Well it might not be broken but I have a few ideas to make it more fun and a bit more balanced.
DMR
I personally dont have a problem with the DMR, its supposed to be powerful, its supposed to be the bridge between a BR and a Sniper Rifle. But after playing with both the DMR and BR, I see two problems 1 Maps which i will get to later, 2 the DMR’s lack of recoil. A Designated Marksman Rifle would be using a large round, and should probably function like an M14. If you pace your shots you can put a target down easily, but if you want to spray the gun using its quick fire rate, you will experience vertical climbing, since you will be firing faster then the gun can reset. This is in no way Blooming. The bullet will go where the cross-hair is pointing, but it will be up to you to fire accordingly and put the cross-hair where it needs to be. Cause its firing bullets and not lazers.
Power the same, Fire Rate the same, Recoil Vertical(similar to M14)
Boltshot
I use the boltshot a lot. Its a useful weapon, but understand that it will just as easily punish you for missing. But what I do to save ammo is Charge it and if I see my target run away on the Radar I will Y Y quickly to save ammo. Since a direct shot is most likely a kill (not always), how about forcing the player to fire the charged weapon before preforming another function. So when you pull back and hold the trigger long enough to fire the shotgun feature you are locked in.
Maps
I feel the maps in Halo 4 have a similar problem I felt reach had. There wasnt enough environmental diversity for each weapon to thrive in. Snipers thrive in very Long - long range, DMR long - mid range, BR mid - close range, AR close range, pistol close range, shotgun very close range. But if you have maps that are all either Long and Close range, with no in between for people to move through, the AR people cant attack the Snipers/DMR people. Because they are forced to walk across empty fields without cover to even try to get close. You pretty much sign your life away when you leave cover. And because of this lack of diversity the BR is somewhat useless. It cant match the DMR’s range, and the maps dont let the BR user to get close enough to fight back properly.
This reminds me of a Halo 2 Scenario if you are Familiar with Coagulation and Blue base. You could have a Sniper on Red Ridge over looking the map and sniping Blue Base. A person with a battle rifle couldn’t fight back, but they could take a teleporter into the Rocks, ducking and weaving from rock to rock until they were at the Snipers Door step, with a three shot burst knock at the door. While the Sniper had the superior long range weapon, the players had the environment to work with to bridge those gaps and get close range where the sniper isnt superior.
Both Halo 4 and Reach lacked the Map Design that could provide a balance to the weapons.
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I have a few other little complaints, like Rockets being dropped a lot in Dominion or a Connection preference which there is already a giant thread for, but over all the game is alright.
I wouldnt mind hearing solutions to other problems/issues that could be solved through minor tweaks not including Nerfing everything into a giant pillow fight.