The field I hunt is about 400yards wide and 980 yards across. I set up in the middle so the longest shot I could see is right around 500 yards. Not an impossable range but one that I would certainly have to practice for in the off season. I thought about trying 110 Accubonds and 115 BTs as well, but the super high speed of the 100gr bullets really has my attention. I can push just over 3000fps in my 6.5x55 with my 120BTs but it seemes to like the heavier 140gr SSTs a little better those only fly in the 2700fps range.
Your right it takes a LOT of practice to be proficient out to 500yds, and near perfect conditions to do so. I had a Leupold VX-3 4.5x14 mounted on top. Using the 115gr Partition loaded to 3150fps, and sighted in dead on at 200yds, it was a simple matter of turning the power up to 14 and setting the top of the bottom post where I wanted to hit out at 400yds. This equated to about an 18" drop which made it very easy to also judge range as this is about the height of a deer from top of back to bottom of belly.
Knowing your load inside out is one third of the equation, the next part is having a rifle which is stable in all conditions. The original stock I had changed my POI over the course of several years until I finally had no more adjustment left in my scope. It simply curled up from natural drying or warping, not sure which. Either case I replaced it with a Fajen laminate and once I got it glass bedded, no further issue have arisen. I good trigger is also a must for this sort of work. You cannot be pulling much over 3# if that and keep things on target at those ranges time and time again. It has to break clean and even every time so that your always surprised when it does. I had mine set to just under 2.5# and there was never a time when I felt this was too light.
The last third is you. Your breathing, heart rate, and trigger control will all play an effective role in your groups out to and past 400yds. It might seem only slight, but once a big buck steps out and you see him in your scope, all bets are off. If you have one foot that starts to shake while your settling in for the shot you might as well back away and regain control. That vibration will transfer up through you and effect your shot. Can't sit there and drink coffee either, as the caffeine will jack up your heart rate. There is nothing like wanting the bouncing ball in the scope to stop bouncing, and it just won't do so.
Changing gears here a minute, that 6.5x55 is nothing to be overlooked. I picked up one years ago when Kimber sold a bunch of sporterized 96's. I figured it would be a good one for my daughter to use while growing up and still have plenty of backbone once she matured. I worked with everything I could find to get the thing to shoot 5 shots on a paper plate at 100yds. After replacing the scope, the stock, and going through every make of bullet on the shelves I finally hit a load which produced consistent 1" accuracy at 100yds.
Thing was I had to be somewhat careful with the loads and not push the pressures. So I settled with the 140gr A-Max plodding along at just over 2550fps. I figured that this would be a fine load for 100yd or so shots in the woods, which it performs outstandingly. What I didn't know at the time was also how well it reached out across 3-400yds of pasture and reliably dropped feral hogs. I found this out while testing the bullets before letting her hunt deer with them. Since we have suck a widespread hog problem they make great test media. Time and time again I stretched the yardage out with them until I couldn't judge the drop reliably. Every hog I connected with hit the dirt on the spot. Hogs are tough and tenacious, and if not hit solidly will make even the best of power and bullets look puny. The 6.5x55 is a fantastic little round and will amaze you with what it can and does do all out of proportion to what you would think it can. What has totally blow us away is recently we purchased a couple hundred rounds of the PRVI 139gr SP ammo. We figured it might work out for the hogs and not cost an arm and a leg to shoot up. It has surpassed any attempts of my handloads in accuracy out to 200yds and even my 9yr old grandson has used it on deer and hogs, and shot less than an inch group more than once at the range. Don't pass up giving it a fair shot.