Tubb flat wire buffer springs.

[SDW]

I had one in my rifle lower for a while. Worked fine with 5.56. But it would not cycle 6.5G Hornady Black reliably in my Grendel upper. Which, by the way, is a 20" rifle gas setup. Non-adj gas block and standard M16 bolt carrier. So I had to swap in a standard rifle spring. Worked fine with the standard spring.

Might work fine in other people's rifles though. And I'll keep it around to try again once the upper is a little more broken in.