Automotive O rings are still pretty high temp so I do not think they will melt, so as long as you find one the right size I don't see what it would hurt even if it melts.
If you can find a AR O ring I would use it but I can see how that might be a little issue over there.
The 6.8 is the fastest growing cartridge and #1 choice for hunting deer and hogs with an AR15.
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