This drawing from Brick, reflects a quick check of overall barrel length,not the length measured from the bolt face
My moderator from Brick is slightly shorter than the one in his drawing above and my grenade ring is slightly more than the 0.100" indicated in the notes. Let's be conservative and say 4.330" plus 0.100" for the grenade ring, giving a total of 4.430".
4.330 + 0.100" = 4.430"
Subtract 0.625" for thread engagement
4.430" - 0.625" = 3.805"
Add this to your barrel length
12.500" + 3.805" = 16.305" barrel length with moderator attached
ETA: I just checked several barrels for thread length. I found 0.623" to 0.630".
So going with 0.630" thread length, like Brick's drawing above, you would have 16.300" barrel length with moderator attached
Now somebody double check my math