Model 700 woes

[PJgunner]

It seems to me, that if the first shots give a very bad group, then the next round of shots gives a good group, that when the barrel(?) heats up, something is altered. A bad scope will sometimes give bad groups and good groups, but a bad group on the first string, then good groups thereafter untils cool off? No bloody way!
The comment about the loose fitting threads, maybe.
However, five will get you ten it's in the barrel. I think a barrel slipped though that was not properly stress relieved. Whether Remington will make it good, considering it's a second hand rifle, I can't say. However, having them replace the barrel will probably be less expensive than from a gunsmith.
Probably the previous owner disposed of it for exactly the same reason. Probably didn't want the hassle of sending it back to the factory.
My only experience of returning a rifle to the factory was with a Ruger #1 in 7x57. It too had a bad barrel that would not shoot anything. My gunsmith did a chamber cast and found the throat was over two inches long, way out of spec. I wrote Ruger, explaining that is was a second hand rifle and what my gunsmith had determined. They had me send it in. It took almost four months to ger it back, but not only had they replaced the barrel, but completely refinished the rifle. Groups are now in the .75 to 1.25" range depending on the load.
Contact Remington and see if they will do anything for you.
Good luck.
Paul B.