Thanks Gus, You answered so many questions for me before I even knew what to ask.The problem comes in when we talk about the use of ALL 7.62mm ammo including poor quality and some surplus ammo VS GI or good U.S. made 7.62mm ammo. Folks on this forum use all types of 7.62mm ammo and that's where the problem comes in.
While I was a NM Armorer for the Marine Corps for some 23 years, three of the four types of ammo we used for NM M14's was 7.62mm. We chambered the rifles with tight NM chamber reamers and settled on minimum headspace of 1.631" to 1.632" as that ensured better function. Now, these were TARGET guns that were not used in combat conditions and especially not with POOR grade 7.62mm ammo.
The first two loads were Lake City NM in 7.62mm in both 168 gr. and 173 gr. loadings. (Now we have to remember that National Match Ammo is made with MUCH tighter specifications than even standard US GI 7.62mm ammo and WAY above poor grade or much surplus 7.62mm ammo.)
The third load was what we called "Mexican Match." I know we made this from 7.62mm ammo, but I confess I don't remember specifically which ammo. I know in some cases they just pulled the original bullets and replaced them with 168 gr. and 173 gr. NM bullets. In other cases, they pulled the original bullets and replaced them with 168 gr. and 173 gr. AND put a different powder charge in them. I also don't remember if they resized the cases before they put the new bullets in place, though I think they did in at least some cases and maybe most of the time. I do remember we resized the cases in the 70's at least. IF and when they resized the cases, they always resized with .308 Win. sizing dies, so that meant they would fit the tighter chambers. I was not an "Ammo Tech," so I did not get involved with loading/reloading ammo. However, this ammo was regarded more as "practice ammo" and generally was not as accurate as our other loadings.
The LAST load we used was a proprietary load that Federal loaded JUST for the Marine Corps for about two decades and it was labled .308 Win. We allowed the FBI and DEA to use it and I think we allowed one of the Canadian Special forces to use it, but no one else to my knowledge. Finally Federal asked us to allow marketing it on the open market and it is now known as Federal Gold Medal Match Ammo.
Now, we tested each NM M14 with ALL four loads to see which one or ones would shoot to our standard of a TEN shot 3 inch group (or less) fired from our super expensive test racks at 300 yards. NO rifle shot all four types of ammo to that spec and it was/is unreaiistic to think it even could happen. A TINY percentage of rifles shot the NM Lake City Loads and the Federal Load that well, but it was no more than one in maybe 30 or 40 rifles and was quite unusual. A good number shot of rifles shot one of the Lake City loads and the Federal Loads. A small number of rifles shot ONLY one type of ammo well and it was almost always the Federal .308 Win. load when that happened.
OK, now on to using the M118 ammo. Originally that was loaded TOO HOT for M14's and they down loaded it so it could be used in DMR's, EBR's and other "special use" M14's. The M118 ammo that is now generally available can be fired well in most GI and commercial M14's because that ammo is more tightly controlled than say Machine Gun ammo. If it functions in your rifle and shoots good, then use it. If it doesn't shoot well, then I suggest pulling it apart, resizing the case in a .308 Win. resizing die and reloading it.
OK, now as to PMC Bronze 147grn 308 that was also marked 7.62 NATO on the box. I really don't like PMC ammo as it can have a tendency not to function right or accurately in some rifles. Some folks have good success with it while others do not.
SO, what I recommend as "standard" plinking/practice ammo for commercial M14 rifles is either Federal American Eagle or Winchester White Box Q3130, 7.62 MM NATO, Full Metal Jacket, 147 GR ammo. BOTH of these loads work well and give good accuracy in virtually every commercial M14 I've ever seen or heard about.