Sub Moa with factory ammo
 

When the right factory ammo for a rifle is found to produce Sub Moa , what is making it the right combination?

Is head spacing and the ogive the main factors ?
What about powder and detonation?

most of it is the bullet, the rifle likes it
RR

Too many variables all have to come into line to really nail it down to just one or two, or ten.

My limited experience with accuracy has been, like Ridge mentioned, find a bullet that your barrel likes, then find a velocity that runs consistently in it.

Since you're talking about factory loads, the reality is that there is no "science" that will tell you which will be the most accurate load in your rifle - you just have to go shoot a bunch of loads and figure out which your rifle likes best. If you were reloading, you could find a bullet that your rifle likes, then determine the "optimal charge weight" or "accuracy envelope" to give you the most consistency in velocity, but since you're talking factory loads, you just have to try different loads.

There are a few things you can look at, like your twist rate and barrel length (resulting velocity) to determine a proper bullet weight then try different loads in that weight range, but then you're still left trying different brands and different loads. I have two AR's with the same brand barrel that I know to be chambered with the same reamer, but one REALLY likes Winchester 50grn Ballistic Silvertips, the other is just so-so accuracy wise with the BST's but is best with Hornady 50grn V-max's. On the other hand, I have a Savage 223 that likes pretty much anything you feed it, and will post 5 shot groups in the 1's and 2's with 34cent per shot Remington 50grn UMC JHP's. Any of them are "accurate enough," but each of them have their own favorite "flavor" for the most accurate factory load.

Finding Sub-MOA Factory Ammo
 

I handload but for just about every high power rifle I shoot at one time or another I've had to use factory ammo. I can tell you that in the (4) high powered rifles I own I've found factory ammo that shoots under (1) MOA at 100 yards for (3) shot groups.

In reloading I've found the most consistent factor in making Sub-MOA reloads has been velocity. My 30-06 likes 150 gr. bullets at about 2950-3000 fps. It doesn't matter if I'm loading Hornady SST's or Sierra ProHunters with RL15, IMR4350, or RL17. Once I get in that velocity range they group under 1 MOA. I've used both Lapua and Winchester brass with CCI & Winchester primers. As long as the velocity is in the range the groups are small. I load .010" off the lands but have loaded to SAAMI spec OAL and gotten excellent results when I got velocity in that range.

When you find good Sub-MOA factory ammo I suggest you go buy as many boxes as feasible from the same lot #.

The attached picture is my 30-06 firing Hornady SuperFormance 150 gr. SST. A friend gave me (2) full boxes of this ammo as his rifle groups them poorly. The group is off a rest at 100 yards.