Although not marked as such, this is most likely one of the many Broomhandles imported from China in the 70s and 80s. The NS marked hammer does not belong to a frame with the serial number 158019 as your frame was made well before the Neue Safetie was invented. While upper/lower mismatches are not uncommon on US/Euro market guns, a mismatched hammer/frame is not at all normal. However, that is the kind of thing that can be expected from a Chinese market gun.
There is a lot of confusing conflicting information regarding these pistols.
There is (practically) no such thing as a "Chinese reproduction/knockoff" C96. Incredibly rare and desirable Shanxi Arsenal pistols notwithstanding, the overwhelming majority of C96 pistols imported from China are German manufactured guns. China developed an appetite for these pistols after the Treaty of Versailles forbade long-barreled pistols (resulting in the Bolo model Mauser). Germany dumped all of its full-size C96 pistols as quickly as it could. Many of these guns ended up in China, where the detachable stocks made them popular with various groups fighting for control of China. As China homogenized as a Communist state, those guns were collected and became military issue. They remained in service through WW2 and long after.
Throughout this service period, the guns were shot extensively using domestically produced ammunition that was both corrosive and lacking QC, subjected to terrible neglect and fell victim to lazy armorer practices. Parts from different guns got mixed up. From factory, C96s were precision pieces, hand-fitted by Mauser's expert gunsmiths. Swapping parts around willy nilly led to issues.
The C96 is a very reliable gun with a very strong action. However, the badly abused guns imported from China gave it a reputation for being persnickety or even dangerous.
There is also a myth that 7.62 Tokarev ammunition will damage or even blow up a C96. This is the result of a long game of telephone - in fact 7.62 Tokarev and 7.63 Mauser are exactly the same (the Russian naming scheme rounded down to .62, the Germans rounded up to .63; the basic load specs are identical. Broomhandles were popular in Russia, hence the choice of round for the Tokarev pistol.). While these cheap former Chinese C96s were widely available, also available was an enormous supply of surplus Norinco and later Czech 7.62x25mm ammunition. This ammunition was apparently loaded a little on the hot side and, by virtue of Communist QC, occasionally double charged.
Run that ammo in a PPSh and you're fine. Run it in a Tokarev and you'll probably be OK. Run it in an 80 year old Broomhandle built from a random assortment of parts, springs that are worn beyond usefulness and a bolt stop that's probably been on the brink of failure since well before it left China and you are asking for trouble. And so the legend of Broomhandle grenades was born.
Winchester white box 7.62 Tokarev is fine for a Broomhandle, although you may not necessarily get great accuracy out of it as the bullets are smaller than they are supposed to be.
OK, so where does that leave you? Well, if you paid over $1000 for it you overpaid. Otherwise, you did fine.
A slight bulge is probably not a problem by itself as long as the barrel is not cracked, but it does indicate that the gun has been subject to some pretty intense pressures. The proof pill for a C96 exceeds .357 Magnum chamber pressure. If something bulged the barrel, it was much hotter than a .357 Mag, so what did it do to your bolt stop?
I guarantee the springs that are in the gun are no good, so go ahead and get a Service Pak 1 from Wolff:
https://www.gunsprings.com/MAUSER/1896-1912 (BROOMHANDLE)/cID1/mID40/dID172
While you have the gun apart to replace the springs, inspect the bolt stop for any signs of peening or cracks. If this part is damaged you should replace it before shooting the gun, because it's all that's keeping the bolt from leaving the barrel extension and coming to say hello to your eyeball.
C96s are very accurate pistols even with worn rifling but with your bulged barrel I wouldn't expect an awful lot. The barrel on the C96 is machined as one piece with the rest of the upper receiver (can you imagine how much that would cost to make today?) but there are options for rebarrelling these. Most commonly, the barrel is cut off and the extension threaded for a Luger barrel.
Alternatively, you can replace the entire barrel extension, but that will not be cheap.
Also, the 8 round Tokarev clips will damage your C96’s clip guide over time. Proper C96 clips are not that expensive so don’t cheap out now.
