Originally Posted by hkcavalier

Count me in the "not worth it" camp.

9mm full-auto is AWESOME. The MP5 is a laser and keeping an entire mag on a body target at 100m is easy.

9mm semi-auto rifle (legal rifle i.e. 16" barrel) --- 1, you're beyond the sweet spot in 9mm barrel length (8"-12"); 2, if you're restricted to semi auto only from a 16" barrel, for Heaven's sake move up to a rifle caliber! If you want a training rifle, go .22LR. Yes, I know it's hard to find but I can easily buy 500 round boxes for ~$32. Try getting 500 9mm for that price.

I find the recoil impulse of 9mm blowback to be worse than 5.56mm.

The only platform where it starts to make sense is an SBR, particularly suppressed. But now you're talking about $400 in stamps and all the complications of NFA items.

I see just about zero utility in the "cowboy" method of matching pistol and rifle ammo in the modern age. True rifle cartridges are so far beyond pistol ones in capability that it's just better to specialize. Just look to the military; they pay a lot of attention to common caliber among like weapons (rifle, pistol, machinegun, etc) but about zero cross-platform.

Not trying to get all mall ninja on everyone. It's cool to have "fun guns" particularly when you have casual shooters as friends or family. But at least for me and mine, we'll stick to rifle cartridges in rifles and pistol cartridges in pistols. I broke that rule with an AK pistol but with a brace it runs right up to the SBR line without crossing it. If not for the brace I wouldn't have bought one.