You seems to be talking about two different applications or uses. Epoxy putties consist of an epoxy adhesive base with metal filler. It's the epoxy that bonds; the filler is just that - filler. If I want to bond two surfaces, I'd use a good epoxy cement, not a putty, especially if there is no gap to fill.
Neither would be my first choice for bedding a rifle. JB Weld works, but it is a two part consumer product, relatively cheap for a reason. I used Devcon titanium to bed a rifle - once. It is hellishly expensive, and I found its working time to be short, shorter than other similar products I had used. For the cost it didn't seem to provide any particular advantage, although volume for volume, titanium is lighter. It still seems fairly heavy to me.
For routine adhesive jobs I use West Systems T-88. If I ever bed another rifle I'll probably use Marine-Tex grey or Brownell's steel bed. If I want to customize the consistency and add strength, I still like traditional Acraglas. Acraglas also has the advantage of hardening well even in very thin coats, something not all epoxies do well.
Paul
