Just as an aside on helmet rims in general. They do come off occasionally.
Whilst training recruits my fellow instructors and I would give an effects demonstration with the "Thunderflash" pyrotechnic which was a large firework used to simulate explosions.
It was a long established practice in which a MkIV helmet minus liner was placed on a hard surface and a Thunderflash ignited underneath it. The resulting explosion would propel the helmet 50 - 60 feet into the air thus giving a demonstration that these were not harmless "party poppers".
On more than a few occasions the rim was blown clean off the helmet which is why it was always best to use a spare helmet.
Not an M1 I know but a very similar rim design and an illustration of the idea of rims becoming detached.
I hope this is of interest.
Regards
Mark
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