Hmm have a question, not 100% firearms but closely related. Maybe someone can help me out.
How long would it take to train someone to be moderately proficient with a weapon in a combat situation? I'm not talking super-effective but know what to do and how to handle his weapon.
To make matters a tad complex, this training would need to be focused on an urban environment.
I know I'm considerably late in joining this conversation, and the OP of this question may never ever see my response. But on the off chance they return, I thought I would offer my two cents. Not because anyone else who has answered is wrong, but because as a firearms instructor who teaches people with zero firearms training how to defend themselves in high stress, life or death situations, I may have a slightly different perspective on how to answer this question.
In a combat situation, the weapon is by far NOT the biggest piece of the equation. One of the slogans of the company I work for is "Any gun will do if YOU will do." What that means is, no matter what kind of weapon you're carrying when that "oh crap" moment happens and your life is suddenly on the line, if you have trained to respond appropriately, you have a good chance of winning.
Training is so much more than just learning how to shoot a weapon accurately. It's even more than learning how that weapon works, even down to its intimate details. Effective training is learning how the human mind and body responds to a high stress, high adrenaline, chaotic environment and situation, when even tenths of a second can mean the difference between life and death. Effective training involves training your body so that when the moment comes, you can set everything else aside, and act quickly, decisively, and effectively--in exactly the right way, every time. It involves ingraining muscle memory (making your body move appropriately without even thinking about it) and yet keeping your brain engaged despite the fear, panic, and chaos of the situation. This doesn't happen in hours or even days.
So yeah, you could teach someone HOW to shoot a gun in a relatively short period of time. But as Drachen Jager touched on, in any sort of stressful, life or death situation, you run the risk of having them be a serious liability. At best, they'd just freeze up and get themselves killed. At worst, they'd spray and pray, and shoot you in the back, too.
With that in mind, if this is for a character who's just providing impromptu back-up or in a last ditch move to get him some self defense skills, training him on the gun could be enough. But if we're talking taking them into a sustained combat situation or on a mission that requires specialized combat skills, anyone who knows better is going to quirk an eyebrow and go, "Nuh uh."
Just my two cents. I'm still learning about this myself, too... and training for it!