.660 is too tight for turkey loads in an 835 or 935. Believe me I know. I have a .660 I bought for card shooting events using light load #8 shot. I tried different turkey shells thru it and always got the classic blown pattern donut. Turkey chokes for the 835/935 range from .670 to .695. depending on the choke maker/design a .670 or .675 will give the tightest patterns. However some prefer a slightly larger more forgiving pattern and for this they choose .690 or .695. The current X-Factor choke Mossberg includes with their guns is .695.