55, and 62 gr will work in 7, 8 or 9 twist barrels, the VLDs will need 7 -8 twist,
a 14 twist should be limited to 50gr and under. An 8 twist will do everything but usually the 8 twist are stainless and you wouldn't really want a short CQB rifle to shoot varmints with if that's what you plan to do with the 14 twist and 40gr bullets.
VLDs and 40gr varmint bullets are for accuracy and 55s and 62s are not so it seems to me you need 2 different barrels, a good accurate 20" 8 twist for the varmint and VLDs and a chromelined or nitro-carburized mid gas 16" 7, 8 or 9 twist for the 55s and 62s.
The bolt and new barrel should be checked with "go" and "no-go" headspace gauges, it should be close but I would check it just the same. If the DPMS you have is not chromelined you could have the throat recut to 5.56, we do it all the time.
The 6.8 is the fastest growing cartridge and #1 choice for hunting deer and hogs with an AR15.
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