Planning on buying a Bushmaster complete upper  with the ARMS S.I.R. system and mounting an Acog on it, the TA01nsn which I already have. Question, this comes with the fixed front sight, 16" barrel, what are the opinions on a backup set of iron thinking of getting this upper withought the fixed front sight and putting a set of BUIS flip up sights on it. Or, would it be better to go with a .040 post and just add the rear flip up sight?

Comments would be appreciated. But, I am going with a complete upper, from Bushmaster.

Thanks

It really is just a matter of how you want it.
I have a TA-31 on mine which is mounted on an ARMS#19, with an ARMS#40 BUIS and an RASII. But that's my set up. I thought about just going with a TA-51 mount and skipping the BUIS. Not sure how reliable the flip up front sights are but I'm sure they'd be accurate for close up work.

I'm assuming that the Busmaster ARMS SIR system has mil spec rails and my mounts for the ACOG will fit with no problem?

Quoted: I'm assuming that the Busmaster ARMS SIR system has mil spec rails and my mounts for the ACOG will fit with no problem?

I'm sure it is fine. If it is not they will exchange but I would bet the rails on these are most definitely in spec. If you have any problems, Bushmaster will take care of you.