The answer is that it depends on a few different things.
1. There are lot of FEG PP, PPK and PPK/S clones out there in both civilian and military configurations, and there were a lot of minor variations in the military models.
At one extreme you have pistols built from surplus parts that sometimes mix components from different military models. CAI has imported a lot of mil surplus FEG pistols and they are one of the worst offenders when it comes to mixing parts.
At the other extreme you have some very nicely made commercial pistols that rival the Manuhrin and Walther PPs and PPK/S in fit and finish.
2. The military pistols were primarily designed for the 9x18mm Makarov round and most of them have aluminum frames with a fair amount of curve in the back strap, while the commercial pistols have steel frames with a fairly straight back strap.
The FEG "AP-9S" is as PP sized pistol in either 9mm Mak or 9mm Kurz (.380 ACP) with a steel frame.
The FEG "APK-9S is a shorter slide PPK/S sized pistol, again in 9mm Mak or 9mm Kurz, again with a steel frame.
The FEG AP-7S and APK-7S follow the same nomenclature rules, the "7" just refers to the 7.65mm Browning (.32 ACP) caliber.
3. Almost everyone gets the names wrong.
The "PA 63" is a common military variant, and the military pistols often have the "PA" designation, while the civilian pistols use the "AP" designation. But everyone seems to call them PA 63s when a lot of them are not.
4. There is a fair amount of variation in the civilian/commercial models even within the above models.
FEG was very accommodating and they would make the pistol to the specifications of the importer.
At one extreme you have the Interarms marked pistols that Interarms ordered to sell along side and for awhile in place of the PP and PPK/S pistols after Manuhrin stopped making them, but before Walther established US importers for it's PP series pistols.
The fit and finish on the Interarms pistols is very good indeed and they owe no apologies to anyone.
On the other hand, companies like Tennessee Guns Inc, also imported them, but with a lower level of fit and finish. They are not bad guns functionally speaking, but they are not in the same class as the Interarms pistols.
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Below you'll see (top to bottom):
Interarms imported AP-9S;
TGI imported APK-7S; and
a pair of Manchurian made Walther PPs.
Here you can see some of the differences between the AP-9S and the Walther PP.
The FEG pistols use a slightly larger grip frame, which was sone to accommodate the 9x18 round both in terms of length, but also in terms of adding a bot more mass to make the recoil more acceptable. In the US the 9x18 and 9x17 round are loaded to about the same velocity, but in Europe the 9x18 had another 100-200 fps more velocity. The slides on the FEG pistols are also slightly heavier to deal with the greater recoil of the 9x18 round, and both the slide and frame dimensions are retained on the .380 ACP and 32 ACP models.
Below is an Interams imported APK-9S above a TGI imported APK-7S. You'll note the less polished finish and the slightly blockier lines and wavier flats on the slide of the TGI pistol:
Here you will notice the lack of mill work on top of the sight rib on the TGI pistol compared to the 7 line antiglare treatment on the Interarms pistol.
One of the other noticeable difference in finish is the rear of the slide. On the TGI pistol it's left very sharp - sharp enough to draw blood - where it's been rounded and polished on the Interarms pistol.
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In terms of performance, the steel frame FEG AP and APK pistols are very reliable - more so than their PP and PPK/S counterparts, but they are slightly less accurate.
FEG uses slightly larger chamber and throat dimensions than the Walther pistols and that results in greater reliability, particularly with cast bullets, but it slightly reduces the muzzle velocity and the accuracy. Still, an FEG AP or APK has great accuracy for a pocket pistol thanks to the fixed barrel design and it will shoot far better than most of the goobers who pick it up.
The AP-9S, APK-9S pistols and PA-63 pistols all use the same magazine, and it's the same magazine for both 9mm Mak and .380 ACP. The AP7 and APK-7S also share the same magazine but they hold one more round and have one more witness hole, along with slightly narrower feed lips.