Hammer heads have fins at the start of your barrel (basically a changeable section at the thread), allowing you to bore your barrel to the paint you are using. Some people swear by over boring others the opposite , so say your paint is .684 when you size it, people may use a bore of .692 known as over boring , or other May use a smaller back , for a more snug fit.
But tbh this can be quite pointless if using site supplied paint ,milked shield for example, as there paint is so cheap and nasty there is no consistency , as paint is a massive factor in accuracy.
I had an a5 about a 2 years ago , and I used a j&j ceramic barrel and the paint at delta force didn't even fit giving me barrel blocks as there paint was soo swollen
So, a hammerhead can greatly improve accuracy ( as will freak insert barrels or bored two piece barrels like shaft 3/4s or dye ultra lights ) if it is bored well to the paint, hope this helps mate