1st shot of my 1" groups thrown everytime

[crowminator]

Hey everyone,
I have an AAC 7.62SD suppressor mounted on my M1S upper in 300/221 Fireball. Everytime I shoot subsonics the first round consistantly lands 5 inches low and 5.5 inches to the left. I can shoot the same bullets supersonic and this does not happen, with or without the suppressor.I am shooting sierra 180 grain bullets in a 1" six shot group at 100yds not counting the flyer. Today I tried some grease in the blast chamber and got the same results with the first shot being a flyer. I have fired several hundred 110gr and 125 gr supersonic bullets with the suppressor attatched with no problems. Has anyone ever seen this before?

[Pitt300]

I shoot the exact same combination & have not had this problem.

[too_many_hobbies]

I run a 762SD on an Oly upper with 16" barrel and shoot only subsonics. I was experiencing the same problem. First shot of a 5 shot group would chrono about 70 fps slower and be 3 to 4 inches lower than next 4 shots. It was obvious that I was doing something right because the extreme spread of the last 4 shots would often be less than 10 fps and group less than an inch at 100 yards. I would reload 5 more and return to my range and the same first shot low and slow would happen again. This happened with and without the suppressor.

I was new to reloading and scratched my head for a while on this one, but finally solved my problem. It turned out to be as simple as screwing the die down a half turn to set the shoulder back a little. All is well now...

Edited to add: If your problem is not headspace as mine was, I bet it is still a tolerance that is changing with temperature as something expands. I have found that subsonic ammo can be really finicky to small changes in reloading dimensions.