Personally, I love the 7.62 NATO round. How much? Well, I own M1As, FALs, AR-10s a MAS-49/56 (one of the good ones thanks to careful research and examination before buying one) and a PTR-91 GI. While my personal favorite is the M1A as being the top performer in a battle rifle, the PTR isn't a bad rifle either. Mine was good to go out of the box and I got a pretty good deal from CDNN at $899 with 10 surplus mags and a sling, scope and mount of questionable origins but I didn't buy it for the extras other than the mags. Actually, I think they are selling the GI models for $799 right now so that's an even better deal but it doesn't come with a lot of the extras mine came with earlier this year.
Now while I already stated that the M1A is the top performer in my book, let's concentrate on the HK design for this post. They are accurate, reliable and built to take abuse. Not a bad deal in a battle rifle. However, there are a few things that can be improved with the PTR rifles.
First, the slim handguards aren't the best at turning back the heat. I put on a wide handguard on mine that does a better job of reducing the hot handguard problem as well as covering the hot trunion area of the receiver as well. The down side is that it's not as fast to actuate the cocking handle. The solution was an HK-21 machine gun cocking handle that is larger and easier to grasp. Works great with the wider handguards.
Next is the safety lever is next to impossible to operate without shifting your grip unless you have the thumb of an orangutan! The solution here is the extended safety lever from RTG firearms. It's not perfect and I'd rate it on par with a metric FAL safety lever that still falls short of the AR-10 selector lever or the safety tab of the M1A but still FAR better than the stock selector lever!
Let's face it, the magazine release on the HK91 style rifles to include the PTR SUCKS!!! Unless you have the index finger of the same aforementioned orangutan, you are going to be shifting your grip a LOT to get at the magazine release. This is due to the fact that the original magazine release similar to the M1A and FAL on the original G3 rifle had to be omitted in order to conform to the laws preventing a G3 full auto magazine release being used on semi-auto only rifles. The solution isn't what I would consider something for a home gunsmith to attempt but you can drill into the receiver with a small pin and bushing that will allow you to use the paddle magazine release tab as in the original design. I have plans on sending mine off to Bill Springfield to have this work done. Once you have this done, removing the magazine is no harder than a FAL or M1A.
Another issue is the sharper recoil of the HK when compared to other rifles. This is due to the delayed blowback design rather than the gas operation of other designs. The solution is either a heavy buffer like those offered by RTG firearms or even a German 2-stage heavy buffer available from RTG or HKParts. The heavier buffer slows down the bolt carrier slamming back and makes the recoil a little more in line with it's gas operated counterparts being a bit more gentle on the shoulder. Not a big deal if you are used to heavy calibers but still an improvement.
There is also the issue to most American shooters of the lack of a bolt hold open device that will lock the bolt open after the last round is fired. Sorry, but that's part of the design and there isn't a solution that I am aware of to counter this. This tends to be a European thing like the British L1A1, AK or FN FS2000 to name a couple designs that lack this feature. With the L1A1, a simple swap of a bolt catch is a fix (I did this with mine) but with designs like the HK91, AK or FS2000, it's just not in the cards. Sorry.
As far as the HK design being not "reloader friendly", there is some truth to it. Yes, the fluted chamber does leave some ugly marks on the brass and to be honest, I have only fired non-reloadable FN surplus ammo through mine and being Berdan primed, reloading hasn't been an issue. However, after cleaning a few pieces of fired brass, they make not look pretty but the chamber flutes are not something I would see being an issue to reloading. The biggest problem is in finding them again! Mine throws brass about 7 yards away in the 1-2 o'clock position. Again, the solution is from RTG in a brass deflector that prevents the case creases against the ejection port (mine hasn't had this problem) but also tends to deflect them downward and a lot easier to find. Something to consider if you ever do plan to reload.
I will disagree with ttarp in that I don't care for the sights. Yes, they are simple and rugged but you will either need the original tool or a pair of forceps and a screw driver to adjust them, particularly when it comes to your initial adjustments for elevation. They are a colossal PITA to adjust when zeroing but once zeroed, are pretty straight forward as long as you are not trying to adjust your dope for wind at longer ranges. Sort of like an AK or FAL, you set the range then hold off for wind conditions. Compare that to the iron sights on an M1A or AR-10 where you can dial in elevation AND windage, these sights come up short. I guess living in North Dakota where ranges are almost always long and wind isn't uncommon, I ding designs like the HK, FAL, AK and others in this regard.
All-in-all, the PRT-91 or HK91 design isn't all that bad as long as you can either make modifications like the safety lever, magazine release (my 2 biggest gripes with the design) and what not or can live with things like cruder sights, lack of a bolt hold open device or rigorous ejection the design displays. Oh, and as far as the weight, to quote "The Thirteenth Warrior", get stronger!
The price of the PTR-91 is VERY appealing for a battle rifle that has a very good heritage and would make a fine SHTF rifle. We look today at the AR-15 as being a very modular deign (and it is) but HK was making modular rifles in the form of the HK91 long before the AR-15 was even thought of when it comes to being modular.