Personal - My personal first choice for caliber, inexpensive magazines and reliability, UGLY but funky-retro space-gun cool. Made for large individuals with large hands with heavy winter clothing on (short butt stock). My favorite trick with the HK91 here at the heretic abode is to add a bit of braided para cord to the charging handle, it makes a quick grab even with thick winter gloves.
Function - Reliable function second only to the AK, so reliable that it is often referred to as the "Kraut Kalashnokov" or the "AK of the West". I have personally fired my HK91's in sub-zero temps, and also sopping wet with slime in 100 plus temps with the only malfunctions based on ammo. Weak charges will stove pipe, and bullets over 190 grains can cause excessive recoil and shake the next bullet out of the magazine in front of the bolt causing a double fed or have a cartridge folded in half by the bolt face as it turns sideways.
Design flaws - No bolt hold open, poor ergonomics for small hands and arms, poor heavy trigger (without modification), fluted chamber engraves brass this does not make it un-reloadable contrary to stories you hear on the internet. The PTR91 “GI/Special edition” will have the odd brass chewer that dents the side of the case - an ejection port buffer fixes this and changes the direction of ejection to forward 2-3 O’clock.
Rifle has extremely violent fired case ejection - woe to anyone on the right of the shooter to catch one on the noggin! (I have a scar on my forehead from a friend who moved while shooting from a bench).
Felt recoil is higher with this delayed blow-back system.
Press-formed sheet steel construction is heavier than some other metals (and stronger) and because it is formed into shapes a large enough blow or crushing force to the steel can cause deformation that can lead to malfunctions, most knocks can be “ironed out” some with common tools and severe dents with special internal forms. With decades of military service including Africa, the design holds its own in even the worst of conditions and care, including possible dents.
There is some debate that states it is possible that the G3/HK91 was specifically designed to work with brass cased cartridges, and that firing steel cased ammunition can damage rifle parts and excessively wear the throat of the barrel. It is possible that if the steel case shows signs of excessive amounts of hot gas jetting past the flutes in the chamber and into the case shoulder this could erode the metal in that area. There was some indication that the German engineers actually designed the G3 rifle to handle not only full power cartridges but also cheep steel cased ammo with a very wide range of acceptable powder burn rates in the 4895 category.
Quality - While the construction of the frame is of relatively inexpensive precisely formed steel, contrary to internet ignorance "stamped steel" does not equate to low quality but rather inexpensive production, most car frames are made of formed steel, even the "muscle cars" of US production were of formed steel. Both the HK and PTR clone's internal parts are very high quality construction, interchangeable and examples of first world production. The last of the PTR rifles to arrive in the collection had a Rheinmetall AG bolt carrier!
Caliber/Ammunition - .308 an accurate and effective cartridge, one of my favorites, easy to reload, but is a large full power cartridge and some consider it excessive in recoil (this recoil is heightened in the HK system). .308/7.62 is one of the most popular cartridges for rifles in the US and high quality commercial and surplus ammunition is widely available. While on a related subject the HK91/G3 uses one of the best designed and reliable .308 box magazines ever produced, currently there are several high quality choices the older steel (heavy but very tough), aluminum, and then the polymer magazines (the only magazine from this company I would recommend and the Thermold magazines count for US compliance parts).
Use - Accuracy out of the box will be good to excellent (suffering somewhat from the heavy "drop proof" pull of the original trigger), the trigger can be adjusted or worked by companies like Williams Trigger Specialties (an excellent "set trigger" I use is available) and Bill Springfield. There are services available where the action can be stiffened and attachments such as the SuperTune to dial in loadings. The HK91 uses the least expensive detachable magazine available in the US market, there are steel, polymer, and aluminum magazines readily available. I am on my original locking rollers and have cooked off thousands of rounds (over 7k) on my current original HK made rifle. A must addition to this rifle on top of the para-cord handle is the Tac-Latch to replace the removed magazine release from the back of the magazine well (the Tac-Latch returns the rifle to the easy either-hand/two hand operated magazine release).
Current production - PTR is considered the highest quality clone in current production, repair parts are widely available, but HK stopped official production years ago forcing the existing HK produced rifles in circulation into the "collectable" market.
I consider this rifle a first and primary choice for a current MBR, high price is a possible concern with the true HK build collectable rifles, but when considering the inexpensive magazines, the total price can be lower than expected (I consider 8 a minimum of magazines per rifle). Because the Special Edition “GI” PTR91 clone is the least expensive .308 except the .308 AK from Saiga and the used magazines are so inexpensive it is difficult to find a better deal.