I have a usp tac 45 with a KAC suppressor on it, which leads me to my question.  My buddy saw mine and really liked it.  He was offered a good deal on a full size usp and bought it.  He now wants to get a threaded barrel for it so he can get a suppressor.  Will the barrel from the usp tac fit the standard full size usp?  Or do you recommend some other aftermarket barrel?  Where can he order one from?

It won't work, the barrel lugs are different. Try Google. There are several barrel makers who might make what you're looking for.

Try Jarvis....I have one made for my USP .45 FS and love it.
Best of all, you can order it with the threads you want to match your can.
Standard rifling as well so lead is Ok through it....but watch that through a can.

IM/PM me if you want more details.

Echo

Thanks guys, I will tell him to give Jarvis a call.
I have one of their barrels on my 1911 threaded to take the KAC can from my tac.  It is perfect.

You're wrong, it will work.

Been shooting my USP .45 with a USP.45 Tactical barrel in it for thousands of rounds.  It is exactly the same spec.

That is nice to know.  I wasn't quite following why someone said it wouldn't work.  I had always thought they were almost the same pistol.  At any rate my buddy is ready to start looking into suppressors for it, so I will start getting him some prices.  I can get him the hk tactical barrel at cost and much quicker than one from Jarvis, so he will be happy about that.

Recoil spring, barrel length, and trigger are the only differences.

But a standard USP .45 with a tactical barrel will run great, with or without a suppressor.

The USP Tactical barrel is a direct drop in for a USP .45 caliber pistol.  Where that one guy came up with the barrels being different in the lug section is anyone's guess but it is totally not true.
You can order a USP Tactical barrel from Midway and drop it right into a USP .45 pistol.  Just make sure there are no burrs around the inside, outside, or in the slide's barrel opening.  Run a cotton ball around these areas and if any cotton snags, polish with nothing more coarse that Crocus cloth and do not use power tools.
While HK does list a different recoil spring for the Tactical, hundreds of people have run the standard recoil spring without any problems.  If you were to run into functioning problems with the can then try the Tactical recoil spring but I'll bet you won't.

Jarvis makes a good barrel and you can shoot cast with them, but if you want a HK barrel you can get them from HK or Midway.  Personally, unless you are hung up on keeping the pistol 100% HK, I'd go with the Jarvis barrel.