Jimro, the USN Small Arms Match Conditioning Unit made the first of those chamber inserts for 30 caliber M1's so the NATO round could be used. That was before they got NATO chambered barrels from Springfield Armory in MA. Those that needed to be driven in with a mallet smacking a tool to seat them (tight chamber where the insert fit) stayed in; sometimes the accuracy was pretty good. Other inserts that went in more easily oft times came out with the fired case in the first few rounds fired. The idea was quickly scrapped as it was too dangerous and accuracy wan't all that great over all; worse than 30 caliber ammo.
Although a NATO round chambered in a 30 caliber Garand chamber typically fired and ejected with the case shoulder an most of its neck blown out to chamber specs (1/16th inch of shoulder and no neck at all), it was just beyond the cliff edge of safety. Many civilians buying those inserts from folks making them who didn't totally understand the safety issues oft times had them come out. So, I'd melt down all those inserts and make nails out of them; they would better serve humanity that way.
Note that 190 load the USAF and USN used was somewhat higher in pressure than the 50,000 cup spec for NATO ammo. Op rod's ringed quite nicely and the felt recoil with them drove the M1 back noticeably harder. Op rods had to be perfectly bent and fitted to much tighter specs than military standards; there were only 2 or 3 'smiths in the country back then who fit them correctly and they would last for the life of the barrel or even one or two more barrels. There's only one still alive as far as I know and he's in his 80's now. "Op rodding" a Garand has become a lost are as far as I know.
I give credit to those 7.62 NATO Garands (most used by ex-USN team members) shooting higher scores in the long range service rifle matches at the Nationals than what the M16/AR15 versions did shooting 22 caliber bullets. The US Army got the NRA rules changed so they could use AR10's in 7.62 versions 'cause nobody could get 5.56 NATO ammo shooting as accurate past 600 yards as the 7.62 NATO stuff did in Garands and a few M1A's.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master