This is a good topic, one that is actually critical in the field. I have tested this as well and will say in factory rifles, this can be critical at extended ranges as the first shot can vary quite a bit from the following shots.
THe reason, well, obviously machining quality, barrel quality, bedding, all that comes into play.
I have also noticed that in a properly built rifle, one that has a square receiver, a barrel that is threaded on axis to the receiver axis, a chamber that is perfectly on axis with the bore and receiver and in a rifle that is bedded properly in a solid non stressed method into a solid rigid stock, the first shot will generally fall into the moa ability of the rifle.
What the hell does that mean????? Well, if a rifle will shoot 1/2 moa on average, that first shot will land with in the group of this size. Looking at the group from your rifle, it is a perfect triangle which tells us that things are square and bedding is good and your ammo is straight and consistant. YOu must be a hell of a handloader and your gunsmith must be world class!!!
, well we know the first one is true......
How do I prepare for the big game season. Well, like you I do load development long before the season begins. Then once that is done, the vast majority of my shooting is done for the year. From that point on, I work on my drop chart, tweaking here and there to match it up as perfectly with actual bullet flight as possible.
Then about this time of year, when the temps start to get down into hunting type temps, I will take my hunting rifles out with customers rifles when I test them before shipping. After shooting the customers rifles(work first!!!) I will pull out my personal rifle with only two shots. Find a target of opportunity. What I look for is a well defined rock, roughly 4 to 8" in diameter for a target.
I range it, check the wind and conditions if ranges are past 500 yards, run the numbers and get the hold. I will then settle in with a fouled cold bore and take the first shot and follow it up with the second as quickly as I can get it in the air similiar to what you would see if hunting big game and a second shot was needed.
I then record the impact points and will compare that to my drop chart predictions. By this time of year, things are generally THERE. By that I mean impacting less then 1/2 moa from point of aim. If I can do this from field shooting positions I am happy. I will test out to ranges far past where I will shoot at big game. For example, I have been testing my 338 AM out to nearly 2000 yards this fall but have no intent to push things this far with the rifle, even though in the right conditions it makes this range seem pretty easy.
Last trip out, I broke the right and left edge off an 8" rock at 1980 yards with the first two shots, from a cold barrel. Petted her warmly, call her some very nice things and put her back in her warm case!!!!
I never shoot from a clean cold bore. In fact every time I clean a rifle, when she is done, I oil the bore lightly, take a fouling round and go out behind the shop and sink a round into the old cotton wood stump so that I never have to worry about a fouling shot in the field. Would it make any difference, I am sure it would because every shot I fire from a freshly cleaned bore is through an oiled bore, ALWAYS, never a dry bore so one can only expect if nothing else velocity will be effected because of the oil in the bore.
Anyway, good topic, I believe in factory rifles, this is far more critical then in a properly built custom rifle but its always something that you need to know about your rifle, just goes back to being intimately familiar with your gear before heading to the field.
Looks like big grey will be eating well this fall for you!!! Remember to send the pics!!!