[quote=@Foster]
<Snipped quote by Silverwind Blade>
iirc, survival rifles are tucked into a compartment under the ejection-seat.
[/quote]

I've looked into it in the past, and it's really hard to find definitive 'yes' or 'no' answers if they're part of survival kit. There's lots of accounts about them being looked into, and people mentioning they carried them in helicopters, or in transports, but trying to find [i]anything[/i] concrete about them in fighters is teeth-gnashingly frustrating. 
That borne in mind, I'm going to say let's just stick with handguns for the meantime, and we can burn that horse when it comes to the flaming bridge later. 
TBH, with the decades we live in now, there's a whole mess of weapons that are small and suitable enough - MP7's, MP5K's, TMP's, etc etc, so I'll probably just invoke some rule that the Expeditionary Force decided to standardise on something like that for the sake of practicality and ease. 
As I said though, it's not hugely important in the narrative - it's a gun that shoots things, and as we're not using stats, it's just stylistic fluff anyway what kind of gun it is. And since we're living in the fictional world, it can be whatever kind of gun makes practical sense. Something small and adaptable would fit.