Interesting. If the spring is more powerful perhaps you can go up over 150 bar on the regulator which would increase velocity and power. As you undoubtedly know, slug velocity and thus fpe tends to be a bit lower than pellets. 37 on slugs is probably equivalent to a bit over 40 with pellets (heavy ones). My P35 does not care for JSB 25.4s but your X might. But if you really want to shoot slugs that won't matter. Good Luck. My guess is the P15 spring is equivalent to the P35 spring.
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