The art of 'infusing' will be your best friend when it comes to leveling up your gear in Destiny 2. Much like how it worked in the original, it comes with a few rules; like only being able to infuse a higher level piece of equipment into a lower one.
But why is that? Wouldn't it make more sense to feed a higher level item with the shards of many, weaker weapons? Well sure... but that's not how it works. Infusing means to feed a high-level item to a lower one, making what was old new again - in a sense.
So what about that time you tried to feed a piece of 270 gear to your 266 rocket launcher only to be met with the words 'Base power too low'? It isn't some fundamental design flaw or glitch in the system at all. Take a look at that weapon you're trying to munch down, and you'll likely find the solution - there's a weapon mod in there that's responsible for bumping up its Light level.
If a weapon or piece of armor has been modded to sit at a light level higher than the Infusion target, it's destined to fail. The keyword here is 'base'; meaning the item's unaugmented level would have sat below the level of the item you're trying to infuse. It's as simple as that!
So next time you're melting down Legendary and Exotic items to gather the shards needed for an infusion spree, make sure your plans aren't about to the thwarted by a +5 weapon mod waiting to disappoint you.