I agree w/ build your own. Order a Seekins SP10 builders kit and then start figuring out what components you prefer such as trigger, grip, barrel, gas block etc.
Note of caution as others have eluded to, don’t Frankenstein the gun. You will likely have problems such as feeding or extraction issues. Proof specifically mentions their barrels are headspaced to JP bolts. Sometimes something seemingly minor makes a huge difference. Springs and buffers you can swap around easily so not as big of a deal. I use Tubbs flat spring, KAK 5.6oz buffer and Superlative AGB and my .308 SP10 runs everything.
Also, just find one in the PX if you don’t know what you really like yet or want it now. Good luck.
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