The only factory ammo with Sierra bullets that I have tested with is Federal and Black Hills. I have never seen Sierra ammo for sale in fact, I didn't know that they made any. The LEOs that come to our range to practice all use Federal Gold Match ammo that they are issued. I don't know how Sierra full case sized ammo would compare to factory ammo that I have purchased, but since factory ammo is using new cases, I would expect them to be full case sized. I have measured the factory ammo loaded with Sierra Match King bullets against hand loads of similar specs that I have reloaded using neck sizing, velocity tuning through powder measuring, and seating depth measured but COAL and via the ogive.
I shot all of these rounds using my Savage 10 FP using a bipod and rear bag off a bench at 100 yards. I've put almost 5000 rounds through that rifle and surprisingly the best group load averages today are better than when I started.
I will admit that probably means that trigger time has improved my technique more than the rifle accuracy has degraded.
Here are my results - all measured using the On Target software.
The Federal Gold Match with Sierra Match King 168 grain bullets (buff and blue box) rated at 2600 fps measured at COAL 2.800 averaged 0.688.
The Federal Gold Match also with 168 SMKs (newer maroon and gold box) rated at 2650 fps measured at COAL 2.800 averaged 0.614.
Black Hills with 168 SMKs rated at 2650 fps at measured COAL of 2.800 averaged 0.682.
These were some of the most accurate factory ammos that I tested.
My particular rifle seems to prefer 168 match bullets.
Hand loads were tuned with a variety of different powders and data for equivalent velocity and depths using neck sizing instead of full sizing, and measuring using the ogive for seating instead of relying on OAL measurements (although I do still measure and record COAL for comparisons). The results for loads that are closest to the available factory ammo that I used are:
With Reloader 15 powder, 168 SMKs, 2650 fps at COAL 2.800 averaged 0.657.
With Varget powder 168 SMKs, 2650 fps at COAL 2.800 averaged 0.733.
With H4895 powder 168 SMKs, 2600 fps at COAL 2.800 averaged 0.580.
H4895 with the same specs but at 2625 averaged 0.587.
H4895 with the same specs but at 2625 and COAL 2.815 averaged 0.538.
H335 powder 168 SMKs, 2600 fps at COAL 2.800 averaged 0.595.
H335 with the same specs but at 2650 fps averaged 0.836.
N140 powder 168 SMKs, 2600 fps at COAL 2.800 averaged 0.452.
N140 powder with the same specs but at COAL 2.805 averaged 0.436.
N140 powder 168 SMKs, 2650 fps at COAL 2.805 averaged 0.395.
The N140 powder is the latest that I have been testing and it clearly is a good powder for my rifle. The average of 100 SMK groups shot to date, good and bad, is 0.537 so it is outperforming the other powders by a considerable margin. The average of 153 groups shot with 168 grain Nosler Custom Competition bullets is 0.518. so my rifle seems like the Nosler bullets even more than the Sierras.
As you can see, the data may have led me to believe some of those myths might just have had merit since varying velocity and seating depth seemed to help tune the barrel/stock combination for my rifle into an apparent node or two along the way and improved considerably on the accuracy I could get with the factory ammo that I could buy.
However, myths or not, using neck sizing, measuring against the ogive and adjusting velocity by adjusting powder measures seems to allow me to improve on available factory ammo for me and my rifle.
While I won't necessarily believe the myths now that you have clarified their questionable factual underpinnings, I'll probably keep using the techniques that seem to work for me to try and keep improving my rifle's accuracy since it seems to work given that I am probably doing something else wrong that mask the more theoretically valid approaches.