The arm parts seem to be just a tad too wide to fit in without having parts slide around or, in the case of the bicep, just not fit at all. I did find a really easy fix though.
[url]http://i.imgur.com/nK3NlgPl.jpg[/URL]
I've not seen many people talk about or show adding open cell foam lining like this, so I don't know if it's just so common that everyone does it or if NOBODY thinks about it. But anyone else out there building Ultron, if your arms are length wise perfectly scaled but are a bit too wide or loose, try this. These fit so snug on the wearer that I don't need to add in an elbow joint.