That base surely fits like most every other base on an Omega or Encore and does leave a small gap from the barrel at the forward end. Just make sure your screws are tight - the base is likely just fine.

I'll add that B-Square DID make a base for the Encore/Omega that also had the gap but included a tensioning screw on that forward end. The screw had a brass tip and pressed against the barrel. I believe Burris also makes such a base for dovetail styled rings. That gap is normal.