The 03, 04, and 05 prostocks are alike performance wise. To my knowledge they all run the same lower tube internals. the bolt and back block design on the 05 is different though. The 5 came with a delrin bolt and has a "p-block" design for the back block. while using the stock back block you can only use evo length bolts. Main difference is milling and weight. Wgp makes great products for the price. Also throwing some new springs in the stock internals (maddman spring kit) can give you some really great performance out of a wgp
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