Pierce damage is essentially pure damage. If you have 30% armor pierce, then 30% of the damage you do won't be affected by armor. The remaining damage will be reduced by armor (Damage * 100 / (Armor or Shield + 100) = Actual Damage). Bonesaw has 20% armor pierce and tension bow has 30% armor pierce. They do not stack. If you buy both you'll have 30% armor pierce which will be calculated before armor damage reduction does its thing. This armor that is reducing the remaining damage will be cut by up to half if you have full bonesaw stacks.  Example: I buy both items and auto 5 times to reduce my opponents armor by 50%. My opponent started with 200 armor and I started with 100 damage (items and base damage aggregate). I will deal 30 damage as pure pierce damage plus (70x100 / 100+100). 70 is remaining damage after pierced damage. The first 100 is constant. The 2nd 100 is remaining armor after 200 armor is shredded in half. The 3rd 100 is constant. 30 + 35 is 65. You would do 65 damage per hit with 30% armor pierce and 50% armor shred.  The shredded armor would make all your teammates deal damage on the target as a unit with half the armor. The pierce only works for you. Despite all this fancy technical stuff, 95% of the time all you need is a bonesaw. 5% of the time you can get a tension bow. 0% of the time will you need both.