Besides CMMG, ArmaLite, BCM and PSA midlengths have triangular end caps. Daniel Defense is unusual in that it uses round end caps for midlength. Almost everyone uses round end caps for carbine length gas.
Actually, that shape you refer to as a triangular end cap is actually just part of the handguard itself, rather than a cap. I still have the handguard off the rifle and took a couple pics of it separated, and joined together. Just for the fun of it, I also included a couple pics of the bolt that I just took now that there was some actual sunlight I could use. These pics look better.