Working on my trainer that will spend part time with a drop in conversion in it. Remember a thread years ago discussing which twist rate would give the best results between calibers. What should I go with on a 16"? Anything I should consider on lining?

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Your best bet is to go with a dedicated upper. None of the AR twist rates will be ideal for 22LR. Typically 22LR barrels have a 1:16" twist.

This came up recently.  Due to the imperfections in cheap .22"LR bullets, the faster the spin, the more inaccurate.  Best is a dedicated .22"LR upper.  Next best will be a 1 in 12".  Next would be a 1 in 9", followed by 1 in 8", etc.

I still use a .22 conversion from time to time even with owning three dedicated .22 uppers. The interesting thing I have found, and I have no explanation for this, is that I have a rifle with a 1-7 twist that will shoot better than another with a 1-9 twist. Then I test a 1-12 twist barrel and it may not shoot any better than the 1-9 depending on ammo. Its really odd and the results are not consistent at all.

If I put my scientific thinking cap on the only conclusion I can come to is that if we had 100% consistent ammo to test with that the results might show the slower twists to be better. Meanwhile we have a very inconsistent ammo selection being shot in a barrel that is not made for that ammo and there are just so many other factors that come to play that barrel twist may not make much difference in the real world.  I would further suppose that if you did shoot just one brand/loading of .22 and really tested then you might find some small differences based on twists. Meanwhile, barring that possibility I have concluded that there are just too many variables and that I can no longer say with any certainty that a 1-12 is always going to shoot better than a 1-9 or a 1-7. But, in general I would guess that it likely would.

My conclusion based on some years of experience and thousands of rounds of ammo fired comes down to this:  In most circumstances for the average person shooting a wide range of standard ammo a 1-9  barrel is probably the best overall pick. The love of the 1-7 comes from the military using it. They chose it for a particular loading which the average person will likely never shoot. On the other end the old 1-12 or 1-14 twists (which are difficult or impossible to find in new barrels) were dropped by the military due to the ammo they were using. The 1-12 is popular in bolt action .223s as varmint hunters (which is where this round really got its start)  typically shoot the lighter bullets like the 40-52 grain. The 1-9 is a great compromise as it works best with the most common ammo the vast majority of typical civilian shooters  use, the 55-62 grain loads

In short, I wouldn't pick a slow twist like a 1-12 just so I could use .22 as well as 5.56 with it. Get the barrel you want for 5.56/.223 and then enjoy your .22 in it. OR, if you really intend to shoot a lot of .22...get a upper dedicated to that caliber, you will be happier with the results and you wont lead up a good 5.56 barrel and gas system.