I own three M1A rifles, one of them is a Bush rifle which is a Scout rifle without the tricky handguard located scope platform. All of them have been hand built by me.
I also own a PTR-91.
The M1A, when assembled correctly, is a precision shooting rifle.
Even the Bush rifle is more than capable of sub minute of angle accuracy at ranges to three hundred meters.
All M14 type rifles are also quirky in their own individual ways and they require more maintenance and care than any other semi automatic .308 caliber rifle I have ever worked with.
The reward is accuracy on par with most off the shelf bolt action rifles being offered on the market.
With proper care and regular maintenance and rebuild, and provided the cast receiver does not fail, this is a million round rifle design that shoots with precision accuracy.
The PTR-91, and any well assembled G3 variation for that matter, is a full power battle rifle caliber version of the Kalashnikov.
These rifles work well in all environments and shoot with more than sufficient accuracy for any type of general purpose use.
They are really basic designs, forget fancy stuff like last round bolt hold opens and repeatable, precision sights.
Even the 4X and 6X Hensolt(sic?) telescopes designed for these rifles are rugged but nothing really dramatic or precise as far as optical sighting aids.
Unless you are willing to spent scadloads of money for a hand built precision shooting version of this rifle style, the basic rifle design will not win the money pot in formal match shooting.
What this rifle does is work and work well, without extensive maintenance and tender care, and if you have access to rebuild facilities the weapons will last over fifty years of solid day to day use if they are rebuilt every 20,000 to 30,000 rounds.
With regular rebuilds and provided the receiver does not fail, this is a million round rifle design that works without a hiccup when needed.
Now you know the difference.