I would hope nobody would waste the money on switching out their current 1/7's for 1/8's. It's just not worth it in my opinion. If you're looking to buy a new barrel though, I wouldn't choose it based on whether or not it's 1/7 or 1/8. In my personal opinion, there are other factors more important. If you're going after a hammer forged barrel then it's most likely going to be 1/7 but if you're looking for a Stainless barrel, it's most likely going to be 1/8.
That being said, twist rates just can't be disregarded. There isn't a magic twist rate that will shoot every weight as accurate as they can be. A think people tend to make it more dramatic than it really is though. Most people are going to be shooting inside and at 100 yards and at this distance, a 1/7 barrel is not going to throw a 40gr pill off the paper. I've never seen it happen and I've never heard of it happening. What can happen though is overstabilization and it will open up your groups. Going back to what I said before. The average shooter won't see it because they aren't shooting at distances where it's going to be apparent.
A little history I believe is accurate: When the M16-A2 was put in service, that's when the 1/7 twist was introduced. Not because it stabilized the 62gr green tips better but because it was needed so the longer tracer rounds could hold on to enough phosphorus to burn past the military requirement of 900 meters.
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