Re: Barrel twist & length relationship
As BobinNC noted, barrel length is usually not an input in ballistic calculators because muzzle velocity is the more critical number. However, your question has merit because in many instances, you will find information regarding what twist rate can handle what bullet length (weight) without taking the increase in muzzle velocity from a longer barrel into account.
As long as the linear velocity of a bullet in the barrel is increasing (accelerating), so is its rate of spin. Although I'm sure the info exists, I have no practical knowledge of just how long a barrel would have to be before a typical load would reach constant velocity before leaving the barrel, but I'd guess it would have to be longer than 24"-26". Where this can come into play in a practical sense (as per your question) is that if you wanted to use a round that was just marginally stabilized by a given twist rate, you'd probably get better results from a longer barrel with the same twist. For example, a 16", 1 in 10" twist .223 barrel is not likely to give the best results with 77 gr loads. However, a 21", 1 in 10" twist barrel will do a better job of stabilizing a 77 gr round because you will get an increase in muzzle velocity and therefore rate of spin as the projectile leaves the barrel. So a longer barrel can sometimes sufficiently stabilize loads that fall slightly above the optimum bullet length for a given twist.
I think the main reason it's less common to see barrel length included as a factor in the kind of tables you're referring to is that there are so many good twist rate choices available in barrels today that it's usually not a problem. Just avoid the whole issue by making sure the barrel has a twist rate that is sufficient to stabilize whatever loads you're likely to shoot. I have previously considered your exact question because I had purchased a rifle with a twist rate that was highly questionable as to its ability to stabilize the loads I wanted to use. Fortunately, it does successfully stabilize bullet lengths that, according to tables of the type you mentioned, would not ordinarily be recommended. My guess is that it has a lot to do with barrel length, although I can't rule out that other factors could also be involved. As I mentioned above, the best way around that is NOT to do what I did, and make sure your barrel has a twist rate that has been demonstrated to stabilize whatever rounds you want to use.
