A lot of the following text is tl;dr explanation about the history of the DMR, and how its mechanics have ruined Halo. If you want to get right down to the kicker just look at the bottom of this post.
The problem that game designers are facing these days is that they look towards the most popular series for inspiration too easily, if they need a new game mechanic, or some kind of “original feature” that will set their next release apart from the rest of the game series (ala Armour Abilities and Halo Reach), they don’t bother evolving the idea any further than they think they should.
Take precision weaponry in Halo Reach and Halo 4 for example - previously all precision weaponry were powerful, yes, but their mechanics balanced out their OP-ness (the Battle Rifle fired in bursts in Halo 3, making it near impossible to successfully cross map someone if they’re constantly strafing), but now with the DMR’s inclusion it is the perfect marksman weapon that doesn’t bother with physical drawbacks that the Battle Rifle had to endure.
It is widely known that the DMR is the easiest weapon to use out of the two that I have presented in this argument thus far, and that’s because each shot the DMR fires is a potential headshot, regardless of the bloom, thus that is why bloom failed in Halo Reach. It was designed to turn the spammer’s game into a coin-toss, but they always had that chance that the bloom would randomly net them a headshot.
This is why in Halo 4 they significantly reduced the bloom because the community complained about how the randomness reduced the actual skill required to use the weapon because as soon as two players disregard pacing their triggers, it turns into a screwy, unpredictable mess of a random-outcome match that we all know that Halo used to not be, and should not be ever again.
So now we’re stuck with a DMR that has the potential to acquire a headshot with every shot, but no longer has any of the drawbacks it used to have. It’s easier to use than the BR, is far more predictable, and has the perfect rate of fire to align a shot, fire, and repeat. But they didn’t reduce any of its power. If anything it has practically become a sniper rifle, seeing as the bloom in Halo 4 is almost completely negligible.
tl;dr;
Instead of having to rely on randomness to punish players who spam instead of pace their shots, why not make it so that there is a “switch”, or “cooldown” that disables the chance of hitting that “critical” shot by temporarily turning off its headshot capability beyond a certain stage. Let’s say we’ll be using the Halo Reach’s reticule/crosshair for the DMR - we all know what it looks like. Every single time we fire a DMR in Halo Reach, an inner-circle gets larger and larger until it pops outside of the larger, outer-circle.
This usually coincided with bloom, but we won’t be using bloom as the “balancing” feature of the DMR here, instead, so long as the inner-circle is within the outer-circle, and has not expanded to be bigger than the outer-circle, the player will always be able to fire a headshot-capable bullet.
Let’s say that we’re using this mechanic in this scenario. Player Red and Player Blue both have DMR’s. Player Red has played Halo for a long time, and plays extremely methodically. Player Blue is new to the game, fresh from Halo Reach. They meet face to face, both begin firing at each other at practically the same time, and Blue’s reaction being used to the coinflip mechanics of Reach’s DMR is to immidiately start spamming, hoping for that “jackpot” to net him a headshot. Red however is seasoned, and expertly paces his shots. By the time Blue has almost popped Red’s shields, his DMR is in “cooldown” phase, where the headshot capability is turned off temporarily, yet he continues spamming. Red however has paced his rate of fire, and has not yet gone into “cooldown” phase.
Red is now unshielded, but Blue cannot place the final headshot because his DMR has “overheated”, regardless if he keeps firing by the time he strips Red down to 4 bars of health, Red would have popped Blue’s shields, and successfully landed the final killing blow on Blue with a headshot.
There is no cointoss event. Red paced his shots, and won. If Blue meets Red and spams every time, and Red returns fire by pacing his shots, Red will always win. There is no dice roll. No random outcome.