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1/7 twist is every bit as accurate with 55 grain FMJ's as 1/9 or 1/12. The bullet is the problem, not the twist rate.
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Agreed. Like the majority of 5.56 barrels, my ARs have both 1:8 and 1:7 twist. They shoot 55 grain pills just as well as 62 or 69s.
I have given my 55 grain hand loads to several people over the years that were having trouble sighting in a new gun because of ammo. Without fail, consistent ammo allowed them to complete their sight-in. Never once asked what the twist was on their barrels.
Mil-spec 5.56 ammo is not known for accuracy. It is made to be reliable when conditions are less than ideal.
If you do not reload, then it would be a good idea to purchase some match grade .223 ammunition for sight-in. Once the sights and optics are set, then go to the 5.56 ammo to see what you have. In most cases, the .223 and 5.56 trajectory will be almost identical. A slight elevation adjustment may be need if you intend to use 5.56 exclusively in the particular gun.
In my experience, 5.56 groups are never smaller. If you have a 1 MOA rifle and get 3 MOA with mil-spec ammo, you are doing well, indeed. If you find a brand your rifle likes, buy a lot of it.