While that approach will work okay for pressure testing most likely, you'll see a more representative grouping if you use 10-rd groups. If you haven't seen it, read this discussion of 10-rd groups over at AR15.com:missouriman said:Hello guys, Im planning to shoot 3 rounds of the 27gain and then clean the gun and then shoot the 27.5 gr and so on .with enough time in between each group for the barel to cool down.I think this way I should get a better tell tell result