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I have an MP5 with the only velocity upgrade being a Prometheus 110 spring (stock spring guide, 229mm barrel, etc.), and I get pretty consistant 365fps readings from it. Which is pretty decent considering the m/s rating to fps is about 361fps with 0.20g BBs. So it'd be higher with a longer barrel. BUT, I have a used Prometheus 120 spring I haven't installed yet, and apparently it put out 370fps in a TM M-14, so I don't know what's up with it.
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