Looks like "MAYBE" we have that girl behaving again????
I hate to even post a reply about my AMs after the tongue lashing I got recently about that being all I do on LRH but screw 'em!! I mean that with all due respect.
In the case of the 7mm AM, I have found very similiar results to what you just have posted. My personal heavy 7mm has nearly the exact barrel as yours, only 30" finish length. I could horse the 200 gr ULD RBBT to 3400 fps if I really wanted to but that was not the most accurate load in that barrel. Turned out it was also a bit off max at aroud 3265 fps.
In my lightweight rifle, its preferred load is down to 3155 fps on average but it is also a 27" barrel.
Accuracy of the light rifle has been 1/2 moa to 500 yards, 3/4 moa out to half mile and around 1 moa from there to 1200 yards or so. I think this may have as much to do with the shootability of the lighter rifle(9 lbs) as much as anything.
My heavy rifle, very similiar in weight with your rifle has proven capable of 1/2 moa out to 1000 yards in good conditions when my head is in the right place while shooting.
I tend to not be quite as critical as many extreme accuracy minded long range shooters as far as E. S. is concerned. Obviously if I have a load that is giving me 75 fps spreads, this will not be really good at long range. But, if I can get a load into the low to mid 20 fps range for ES for say a 10 shot average string, I will be plenty happy with that and have yet to have any problem hitting big game at the ranges I have shot to so far.
That said, I certainly can not argue the fact that the tighter the extreme spreads the less vertical stringing you will see down range. That true with any chambering.
One very nice thing about the 7mm AM is that you have a pretty wide performance window you can search for your preferred load in. By that I mean, it has the case volume and ability to push 3400 fps in any barrel length 30" or longer easily. Often times this not the most accurate but I feel this is a bullet jacket issue and that will be changed very soon!!!!!
But even if you have to down load a bit, your still right there at the very top of the performance ladder for 7mm rounds. The 7mm STW will get you around 3000 to 3050 fps with this bullet weight. The 7mm RUM will get you 3100 to 3150 fps with this bullet in the barrel length you are using but needs very high pressures to get to this level of velocity.
In comparision, 3140 fps in the 7mm AM with a 32" barrel is such a mild load, I would suspect the cases would rechamber after firing and rechamber easily from the lack of high pressure.
When the right bullets come available, then we can throttle up and see what she will really do!!!
WC872 also seems to have very distinct vibration nods and also a relatively narrow pressure range where the powder will burn really consistantly. I think that is why we see accuracy drop off on the lower end of pressure as well at times and certainly can see some velocity spreads increase.
Anyway, take away that third shot and it looks like your playing with 1/4 moa at 1/2 miles. Not to bad at all!!!! Can not wait to see how you two boys do in Idaho!!!
One of these years I need to come down and play with you as well. I could bring a 338 AM, you your 7mm and roy his 270 and we could really have some fun!!!!
Again good post,
Good luck this season.