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Avoid any PDI spring, they are very unpredictable these days. I've dealt with PDI 150% springs that are easy to install (6 3/8" long) and shoot around 350fps, all the way up to PDI 150% springs that were impossible to compress and install (7" long), that ended up putting out over 420fps. Never know what you will get, but know that your V2 mechbox will be in jeopardy (even a reinforced one) if you go that route.
For your setup, I'd suggest a spring along the lines of 110 or 120 (meters per second)rating. Systema, Guarder, Prometheus, etc. They are much more predictable in their manufacture, and will perform well. And keep in mind an inner barrel length increse will (unless "barrel suck", which in reality the inner barrel having more internal volume than the cylinder does, air pressure will drop in the barrel and create a small vacuum behind the BB before it exits....... not a common problem, just a tip) knock up your fps up. Case in point, my 365fps MP5SD got itself a 10cm longer stock inner barrel (229mm to 330mm), and my fps jumped up to 387fps.
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