I had a 639 that shot exactly like yours. 8 inches at 10 yards at best.
Posted the issue here.
A nice member advised me to measure the headspace with the gun in battery (unloaded).
I could move the barrel about .025" fore and aft, IIRC.
Posted my measurement.
He mentioned my 24th Infantry avatar and said "your Christmas just came early" and shipped me a complete top end, gratis.
After the fix, it was all good. 100 rounds rapid fire.
Thanks, Nyteman44!