Personally, always liked the Halo: Reach AR. Major part of it is my bias towards the game since it was the first Halo I really got into, then going to play the rest. Nonetheless, the Reach AR was a very odd pick in terms of actual lore. To pick the design of a weapon that first appeared at one of the earlier events in the Human-Covenant War(in terms of the games) was received a mixed bag. As a design choice I don't mind it, though in a lore aspect, maybe they'd write it off as an upgrading of retro tech. Or it could also simply be an upgrade with an older look, with the UNSC and the people who manufacture it's gear leaning towards an older style.
Even so, its an interesting direction. I'm curious what else is going to change with it.
..lets just hope that the designs of some traditional guns gets a change as well. Not admittedly a fan of the first rocket launcher Halo 5 had to offer. Though they did add the SPNKr, I would definitely prefer just the SPNKr.