I always research and determine things for myself. Reference a stainless steel RSA (not just the rod) my opinion based on my recent experience with STAINLESS STEEL GUIDE RODS, INC. they do as advertised.
There's no question M&P Shields have a reputation for being hard to rack, especially locking the slide back via the slide lock. There
have been a number of posts about this.
First, other than being hard to rack, I've never had a problem with the RSA that came with my two Shields.
However, based on a poster's experience here/ found the replacement stainless steel spring easier to rack/ I ordered one form STAINLESS STEEL GUIDE RODS,inc and it slipped in easy and racked easier. A hundred rounds > flawless.
I've just a few minutes ago ordered a second one from them for my other Shield.
Poli Viejo