The 300 is just their attack level. Their damage is measured by impact. That's why people still use guns like fatebringer on dlc stuff like crota. Even though it is only 300 attack, the impact is still killer and still plenty enough to wreck some hive. 300 just means the weapon can't do 100% its own potential damage with higher level enemies. But if it is already more powerful than most dlc guns in terms of impact, it still doesn't matter, it will likely either come out be just as good or in some cases(like fatebringer) better than dlc guns. Which is why I held on to my high lord fixer. It's like a fatebringer without the arc damage. Add in its explosive rounds, it wrecks.