Thirty years? Ha! Mine might be only a couple of years older than I am.
I lived in Iqaluit/Frobisher Bay for many years. US forces stationed there did quite a bit of shooting in a valley, and left everything lying on the ground. The Browning belts were rotten, the thousands of cartridge cases were oxidized, but the M-1 clips had light even rust, and most cleaned up very nicely indeed. I look at my stash and feel sorry for those who have to buy them at current prices.
I didn't have the same opportunities you had, but the area where I live in BC is only about 20K from the Vernon Base.
When I was a youngster, there were places that had been used for Field Training Exercises. We found hundreds of discarded chargers for Lee Enfields, as well as many more fired and ejected blank cartridge cases. We found all sorts of "junk" that were treasures to us as kids.
In some areas, there were a lot of unexploded munitions, especially mortars. Many civilians were killed picking this stuff up, sometimes just moving the pieces caused them to explode. I knew one fellow who had found a couple and was throwing them at rocks.
As for M1 Garands, mine is a Pietro-Breda made FKF, Typo 2, chambered in 7.62x51 Nato, with an Italian 56 date barrel and unissued stock.
This rifle has had the recoil/return spring replaced with a heavier Wolf spring and has digested at least a dozen different manufacturers' surplus ammunition.
The US Lake City Match and Hirtenberger are the hottest of all of them, with Canadian IVI being at the low end.
This rifle digests it all without any feed or extraction issues. I wasn't worried about the op rod or gas system as they were purpose-built for this ammo.
Of course, the sights have to be adjusted when shooting different lots, which is normal.
It shoots most surplus acceptably with a noticeable preference for the Lake City Match.
As for en blocs, I was extremely lucky about 35 years ago when and old REME who had been stationed on the Alaska Panhandle during WWII, gave me a several dozen en blocs. The unit he was with was issued Garands and Pattern 17 rifles because it was all supplied by the Americans, who shared the base with them.
They are definitely better than anything I've picked up after.
I gave many of them away, when I sold off the various M1 Garands I've owned over the past 50 years.
My first came from a mail order house in the US called "Klein's" It cost me a week of wages at the time, but it was worth every dime.
Afterwards, Lever Arms would bring them in. Alan didn't care for the Garands but he liked making money so he tried to keep a couple of hundred in the warehouse at any given time. That's all changed now.
I remember one particular batch of National Match M1 Garands he brought in and I had the job of cleaning them up. They had been stored for quite a while and most were well worn, but still safe to shoot. A few of them looked like they came straight from the armorer special build shop. I managed to get one of those.
I was also lucky enough to get a couple of cases of Match ammunition, which the rifle shot extremely well.
Sadly, that rifle went the way of the willow to help pay off my divorce. I miss that rifle a lot, the ex-wife, not so much.