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My "advisor" says a 13x1 would be probably the best you could do as you need to have some shoulder to pull up on. This thread would be way better than a 1/2-20 as the metric one would take less meat...

There are two issues around this.
The first one as Tim mentions is that wall thickness becomes marginal with a half inch thread. This probably isn't a huge safety issue as muzzle exit pressure is...

I wouldn't ever choose that size for a 7mm barrel, especially a hammer forged/ button rifled barrel.
I'd suggest at least a 14mm thread in this instance.