I agree with Jackfish.I have some Buffalo Bore 350 Speer ammo and it is the same way with the small primer pocket and I here Garrett ammo is the same.Don't know for sure but I think so.

I also load the 550 Crater in my Guide gun with Winchester Brass and CCI primers and H-322 for 1500 fps.It is not an overload and the kick is more of a shotgun push to me than the snap of a 300 Win Mag or even a 405 Kodiak pushed to max.

Velocity from a chrono is a far more accurate means of pressures than the pressure ring but I also use it to see for myself and to date there is nothing concrete time after time to tell in my opinion.

I do know a guy who loads with H-335 and the 550 and uses the same amount of powder as you for approximately the same velocity.Your load should be just fine in my opinion.

Jayco