Who is the guru of making carbines run?

Well, this was the first time I've seen that it should be larger than 10-1/4". So... But the man's opinion does carry some weight if he was an armorer. Yet if a lot of peoples carbines are working and their spring is 10-1/4", it's probably nothing to worry about. But all I know is when I tore mine down, it was only 10. It was a long time ago, but I'm pretty sure I ordered it from fulton. And it didn't see so many rounds that it should be worn down. Not to my recollection.

I have always tried to keep a recoil spring that was 10 1/4", but no shorter than 10 1/8". I had a 9 7/8" spring that worked OK on Aguilla (underpowered) ammo, but would not feed from the top of a 30 rounder.. In an AR15, the carbines and M4s are vary dependent on correct recoil springs. I generally get a USGI M4 spring NIW when I can find one for CAR-15 builds.

I'm tearing My Dad's carbine down now and his recoil spring was only 9-3/4". Ugh. Well, at least I know this might be a good reason why it wasn't running well. Now, I think I'll order some this week.