After doing some research on the straight blow black systems, I've learned that a 5.5 oz buffer is one of the best reliable systems you can run on a AR9. I have a Blitzkrieg Components / CTS 9mm AR-15 AKTIVE Buffer that is 5.5 oz on one of my 9's and a 8oz SS buffer on another, and have battery issues with the 8oz same BCG on booth. I have some more range testing to do but if 124 grain keeps failing I will try a spare H3 that I have with two dollars in quarters, until I can get my hands on a hydrolic buffer.

H buffers came around to address cycling issues in carbines. Colt used a heavier buffer to improve reliability.
Different calibers and gas systems required different buffers.

Colt started having bolt bounce issue on full auto with the socom profile barrels.

I have two 9mm blow back PCC's one is a QC10 glock and the other is Colt style CMMG MK9....both are running perfect with 5.5 RRA buffers and standard CAR springs...……….that said, I should only 115 ball most of the time, some 124 but with no issues as well...…….That QC10 is the most reliable firearm I believe Ive ever owned. never a hiccup of any kind...…..thousands of rounds down range in the last three years...…...the LRBHO sucks and I removed it awhile back.