline blasphemy due to two factors: (1) progress for the sake of progress invites Chaos to take root due to the Chaotic nature of invention (2) the duplication of already exsiting arms and armor to improve them is seen as wasteful since perfection was achieved centuries before (making everything that is new seem to be highly inefficient; this can be false). By similar authors, the decontsrution and study of a weapon (outside of maintenance) is considered heresy itself by onlooking tech priests, since they are the keepers of the secrets of the Omnisiah.
Basically, tech-priests are stereotypically greedy knowledge hoarders since knowledge has led numerous times to disasters involving Chaos, whether directly or not. Those within the Adeptus Mechanicus are encouraged, albeit indirectly, to ever-improve what has been perfected for the sake of mankind; hence the permission to create their own Artificer Armor with the Specific Wargear talent Nicollo mentioned.
Tips for Persuading your GM:
Tip 1.1) Never ever, push, nag, or otherwise rule-monger your GM for anything. It is a surefire way to get your characters killed a lot, not to mention it will form bad blood between you, the GM, and sometimes the other players. Reputation isn't just a game mechanic; infamy in one gaming group can spread to others you've never played with before.
Tip 1.2) The book is a guideline. While some things are there to keep you from breaking the game, others are only there to imply an option (like the crafting rules). If the GM rules that crafting is not allowed, then accept it and simply go the legitimate Signature Wargear path. You'll get what you want most of the time (you can add flavor like appearance and stuff), and he can't deny you the item if he's permitted another player the same talent (and if he does, then walk away from the group if he doesn't give a good explanation; that's blatant discrimination).
Tip 2) An Untouchable from Radical's Handbook might be able to weasel their way out of reason 1 (in Regarding Fluff above) by keeping their inventions "pure" through their lack of presence in the Warp and natural Warp suppressing aura. The real trouble comes with the second reason.
Tip 3) While reason 2 in Regarding Fluff is harder to circumvent, good roleplaying can go a long way. I don't mean jumping through game loopholes or reckless suicide; I mean the reluctant salvation of an otherwise doomed teammate or NPC despite your character's dislike of them. Stuff that makes your GM think, "well maybe they deserve something for that awesome scene." This normally gets you an item or token, but not crafting rights. What you need to do is justify in-game why your character would try to improve on perfection (character background), and show that the rules are not as strict as he thinks using "fluff" a.k.a. lore. Look into the Adeptus Mechanicus lore to find such things. Note, it might take some digging, but stick to the Codex's if you can.
Tip 4) One thing I have NOT seen anywhere in the books I've read is whether or not all types of Space Marine are permitted into the Chapter Forges. One would think that they would be to turn in or repair their equipment, but perhaps the tech-priests prefer to guard the secrets of the Omnisiah. I know that equipment is turned in after each mission for inspection, but the rules on Signature Wargear specifically state that the Signature Wargear is PERMANENTLY in the Battle-Brother's possession (and "always at their side"). This implies that it is also with them while they are not on a mission; they would take great care with the Wargear due to the trust of the Chapter. That ALSO means they would accompany it into the Chapter Forges for repair or maintenance (proving that all Astartes have access to the Forge to some degree).
Tip 5) Don't optimize your character on simply combat (take some talents and skill for it, but not all). Take legitimate skills or talents that could eventually push your character's knowledge to permissable levels to justify crafting. An added bonus is that while your combat Min-Maxed Battle-Brothers are completely stumped when you run into some Archaic Tech (minus your friendly, neighborhood tech-priest), you can not only identify it, but ALSO get it running. That could lead to some awesome honorary induction into the tech-priests since you can appease the machine spirits of old!
Tip 6) Suggest that crafting is merely fluff. Take the Signature Wargear talent, but ask your GM to let you customize its appearance. You'd get the necessary parts to create the item through Requisition anyway so I pretty much see crafting in Deathwatch as fluff due to the Requisition system. If you find special materials along the way, ask to take them and incorperate them into your item (Kraken fang, Fenrisian wolf pelt, the blood of your mightiest enemy, etc.). If you're lucky and have been kind to your GM, maybe he'll give you a small bonus for roleplaying well. If not, well, you still look like a bigger badass than your Battle-Brothers!
For GM's: I've considered crafting to be one of my favorite aspects of any rpg, whether I'm GM'ing or playing. Making something otherworldly should be pretty much out of the reach of most players until later levels/ranks. I say pretty much because a Rank 1 of ANY Space Marine type could craft a Relic if they rolled well and spec'ed only toward their Trade skill (DC Hellish is a -60, which is still achievable with a starting stat of 50 and the proper advances), but it would be a 1-9% chance of success. A Rank 1 should never be running around with a Relic of any kind unless it is solely a delivery mission, and they can't use it at all (alright, maybe one swing of an axe in a dire situation). Relics are supposed to be mythical, but the standard Astartes Chainsword isn't a relic. Permitting a player to craft their own weapons or armor that they can permanently keep should be acompanied with the Signature Wargear talent. If you're wondering what they should use to purchase crafting components, just assume they have the components because they took the talent. If they use that specific piece of equipment for a very long time, consider giving them a bonus as its legend begins to grow with the Battle-Brother. After all, a relic started off as merely a finely crafted piece of equipment. Some of their powers are attributed to mere belief in their legend or "pleased machine spirits" that developed over time. A prime example of this is the Power Armor History in the Core Book. The things (suits of armor) have been around for so long that they seem to have their own character. That chart is for all Power Armor, and they aren't even Excellent quality much less Master Crafted. Just think of the implications with other items!
Below are the three Talents that will enable you to PERMANENTLY keep any Wargear using Core rules.
Signature Wargear: A tool of war has served the Battle-Brother in countless victories, and is always at his side. Select one item from
the Armoury (excluding unusual ammunition) with a Requisition cost of 20 or less. This Requisition limit may include Upgrades and/or Craftsmanship modifiers in its calculation. (For example, a Master-Crafted Flamer.) This item is permanently in the Battle-Brother’s possession without the need to Requisition it, effectively becoming standard issue for him. He must meet any Renown requirement for the item. A Battle-Brother may purchase this Talent multiple times in accordance with his Career Advances. Each time, note the item it applies to such as Signature Wargear (Master-Crafted Flamer).
Signature Wargear (Master): Prerequisites: Signature Wargear for the item selected
The Battle-Brother has forged a bond with his chosen tool of war, and can perform impressive feats with it. The maximum Requisition cost of his Signature Wargear increases to 40. (At the time this Talent is purchased, you may choose to apply additional Upgrades to your existing Signature Wargear, or replace the Signature Wargear item with a new item worth up to 40 Requisition.) Choose one applicable effect from the table below and apply it when equipped with the Signature Wargear. (The table is under the talent in the book.)
Signature Wargear (Hero): Talent Groups: Weapon, Armour, Relic, and Wargear (includes Servitors)
The Battle-Brother has been permanently entrusted with artificer armour, a Space Marine relic, master-crafted weapon, or some other prestigious and rare tool of the Adeptus Astartes. Select one item from the Armoury (excluding unusual ammunition) with a Requisition cost of 70 or less. This
Requisition limit may include Upgrades and/or Craftsmanship modifiers in its calculation. This item is permanently in the Battle-Brother’s possession without the need to Requisition it, effectively becoming standard issue for him. He must meet any Renown requirement for the item. A Battle-Brother may purchase this Talent multiple times in accordance with his Career Advances. Each time, note the item it applies to such as Signature Wargear (Artificer Armour).