Borepatch's infatuation with blued steel and walnut as a preference to modern plastic and stainless steel is something I share with him. I like older designs, although I don't own anything as old as the Martini. My current favorite is an old Palma rifle built on a 1903-A3 Springfield receiver, sometime in the late 1950s or early 1960s. Blued steel and wood, Olympic sights, chambered in 30-06. It may not be a practical choice.
I do own an AR. It's easy to shoot reasonably accurately, it's fun to take to the range, it's easy to clean, and ammo is available. Today, it's practical.
The question of practicality, however, is both subjective and situational.
In a modern military unit with a functioning supply system, AR pattern rifles and .223 ammunition are a practical choice. Ammunition is factory made, every round made to tolerance and highly reliable. Repair parts and replacement rifles are made to modern tolerances out of quality raw materials. Keeping the army at the end of the pipeline supplied is what makes them practical.
If things fell apart, how long would ARs be the practical choice? Only as long as the ammo lasted and the rifles didn't fail. You can back up from there. Any rifle (or handgun) that uses a brass cartridge depends on a functioning industrial base. A bolt action rifle, like a Mauser or a 1903 Springfield, is a simpler technology than an AR. But it still requires modern steel, a reasonable level of machine tools, brass cased ammo, and trained people to produce. If things fall apart, bolt action rifles outlast automatics, but only as long as the ammo lasts or the rifle remains usable. It may be more likely that an old bolt action would be repairable, but the ability to make new ones would be lost. Ammo would eventually run out and even reloaders need empty cases, primers, and smokeless powder, along
with the equipment (presses, dies, scales, etc.)
I think the level of technology that survives a collapse long term is probably about the flintlock stage. The recipe for black powder, hand cast bullets, and an understanding of how to forge a barrel is what is required to make a functioning musket. A single shot flintlock is a practical weapon when anything with a higher rate of fire is gone.
If we lost the ability to forge, it's back to long bows and metal tipped arrows. With as much metal as we have lying around, I think it unlikely we would fall back beyond this point. This is also the point where spear heads, and swords, at least made from existing scrap, would persist. That's not the practical point until the rest is lost.
Back to Borepatch and Filthie, the Martini-Henry rifles the British carried at Roarke's Drift were only practical as long as they had cartridges to load. But they were absolutely practical until that point. After that, the Zulu spears were the practical weapon. The battle ended before the British ran out. Otherwise, it would have gone the way of the battle along the Little Bighorn River.
Kim DuToit used to occasionally run a post along the lines of, "If I sent you back to 1800 to go exploring, what rifle would you want to take?" The scenario would take different tacks. How much ammo is reasonable? Are you ever coming back? Do you have wagons and pack animals? What about handguns? It was an exercise in thinking about practicality. If you could take a modern weapon back, which one and why?
You saw lots of thought put into it and people came out at different places. Pump shotguns were popular choices. With an ammo selection that included everything from birdshot to high brass slugs, they were a practical choice. So were lever-actions, sometimes in intermediate cartridges that could be loaded in a matching handgun. .357 Magnum, for example, or .45Colt. Another effort to be practical. All of those ideas are only practical until the ammo was gone, then the rifle is just an un-ergonomic club.