PTR91's are made on a combination of HK and new machinery. They bill themselves as having better barrels than HK. I think not, HK used hammer forged barrels that were tough as nails. Heavier and thicker does not mean better.
You need to get an understanding of the roller delayed locking system. There is no gas system to foul but there is a delicate balance inherent to the design. Look up "bolt gap" and "false gap". Get a set of oversize rollers from robertrtg.com. Someday they will be needed.
Chamber flutes are of the most misunderstood aspects of the system. The flutes permit gasses to escape along the body of the case. The case floats on a cushion of gas. HK flutes the chambers quite deeply. This is a characteristic that balances with the thick case walls of the 7.62 X 51 NATO spec ammo. It is also a feature that makes them unsuitable for many .308 loads. PTR has made barrels with a wide variety of flute depths. The deeply fluted ones act like HK's and only like the NATO ammo. The shallowly fluted ones have a definite bias toward US commercial .308 and do not like the NATO ammo.
CLEAN THE CHAMBER LIKE CRAZY. The key to reliability in these rifles is having a clean chamber.
I would start with NATO spec ammo and see if that performs well. If not (and the chamber is clean) move to .308 ammo. It will likely have a definite preference.
Get a stuck case removal tool. You will eventually need one if you shoot .308. Not a big deal, tool is affordable and easy to use.
Steel case ammo seems to work pretty well on most of these rifles. I prefer Silver Bear as it does not gum up the flutes like lacquer or polymer coated cases can. If you use lacquered cases, give the chamber flutes extra TLC (gasoline cuts the lacquer well).
Stock up on the Aluminum magazines while they are sill affordable. They were as cheap as $1 at one time. $5 is more common now. When they are gone, they are gone.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball