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Things like piston head bearings, spring guides, compression leaks, hop rubbers, barrel lengths and barrel bores can affect the fps of a given rifle.
If you know for a fact a given spring is say 350fps under ideal circumstance (like the ratings on the ASC spring chart), then you're golden because the fps can only go down from there, it's rare to go 5-10fps over.
But if your spring is 350fps in a given gun, and you don't actually know what the spring is rated to, then that spring could push anywhere from 300 to 450fps in another gun.
But if it's a stock spring, then you've got a very reliable 5-15fps window to work with. Like if it's a classic army stock spring, they're all within 310-330fps
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