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human
[Stargate:Atlantis] Why did Lt Colonel Sheppard switch to a G36k?

Why did Lt Colonel Sheppard switch to a H&K G36k carbine from Season 5 onwards?
gpt
Depending on how much pull he has, it's either because A) he thought the K variant G36 was a good compromise between the range of a full sized rifle and the compact form-factor of PDWs like the P90 or B) someone higher-up changed the Table of Equipment, meaning his security teams started getting issued rifles whether they liked it or not. Stargate units seem to operate in a weird niche between more conventional forces (who get what they're given and eat shit if they don't like it) and special operations (who get a lot more leeway in terms of choosing their own hardware, within reason), so either is possible.

If memory serves, that time period also corresponds with when Earth-based Stargate Command teams started using more rifles, so a high-level change seems like a likely cause. The most likely reasons for that would be wanting to get SGC on more common footing with both the rest of the American military and other NATO members coming on board with international initiatives (most of whom don't use 5.7mm PDWs) for logistics reasons. On top of that, the SG teams were originally conceived as an exploration force first, fighters second (with exceptions like the Marine SG teams, who rolled with 5.56mm rifles from the start), so a PDW made sense for armament, but as time went on and SG units got into more and more fights, it would just make sense to switch to something with a bit more range.

(If memory serves, the Doylist reason is that the Canadian military was buying up so much of the available 5.7mm blanks that the producers had to switch people to other guns, like Sheppard's G36, Carter's tricked-out mixmaster M4, and so on.)