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Mike the stock spring was heavy and smaller (about 6mm) compared to Nelson spring sets, with a matching dialler. I replace mine with a hammer and dialler that takes nelson/madman springs. The belsales REX is a good off the shelf kit - but you need to drill the thread out of the rear body for retaining screw. I have a few spares like hammers here that may work if you want to go the DIY route. Another usefull mod is drilling the body to allow the sear grub screw to be adjusted from outside. Early snipers had to be adjusted from bottom, which meant taking the grip frame off every time.