26 inches is too long for a .223. There have been some tests conducted where the same barrel has been cut to 24" and it produced higher velocities (IIRC +30fps more).
The highest velocity is reached with a 24" barrel and if you cut it down to 20" you will lose around 120fps on avg. But cut the same barrel down to 18" and you are losing around 200fps.
I would cut it to 20" and then add the suppressor.
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