not very much. the best thing we have found is the bowjax limb supressor set-up. i don't shoot any supression on my tenpoint and have had no issue. compared to my vertical it sounds like a train horn but at 315 fps its not an issue.
Depends on the brand and model of the crossbow. Some of the Parkers you will notice a big difference. Many others you would be wasting your money on trying to quiet them down. Buy one set up from the factory with them on there already.
The new PSE Fox Fire has the string stops and it makes a huge difference and they have a kit available for the vibration dampners for your limbs as well.
Which Parkers? I think he has a Parker. He is also concerned about the power being redirected to the bow and causing failure. He just got it, so he may be able to add the factory style. He is also considering the D-I-Y string stops- he is very handy with tools.
Check the Cabelas website they have ad ons for them. The Cyclone model comes with it and does make a big difference. I think it is called the Parker red hot supressor kit for different Parker models. They do work on the Parkers very well!
Comment