I appreciated my HK91 for a long time. I had no truck with "poodle shooters," we all knew that real men used .308. Wondered what all the ruckus was about at one early three gun competition when the .300 'whisper" AR15's were disallowed. Good riddance.
Then, I was young, knew all I needed to know about guns because we read and talked about it among ourselves, and had heard all we needed to know about the M16.
I had that HK for 18 years hunting with it, and also served in the Reserves, being trained on the M16, A1, and A2, among others. I sold the HK because the comparison, even subconsciously, began to be obvious. The .30 battle rifles as a class are ALL heavy, bulky, and have a lot of kick.
Once I retired from the Reserves, soak time. Time on the internet. Time researching the M16. Like Waco, it wasn't all that it was said to be. When you keep searching for the facts, they will come out.
.30 cal battle rifles have a place yet today, but not in the hands of the bulk of soldiers. They have issues in comparison, HK, M1, FNFAL, whatever. That is why they are all quickly approaching the date where they are C&R's - they went out of service decades ago because they don't do the job as well as the intermediate MSR's.
It's why the M16 holds the top places in Service Rifle competition. And many cost half as the gunsmithing isn't nearly as complicated.
For the soldier, I found I could go all day in full battle rattle and the M16 wasn't the noticeable part. Hunting light with the HK, it was the focus of the work. A heavy gun with significant recoil does affect us more than we'd like to admit.
I sold the HK, and eventually built my own AR. I can't say I'm a fan of 5.56, I chose an alternate cartridge with 50% more downrange power. But, it's still an AR, and because of that, it has superior ergonomics.
If a .308 level rifle is what is wanted, then the AR10 would be the better choice all around. And one maker has recently come out with a "light" version that weighs another pound less than the conventional -10's. There are other choices that will be more user friendly than the PTR's.