5.56 NATO and barrel twist?

Accuracy? Just depends on the gun, you gun will shoot 55-60 grainers just fine. I don't think you will be able to shoot 90 grain bullets out of a magazine, I think a 90 grainer will have too long of an OAL to fit in a mag. Don't quote me though it just depends.

Barrel life? Your barrel will wear out at about the same rate with any ammo, the biggest killer will be heat and ultra high velocity ammo, but bullet weight won't really matter.

Yes you can spin bullets apart, I haven't seen it done in person but my dad had it happen to him. It waas a .223 (if I remember correct) that had a fast twist rate, he tried working up a load with some 40 grain varmint bullets, something like a Barnes varmint grenade, and they had such a high velocity in such a fast twist rate that they were coming apart somewhere in flight because he couldn't hit paper and there was no dust poofs anywhere around it. As long as you don't shoot light varmint bullets you should be fine, but your gun may be able to shoot them regardless.

The faster the twist rate the heavier the round that can be fired without tumbling. 55gr in a 1:7 will still do enough damage to either game or people to kill or seriously maim them but works better with graisn such as 62 and can go up to 90. While the lighter the load the slower the twist rate should be.

You won't have any problem with 55 grain and up.

You run into problems when you try to push light bullets at high velocities. Varmint loads with something like a 40g Blitzking pushed to maximum in my 1:12 twist .223 will vaporize out of my 1:8 twist rifle.

Using a lighter bullet can cause the bullet to spin too much and can cause the bullet to "fly" this means not be as accurate not fly apart. From what I understand it is because a smaller bullet moves much faster and has less area of contact with the rifling, and will damage the bullet and can cause bad flight patterns. This will not always happen and my never happen to you but it very well could.

Some people will claim that a fast rifling twist with a light bullet can make the bullet fly apart in mid-air. I've never seen that happen, but it's been some time since I've been to a range.

My son has an upper that we built. It has a 20" heavy YHM barrel with 1:7 twist. He gets touching bullet holes at 100 yds with hand loads with 55 gr Nosler Ballistic tips.

No. It will have little to no impact on barrel life. It SHOULDN'T have an effect on accuracy, either.

Most rifles are more accurate than the people shooting them. And if you are one of those few who actually IS more accurate than your rifle, I'd be interested in knowing the differences you encounter.