This was originally distributed to the top 100 ranked players in the Informant Event, based on the number of Informer kills.
For a short time after Update 10.6 (2013-10-30) it could have been acquired via transmutation. This was due to a bug where all mods (including unreleased ones) became obtainable through transmutation. This has since been fixed, but players who obtained the mod this way were allowed to keep them.
During Devstream 17, 18, 43, 49, 59, 94, 107, 112 and 116 one lucky winner was given a Primed Chamber mod through random selection. Two winners were randomly selected during Devstream 121.
The option of being given either a Primed Chamber or a Legendary Core was given by [DE]Sheldon at TennoCon 2017 to five people who caught MTG cards he threw out into the audience. He later signed these cards as proof of the prize.
Two Primed Chambers were given out during QuietteShy's Twitch stream via random selection on January 16, 2019.
This mod can be purchased unranked from Baro Ki'Teer for 2,995 Ducats 2,995. Note however that Baro Ki'Teer's stock changes with each appearance, and may not have this item available every time.
Despite its name, Primed Chamber is not related to Primed Mods, and was in fact added in the game long before the first Primed Mods. As such, it can be equipped simultaneously with Charged Chamber.
Primed Chamber is multiplicative with base-damage mods. For example, a Vectis installed with a max ranked Primed Chamber and Serration will deal 225 × (1 + 165%) × (1 + 100%) = 1,192.5 damage.
If used with Ammo Efficiency bonuses, it will provide bonus damage for the first few shots before a single ammo is consumed. For example, using Energized Munitions (75% ammo efficiency at max rank) will give snipers 4 shots with bonus damage.
This mod goes particularly well with the Vectis as it has a one-shot magazine, giving the bonus to all shots fired.
[DE]Rebecca has stated the following on the topic of people being upset over Primed Chamber being exclusive:
Ah, we did not say the Primed Chamber mod is Exclusive to the Drone weekend event, it was just the first time it was introduced. Sorry for the confusion there! I imagine it will appear again in future events.
Due to the release of Charged Chamber it seemed unlikely that Primed Chamber will be released as a drop however the possibility of Primed Chamber appearing in another event has been confirmed in Livestream 17. DE said they were going to have it as a Gradivus Dilemma reward, but "didn't get that far" and are looking for the next appropriate event to add it to.