That is not hair-splitting, inasmuch as high-capacity magazines for Glocks were and are commonplace â€” almost as commonplace as Glocks themselves â€” and remained so even while their manufacture and importation were banned. Most Glock 9mm magazines are usable in any Glock 9mm pistol, regardless of model. Glock makes at least four different 9mm pistols at the moment â€” 9mm being one of the most common calibers â€” and a high-capacity magazine sold for almost any of those could have been used in the Glock 19. Third-party manufacturers make them as well, and have made them for years and years, meaning that AWB or no AWB, finding one is not very difficult. The only difference the AWB is likely to have made is that the shooter would have had a used magazine instead of a new one (assuming he did in fact have a new one), and he probably would have paid five bucks more for it.
Most of the magazines I own I bought during the ban. Most of the magazines I own hold more than ten rounds. With the exception of Glock factory mags, which did get more expensive during the ban, prices were unaffected. Magazines are and were always plentiful. Glock aftermarket magazines were still cheap during the ban. There’s good evidence the shooter used an aftermarket magazine.