I’ve experimented a good bit with springs, including on an intermediate gas system. What I have found is that increasing spring rate introduces stoppages to a functioning but lightly gassed gun. It also does not do as well as one might hope to tame a gassy beast. So there is little advantage, IMO.
I think that the optimal setup is a regular Colt spring, up to a Sprinco White or Hot White for Carbine buffers and Colt up to Tubbs SS for A5 or Rifle buffers. And tune gas to match, or buy a barrel that doesn’t suck. It seems that many barrels are ported to run steel case with an H3 and Sprinco Blue, because every dude on the internet is claiming that his setup is pure gold, so dudes are buying guns and throwing crazy shit in them. Its a snake eating its tail.
Sprinco springs rust. Fast. I get it, you want one because Mike Pannone likes them and they last forever. My Blue, Hot White, and White springs rusted significantly in about the lifespan of two Colt springs. So, they cost more and did not last longer. Yes, that rifle was abused somewhat. You guys in the desert can disregard, along with dudes that shoot in dry weather or at least dry and clean their guns immediately after use. Blue is too much, I’d choke down my gas if Blue runs well in adverse conditions. I liked the Hot White, a lot, because it goes into battery more vigorously, but that energy is coming out of the rearward half of the cycle.
Tubbs SS is too much in a Carbine sprung system. In an A5, the Tubbs is similar in strength bottomed out as a Colt spring, and a bit stronger in battery, as I’ve measured. So its not optimal as a solution, but it does work without adverse effects, in my A5 lower. I’m using a Tubbs SS on a dedicated suppressed rig, simply because it was a quick fix. I choked the gas down when I put the thing together, and it was still a little gassy. The spring works in this situation, but choking the gas back some more would 100% be the right answer. I’ve used other uppers, unsuppressed on this lower, and nothing crazy happened.