Texas is right on.
But to your original question, first, a chest rig with mags is not going to stop anything rifle-related. The "guy on the other forum" is wrong.
Second, go with 1:7" twist so you can have the flexibility of using heavier rounds if you want.
MK262 is OK, but it's accurate at long range, but expensive and not in the current crop of best performers....
If money is no issue, I'd look at Barnes TSX 50gr from black hills, or something like the federal Fusion softpoints, or better yet TBBC...
If money is an issue, I'd just stick with XM193.
There's no magic bullet. Doc lays out the good stuff and info here:
https://www.m4carbine.net/showthread.php?t=19881
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