I finally caught up with my friend Dennis. Dennis shot for the USMC International and US teams. While he did shoot 300m 3P and CISM, he did not shoot the Winchester M70 Intn'l Army Match. He shot a Remington 700 or 40x bedded into a Standard Rifle stock (probably an Anschutz) by the RTE Armorers (2112's).
...so while I can't speak directly to the accuracy of the M70 IAM's, I did get a peek at the accuracy requirements of a competitive 300m rifle. Dennis says that they were striving for multiple 1.5" 10 shot groups at 300m out of the test cradle at Quantico. His rifle shot tested at 2" and he set the half course prone record at that time, which means his rifle was shooting 10's for 20 shots (the 10 ring is roughly 3" at 300M). His teammates Gary Andrade shot a M700 as well and DI Boyd preferred an M70 (that he shot lefty). Their rifles tested similarly and Andrade broke Dennis' record shortly after.
I know the 2112 who was building these rifles and I just recalled a story he told me that is relevant. He had installed a barrel from one of the "name" barrel makers and it had tested the tightest he had seen on a 300M rifle at 10 shot 1". Problem was it was printing one target over. He called the barrel maker who said it was impossible.
