Measure the length of the buffer tubes internally and springs. There were some proprietary lengths out there at one time, I ended up with a rifle cheap that wouldn't run. Turns out that the carbine buffer tube was 3/8" short.
Carbine: 7” tube length
10 3/4” spring
38 coils on spring
Rifle: 9 5/8” tube length
12 1/2” spring
45 coils on spring
