The SAR and AR/ARM recoil assembly is the same length. Likely the same spring rate too. I just measured my AR and SAR assemblies before posting.
I have not had the chance to play with a micro yet.
Since the receivers and carriers are the same in the SAR/AR/ARM it makes sense that the recoil ass lies will be the same length. The only difference between a SAR and a AR/ARM is the barrel length, gas tube length and piston length. The rest is exactly the same.