Help BT4 not getting up to speed
Hi Guys need some help for a freind. He has a BT4 that just refuses to get up to 300 fps, we have pollished the internals replaced orings put in a heavy velocity spring and shimmed it so the spring does not slide back into the stock, he is running hpa not co2. I did measure the stock barrel and was very surprised to see how big it was .710. Any ideas?