The most important thing to look for is the SNELL 2005 label. I have hundreds of racers using M series helmets in NHRA,IHRA,ORSCA ETC. It all depends on the sanctioning body and their rules.I just sold 15 M series helmets(and just as many SA rated) at a ORSCA Event last week and they all went through tech inspection and Orsca FOLLOWS ALL NHRA RULES.The best thing to do is ask your local track official for what they will allow and go with that,after all they will have the final say in if you need a M series or a SA Series.