Choke Thread size

[Zoiezeus]

Hello all, a newbie here, Im getting ready to cut and thread the barrel on my old H&R 12gauge. Ive been a machinist for 20+ years and use to very tight tolerences. I plan on tapping barrel 13/16-32. I have the tap which measure .816" OD. I have a choke tube from a mossy which measures .807" OD, thats .0045" a side smaller, seams like a lot to me. Anyone have any opinions on this?