Just as a notice, nobody ever has to make actual availability rolls for ammunition - excepting truly exotic stuff like Deathdealer rounds and mortar shells. Basically, anything grenades and under in cost/power I'm assuming dedicated runners have fairly free access to unless they end up in the middle of nowhere or are being actively shunned by every dealer in the city.
As for Marduk... he needs the gauss rifle and mod parts, though it'll take Jimmy probably even later then we get back to acquire them. The rifle itself is 24F/13000, external smartgun is 4R/400, chameleon coating is 10R/1000, improved rangefinder is 6/1000, and powered breakdown is 10R/13200. Most of that's pretty simple to get (except the rifle itself) and Tink can install all of it except the breakdown with at least an armorer shop. The last mod requires an industrial mechanic facility, though, which I don't think he's willing to spring for so I'll have to pay an NPC to handle it.