The M118 LR/SR PMAGs are great and my preferred because they work in every single build I've done but they may not be a good option for the OP whose looking for maximum COAL. Aluminum or stainless steel mags usually have the polymer mags beat in this regard. I've measured KAC SR25 mags circa 2011ish that were 2.885" to 2.890" COAL.
For polymer/steel hybrid the Lancer L7AWM had if I remember correctly 2.850" (was definitely longer than the M118 LR/SR) but my experience with the ones I purchased around 2016 has been negative. They work for plenty of people but didn't work for any of my 308 builds.
@Jimbo1 If you're not chasing lands as the throat erodes or trying to jam bullets, the M118 LR/SR PMAGs are a great cost effective choice. Every single barrel I've measured out with AMAX, ELD-M, ELD-X, GMX, SMK, TMK, GameKing from 168gr to 178gr had resulted in a COAL that never exceeded 2.820". Potentially negative - they are taller than 20rd mags.