Sig Customer said Sig Factory Taper Washer does not fit my Sig Spear Rattler 7 3/4 “ barrel because Rattler 7 3/4 “ 5.56 was never intended to be suppressed.

Does this make sense to you?

Check the threads. Those barrels are likely threaded 5/8x24 like a .30 cal barrel. If so, you will need the .30 cal taper washer. This was due to a military contract requirement.

Wow! Did not know about military requirement.
Thank you.