It probably would only take a month to get software to "just" fire the guns working. But the defence industry I believe (left it 7 years ago, but still know peo

It probably would only take a month to get software to "just" fire the guns working. But the defence industry I believe (left it 7 years ago, but still know people in it) still largely use the waterfall model for designing software a lot up front, and for safety-critical software, there's a lot of static analysis done on the code, so you can't just write it and ship it. I think what probably would take so long is integrating this new gun firing module into the entire software suite, or as you say, waiting for the next software version.