i am new to the AR-15 market i am looking to mount a Trijicon Acog ta01nsn on a bushmaster m4a3 With Magpul back-up sights will they co-witness with the acog or not ? or what flip up back up sights should i use if not the Magpul back-up sights.
You will not be able to co-witness with an ACOG. Your back up sights will only be used if you remove the ACOG.
Back up sights are just that. They can’t be used (co-witnessed) with an ACOG. They are there just in case said ACOG fails. 
As others have said, you won’t be able to co-witness with a magnified optic like the ACOG.
I run a Troy folding rear on my 6920. I had a Trijicon TA-44R 1.5x Compact ACOG on the 6920 for a while. I’ve switched that out to a Kahles 1.1-4x CSX, and the Troy works great with that too.
