WHAT DOES FEDERAL GM150M PRIMERS MEAN?

They usually go bang for me and appear to be steady. I've only used their small pistol primers. I have utilised a good number of CCIs likewise, and when They're good, I still want the Feds.I've been reloading for over 35 years and have routinely switched between Winchester, CCI and Federal ( I invest in whatever is on sale) on a longtime loading wi