New String Sleeve Silencers are available in packs of 4. They are made of the same quality material that is used on our most popular string silencer the ultra ll's. It's super soft material and durable enough to withstand thousands of shots. They weigh 16 grains a piece, you could use them alone or use them in combination with the ultra ll's. The ultra ll's weigh only 19 grains, so the combination of these together is the perfect balance for vibration control and weight on your string. Some people just want to add the string sleeves for the speed gain of 2 - 3 fps. You wouldn't need brass nocks if you use BowJax String Sleeve's.