556 Mags

So long as they are not pushed beyond their design constraints, springs should not wear out or "take a set" from being compressed. Springs wear and lose tension from use.
 
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20 rounders I think the best are still NHMTG

30 rounders Okay Surefeed, Lancer and a plethora of USGI manufacturers out there.

P.S. - Springs only wear during movement, not from being compressed. Be more worried about follower construction.