Why would anybody in the 26th century use a 500-plus-year-old gun, durable or not?
It assumes that no-one between now and the time of Halo will come up with a better version of a cheap to make, reliable assault rifle, which is kind of hard to believe.
We still use a lot of technology now that was invented a long time ago. Pulley, compass, bottles, glass and boats to name a few. Boats get fancier but the basic design is the same... same for guns, they get fancier but the concept is still the same. The Ak-47 has been heralded as one of the best all around firearms, as well as the most durable and versatile. It really has not changed much.
The simple fact is there are 75 MILLION AK-47's, not including the more than 25 million of the modern variants, there are 15+ million Mosin Nagant bolt action rifles, which are still being used by middle-eastern snipers against Coalition troops in Iraq/Afghanistan.
Over 20 million VW beetles were made during their production run. Over 900 million Big Macs are sold every year. Millions of people watch Gossip Girl...
Again, just because something works now, is in use all over the world now, is considered a good thing now, it doesn't mean that it will be thought of in the same way in 500 years... And if it does, AK using, VW Beetle driving, Big Mac eating, Gossip Girl watching insurrectionist would certainly be a force to be feared.
But hey, if you guys think the AK (47, 74, 102, 12, etc.) will still be chugging along in the future, that's your opinion. Personally, if I were living in the 26th century and needed an old weapon to use, I'd choose an M14. Better range, better round, more accurate, more suited to a trained soldier.
There are some things where you would want to leave a chunk of metal inside to cause as much pain as possible and others where you want the target to be completely vaporized.