Identifying a stock spring?

[LzChase]

I need to find out what spring my rifle uses (Cyma 702 M24) but I can't find it anywhere. Is there a way to identify the spring? I need a slightly weaker spring as 490 FPS won't be needed. The thing is M140 to M150 springs which seem to be used to give such fps are found in upgrade kits, which means the stock spring must be weaker. Again I need a relieable way of identifying springs.

[Lozart]

Reliable spring testing? https://www.ametektest.com/materials/spring-testing-solutions

 

Actual, real world spring testing? Buy a chrono and get used to swapping out springs. Unfortunately, an M140 spring from one manufacturer can have different gauges of wire, different tempering, different materials etc so there really isn't any hard and fast rule that you can apply to all of them to tell you what your spring is.

 

What FPS are you getting with the stock spring?

I'm pretty sure these are supposed to do 485-500FPS out of the box so you could just cock it and leave it that way for a week. Depends on how low you want the FPS to be.

[Cr0-Magnon]

Especially if you are working on a bolty, access to a chrono will make your life a lot easier. As mentioned above, spring power varies from manufacturer to manufacturer and even from spring to spring (by the same manufacturer) in some cases. For one of my AEG's I was getting 325fps on a M115, so an M120 should have given me 340fps. However buying a Nuprol M120 put me over 400.

[Hudson]

It's going to differ from all sorts of variables, so there's no X=y when it comes to springs unfortunately, there's only telling what it'll do in your specific set up.

 

I would try an M140 or M130 to start with and go from there.

 

As mentioned above, buy a chrono so you can test it at home, and also chrono on whatever weight ammo you're going to be using and base the power output on that, you'll likely find that due to joule creep, you'll get a higher power out of those, and your power on 0.20s will be lower.

[Dratsab]

If you have access to some digital calipers and have a few springs laying around this might be a quicker easier way to tell.

 

Different springs will yield different FPS in different guns so it's not always a reliable way to tell. I validated this guy's findings on a few springs I have laying around (still in packaging) and so far so good.

 

Have a little read:

 

[ak2m4]

@Dratsab a guy in the US sells a basic spring tester that uses a luggage scale, it works but isn't hugely reliable.  A real spring tester that will measure the N/mm costs a couple hundred quid.  Length, coil number and wire thickness are a good way of estimating, of course different brands are based on different markets where they play with different FPS so there's no standard measuring scale.  The closest you can get is having someone measure the Newton force at the stage in which it's released in the gearbox.  I've spent a fortune getting a spring factory to working this out, send me samples so I can test etc...Hopefully have some new M100 samples arriving next couple of weeks although with the higher powers.