[HaplessOperator]

AR-15 variants for service rifles, hands down.

Manual of arms is faster than a raped ape. Runs better in mud than an AK. Runs even better if you keep 'er wet, even in the desert (the "keep your weapon dry" thing is a sure fire way to have it malfunctioning; environmental contaminants can't seize if they're a liquid), and you can slap just about anything on the platform in any configuration you wish and still have a lighter rifle than the next competitor, and you can trivially upgrade or modify essentially any single part of the rifle imaginable.

Need a rifle the size of an MP5? Mk18 has you covered. Need to reach out and touch someone at 800? Mk12 and 90-grain SMK's got it. Want to convert between the two in a private setup? You're two takedown pins and 15 seconds away from a room sweeper to a precision rifle. More exotic workups are easily possible; that same rifle can serve as a 9mm subgun to a .50BMG bolt-action, magazine-fed rifle or anything in between, and with a given lower, fire anything that fits within the 5.56 action length, and if you're not too snooty about upsizing, you've got the AR-10 and SR-25 families of design following essentially the same pattern.

Lefties do fine, too. No proprietary parts switching (which aren't issued along with service rifles most of the time, anyway), no worries about having to physically pull your magazine free if retention isn't required but speed is, allowing you to shortcut the process of gassing your weapon back up, no brass to fly in your face if you're transitioning shoulders (and you will constantly be transitioning shoulders).

With over 500 companies manufacturing parts for the things, and thousands that could if needed, they're better situated than any other country's service rifle for emergency wartime production, especially considering modern design and fabrication methodology for cutting aluminum billet and punching barrel blanks, and the existing aftermarket is probably the richest for any rifle platform in existence. These days, you can turn an AR-15 cheaper and faster than you can most extant AKs.

There's no one rifle that can do everything, but the AR-15 - especially in its modern incarnations - probably comes the closest.

If I were a trifling man, I could also mention that I've never been able to ignore that all these countries with indig service rifles (especially bullpups) tend to have their special operations forces almost universally using some flavor of AR-15 as well.

For the business end of things, an AR-15 is - within most common shootout distances - slinging a round that is going to cause considerably more tissue damage than a 7.62x39, as well, with lighter recoil, faster follow-up on the target, and on a trajectory that isn't like firing a rock from a slingshot.