Looking to have an M4A1 build alongside my MK17. I got an offer to get a Classic Army Armalite M4. I’m not looking to do anything crazy in terms of upgrading, I just want it to last. So my question: Is Classic army a good upgrade platform? Are there any proprietary parts that would hinder the upgrade process? This gun would be more of a backup gun that I can rely on, so again, I’d just want a mediocre internal setup that will just last. No crazy ROF, No crazy trigger response. I just want to build a good all around M4. Thank for looking! Comment if you want to know more specifics!