Greetings! I am working through the "Revolver" modeling and, for the life of me, I can't figure out what is causing the pinching I'm seeing when I apply the Subdivision Surface modifier. Hopefully the pictures that demonstrate my issue upload correctly, so this makes sense. Any assistance here would be great because I'm stuck. 
Hi rrobdomenech ,
That might be, because you have 'double Vertices', or inconsistent Normals. Let's fix those: Select everything (with A), then press M > Merge > By Distance, immediately (with everything still selected!) followed by SHIFT+N.
Most likely that'll fix it, but if not, there might be some inner geometry (Faces, Edges that aren't supposed to be there...something like this:
,
but I don't think that is the case here...
Also sometimes you try to merge by distance, so you select everything with A, then M and merge by distance but nothing happens. Try checking that vertices are for sure on top of one another:
This happens to me sometimes, that I try to merge and the distance is not enough because the verts are not occupying the same space. Try checking for that as well, just in case.
It was doubled over vertices! Selecting all and merging by distance did the trick! Thank you all so much for the guidance.
