Didn't think too much of it at the time, but got to thinking today.  I was at the range two weeks ago, and one of my friends was shooting my UPS .45 and experienced a FTF.  I wasn't paying attention at the time, as I was doing my own shooting.  He told me what happened, and I told him to rack the slide and try firing the next round.  Racked the gun, and the next round fired.  I've had no problems with the gun myself, and the gun fired another 80-100 by my friend without a hitch.  The gun has maybe 300 rounds through it.  I thought maybe he limp-wristed it, but remembered when he racked the slide a live round popped out with no dent in the primer.  Any clue what happened?

P.S.  The ammo was Federal AE 230gr. FMJ

What was the weather like when you were shooting?  If it was cold and possibly even if it wasnt, natural gun oils can gum up the firing pin channel.  Take the slide off, spay the entire thing with aerosol cleaner being sure to get the firing pin.  Then relube with synthetic oil.

I have seen with a friends full size USP 9mm. Shooter was new to autos. Shooter put magazine in pistol and pulled back slide and let it ride slowly forward (a big no no). The pistol failed to fire. I believe the round was not fully chambered and the slide was not all the way forward. This might be the same in your case.

It was pretty cold, about 20 degrees and windy.  The pistol always had the slide locked back when it was unloaded, and I tought him to use the slide release after inserting a loaded mag, so I'm pretty sure he hit the slide release, which would put the gun in battery.  

What kind of synthetic oil do you recomend?

Quoted: ...The gun has maybe 300 rounds through it...

let it break in.
mine had a single ftf in the first 300 rounds, but now i havent had one since 1000 rounds later.

It may have been a bad caseing.

My friend's p220 had a simular problem, but I noticed the slide wasn't all the way forward. As I always do, I took the unfired rnd and stuck it in my pocket to be inspected later. When we were cleaning our equipment later on, I stuck the round in my friends USP .45F barrel (that was by itself, not in gun) and it only fit 1/2 way in. Didn't look damaged or buldged or anything. .  just must have been wider in the center or something. . . .

-Justin

Have you been dry firing it a lot lately.  HK's are prone to breaking their firing pins with dry firing.  However they still shoot (sort-of) with a broken firing pin.  This MAY be your problem.

I have a USP 45acp Compact S/S & it's never failed to fire nor feed. I would bet on the ammo.

I have an HK USP Tactical and in two years of shooting it has yet to jam.  No kidding, at least a dozen guys and gals have tried it with all kinds of ammo.  No jams ever, the only thing I own that has ever done it.