I did it. just go to your save game, you should find this at (C:\Users\"your profile name"\AppData\Roaming\Running with rifles\savegames\"your campaign name") unhide all folders first.
Basically i just open the .person with notepad, and edited like this:

<person max_authority_reached="50.012535" authority="50.012535" job_points="1414.000000" faction="0" name="ANDREW MARTIN" version="66" alive="1" soldier_group_id="0" block="11 15" squad_size_setting="0" squad_config_index="0">
<order moving="1" target="530.983 0 629.363" class="2" />
<item slot="0" index="12" amount="1" key="xm8.weapon" />
<item slot="1" index="21" amount="0" key="medikit.weapon" />
<item slot="2" index="11" amount="4" key="hand_grenade.projectile" />
<item slot="4" index="-1" amount="0" key="" />
<item slot="5" index="-1" amount="0" key="" />
<stash>
<item class="3" index="18" key="vest3.carry_item" />
<item class="0" index="13" key="f2000.weapon" />
<item class="0" index="18" key="aks74u.weapon" />
<item class="0" index="56" key="barrett_m107.weapon" />
</stash>
<backpack />
</person>

i just type the name of the firearms in here :
<item slot="0" index="12" amount="1" key="xm8.weapon" />

yeah i did spawn with some cool gun. BUT , when i put it on armoury it doesn't appear. so much different when i drive the cargo trucks and hit the armoury, and voila! there's some unique gun, it's automaticly registered to armoury or when i pick up stock weapon on other faction, it would registered on armoury too.

My question is, is there some trigger to add some rare weapon to armoury? or else?

Just read the rest of the Mods section, thread by thread - a lot of guys, who posted there have a solution to this question

_________________
"Ad cogitandum et agendum homo natus es"

I work quickly, cheaply and qualitatively. You can choose only two of the three options.

hmm that's odd. it worked on quick matches. but not campaign?

To get rare weapons appear in armory in campaign you could use this viewtopic.php?f=11&t=1916#p11527

Wow thanks still learning about those logic though