REF: Army TM 9-1005-319-23&P Nov 2008, Page 0021-10:
Check free length of action spring. RIFLE ONLY: The free length must be between 11 3/4 in. (29.85cm) minimum and 13 1/2 in. (34.29 cm) maximum. CARBINE ONLY: The free length must be between 10 1/16 in. (25.56 cm) minimum and 11 1/4 in. (28.58 cm) maximum. If measurements are notaccurate, replace. Do not attempt to adjust the length by stretching the action spring.
Your manufacture might be different, in making the jig I used the above.
If you have a 3D printer it is a cheap print. This is too long for a standard print bed, I used my Ender Extended 400×400 print surface for this one.
You can download the STL on Thiniverse
To help check the length of your buffer spring I created 2 versions of the AR15 Buffer Spring Measurement Jig; Rifle and Carbine length. The Center is minimum length and the other is maximum length. Your printer might be different so double check measurements after printing.
With Rifle Spring:
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