There are 2 sides to this problem: the maps and the effective range of loadout weapons. Both must be tuned perfectly to eachother, before the game is even released.
Maps that are quite open in combination with very effective long range weapons tremendously slow down gameplay. (e.g. BTB Ragnarok in H4) The larger the map, the bigger this effect becomes because there is more emphasis on the use of long range rifles.
This can be fixed in two ways. Either create smaller, more closed-off or sectioned maps, or decrease the effectiveness of loadout weapons at long range.
For the maps, there are a lot of good examples out there. Sanctuary, The Pit, Heretic, Construct, Citadel, … All are small and easy to get around on, and feature long sight lines across the map, but also plenty of closed off sections. There is always enough cover unless you run around like an idiot. You basically have no fear to get picked on from across the map without having enough time to get to cover. As always, there is a balance that needs to be struck: maps that are very closed off with no long sightlines make precision weapons useless and that dulls down gameplay.
The weapons need to be just right. They should have high killtimes, but very difficult to get the maximum killtime from. Especially at long range. In general, the average killtime outside of the intended range should be much longer than the average killtime. This can be achieved in multiple ways. A good example is the BR from older games: The spread greatly decreases the number of bullets that hit per shot at longer ranges. Another example is Reach’s bloom, though that is a random mechanic that should be avoided. A third example is bullet travel time, where you need to lead your shot for longer ranges, though that has a poor performance in laggy conditions.
I suggest an 11 bullet, 4 shot kill BR that has a large spread in time and minimal, fixed recoil. That way, the bullets come out slower and your aim needs to be on target for a longer time. And if you want the 4 shot, you should still aim slightly below the head. Much like the LR behaves right now, except with a perfectly fixed spread. If the maps are very open by design, the BR can be nerfed to compensate, with a short bullet travel time of 1-2 frames. (compared to 3 frames in Halo 3. This is viable because of the dedicated servers and the smaller delay) It would also have a short red reticle range. This is a small but important change from H4’s BR.
The DMR would have a similar behavior as H4’s DMR, but with a slightly slower firerate and a smaller reticle. If the maps are very open, again this should be compensated by a 1-2 frame bullet travel time. The red reticle range would be longer than the BR’s.
Novel suggestion: In order to increase skillgap and add depth to the game, perhaps headshots on shielded opponents with headshot weapons should deal some bonus damage as well? That way landing only headshots with the BR could turn it into a 3-shot kill, but doing it would be very difficult and make it a rare occasion.
TL;DR: The developers should play around with the skillful mechanics for the weapons: Bullet travel time, spread over time, creating weapons that are difficult to achieve the maximum killtime with. This needs to be done in conjunction with map design; open maps require less effective long range weapons and vice versa.