You need to go by the specified length of a good quality reloading manual.
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All my 223/556 cases get trimmed to 223 spec at 1.745 middle of the road. I have a LOT of this type of brass and its such a chore I use a dillon electric trimmer in the xl650 to do them all when the bucket of brass gets close to the top.
What you need to be concerned with is how thick is the web. As you shoot reload time resize and so on the brass you remove comes from somewhere. What I do is sort by headstamp, weigh and segragate into 1 grain of weight difference piles. Take one from the pile with the lightest average weight cut it at the base of the case and examine wall thickness. If it looks thin all of that weight gets recycled.
Its still work but it prevents case head seperation as the brass gets thinner from use
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I have lots of 5.56 brass as well. I shoot a lot of 80 gr bullets and want as much bearing surface as possible between the case neck and the bullet, so I trim my cases to 1.755".
I reload in 1K batches and use a Gracey power trimmer. It works like an electric pencil sharpener. You simply stick the case neck in the end, and it trims, chamfers and de-burrs the neck all at the same time. Done in no time at all.
If you are loading bullets with a cannelure and crimping the case mouths, it's best to trim all your cases to an equal length. That way the crimp doesn't end up above or below the cannelure due to varying case lengths. Consistency in your reloading methods helps maintain the accuracy of the ammo you produce.
For bullets without a cannelure and using a light taper crimp it's not a problem unless you exceed the maximum case length.