I started looking at offerings from Atlas Copco and Chicago Pneumatic, and as far the joules rating goes, the Chicago Pneumatic CP7150 is rated at 9.5 joules, which makes it the most powerful .401 air hammer/riveter between CP and Atlas Copco (Copco's top 4x riveter is 8.0 joules). Seems too good to be true for the price of the CP ($65).
I also emialed Snap On what the joules rating of the PH3050 is, and they said they had no rating (probably don't want to share it?). Not too many other manufacturers have actual joules ratings on them.
Anyway, the CP7150 is advertised as being able to do 1/4" cold steel rivets and 5/16 cold rivets (guessing this second rating refers to aluminum rivets?). Does that make it a 4x or a 5x since it can do 5/16" rivets? Or are rivet guns rated by the max steel rivet capacity? I know pretty much all 5x rivets are .498, but I've also never seen a 5/16" claim on any .401 hammer. CP doesn't really advertise the 7150 as a riveter either, but they do include its rivet capabilities in the spec sheets.
http://www.cp.com/usen/whatwedo/powertools/ecatalogue/?family=9&filterby=all
Edit: Did some research and it looks like 4x rivet guns are rated at 3/16" steel rivets and 1/4" aluminum rivets (which is where the rating comes from, 1/4"=4/16", so that would be a 4x rivet gun).
That makes the CP7150 a .401 shank 5x rivet gun. That means it's the weakest 5x rivet gun on market (by at least 2-4 joules, or 15-38% weaker than other 5x guns). All other 5x guns, however, are .498 shank. The CP7150 is a .401 shank, which quite possibly could make it the most powerful .401 air hammer available?
Feel free to correct me if I'm understanding all of this wrong! Rivet guns and air hammers aren't my speciality....