They have a unique floating chamber that allows alternating between shorts, Longs and Long Rifle. I'm willing to bet it would cycle sub-sonics.
I'm sure that KMS is right.
A friend had a Rem 550 (the predecessor to the 550-1 model) that I used to borrow and shoot a lot.
All I had to do was to keep that floating chamber clean and it shot anything including subsonic loads.
I was never able to by my friend's Rem 550 so perhaps it is time I got the new model for myself!
John
