Developing a 500 yard load for 300PRC can be relatively short and sweet - if every round fired has a purpose.
Seeing those groups with relatively different bullets all shooting under an inch, the task is really just a matter of finding a forgiving load - finding a node - and loading there. I know 3 methods to work, if appropriately employed, and none of them take much time, ammo, or energy to complete:
1) Audette Ladder at at least 300, preferably as far as 600yards. Load one to three shots each of 8-15 different charge weights in relatively small increments (.2-.4grn each step, depending upon cartridge and powder dispenser/scale), fire all of them at the same point of aim at 600 yards, keeping track of which load represents which bullet hole (magic markers of various colors can be used to distinguish one load from another, leaving colored witness marks at the edges of their holes in paper). Some adjacent loads will bunch up vertically, while others will gap apart - these adjacent loads which hit near the same water line are in your node. This can be done as single shots per charge weight or as 3 shot groups per charge weight.
2) Newberry OCW fired at 300-600 yards. Same principle, reliant upon vertical POI shift between charge weights to reveal the node, but fired as groups at multiple targets strung side by side. Many folks shoot this as a round robin, one round of each charge weight, repeating 3x at their respective target. Similar to the Audette test, we’ll see some adjacent groups riding the same waterline relative to eachother, and some adjacent groups spreading up or down considerably compared to its neighbors. The error possible here is to complete the test only at 100yrds (many online sites promote this test at 100yrds) rather than 300+, because mechanical errors will overwhelm the vertical dispersion of the group at 100yrds. Same deal - the adjacent groups which ride the same waterline define the node.
3) The easiest of all, which I have confirmed to align with the other 2 POI dependent tests, is the Satterlee Velocity curve. This can be 1 shot each charge weight, or more. I personally shoot 3 shots per weight. This can be done with or without a target, honestly, but I do like to combine my Satterlee test with one of the other two tests. Shooting all charge weights across a chronograph, you’ll see flat spots in the curve, meaning increasing or decreasing charge weight in the flattened areas will yield little to no influence on velocity, and such little to no influence on long range point of impact... effectively, we know those bullets will impact at range on a common waterline.
With these tests, a guy can be done with primary load development within 8-45 rounds, and may not need any secondary load refinement at all (for example, I’ve not done jump/seating depth testing for most of my loads for the last 3-4 years). I spend far more time and ammo in barrel break in and basically as much in fouling than I do in load development.