Re: will heat (barrell)open up groups at 700 yards?
It depends on two things; what kind of barrel it is and how it's fit to the receiver.
Most factory production barrels are not well stress relieved. Which means as they heat up they will bend. The group may shift center but usually doesn't get larger.
Most custom match-grade barrels are very well stress relieved and they don't bend as they get hot. Best example of this is when a friend took his Win. 70 based .308 Win. target rifle with a stainless Hart barrel, clamped it in a machine rest (unrestricted, free recoil) then fired 40 consecutive shots about 20 seconds apart starting with a cold barrel. That barrel was pretty darn hot when he finished. So was the 40-shot group 600 yards down range; 1.9 inches.
If your receiver's not trued square with the barrel tenon thread axis, the barrel will fit hard at one point around the receiver ring. This can cause a slight stress that can cause barrel bending in that direction.
And a round left in a hot chamber over 15 seconds can heat up to have a higher muzzle velocity which means it'll strike high.