It is for carbine length buffer tubes.
According to Strike Industries, this buffer spring is "Compatible with all standard AR10/SR25 receiver extension lengths"... quote taken directly off of their website. So, not just carbine length, but ALL standard length buffer tubes.
King Kong could not cram this spring into a carbine buffer tube!
Tried many many times but this thing will not go into a carbine buffer tube, way too long. Packaging states: "compatible with all standard AR10/SR25 receiver extension lengths". There is just no way - without bending it out of shape or cutting it off, this thing is going in to a carbine length buffer tube! I Will return and try something else.
Sorry Jonathan D. from IL. but you are wrong, not for a carbine tube, stick to being a gear expert not a gun expert.