Originally Posted By rsegrin:
Even the Maryland AR sight describes a similar problem. I will keep researching...
Yes, but that problem (with Chuck's rifle) dealt with a COLT M4 barrel with a variety of upper receivers. Seems the newer M4s use a shorter than standard front sight assembly.
I have no idea why your rifle is shooting high but I do have a suggestion.
Don't use the silly 8/3+1 zeroing method - its been long since dicredited. Use Chuck's Improved Battlesight Zero instead (its in the Zeroing Methods section at MD-AR15.com - you can use the 25yard version if you don't want to adjust your drum to bottom out at 8/3-2, but the Colonel's version is better).
Zeroing this way may be within the tolorances of the rifle (it doesn't 'fix' the problem - just ignores it).
Also note even 'professinals' make mistakes - no matter if its building a new barrel or installing it. If the Santose IBSZ doesn't work then I'd send it to Bushmaster - its their barrel and upper.