There are all kinds theories about what makes rifles and ammunition accurate. Some are well proven, such as having the barrel and action properly aligned, or removing pressure points that interfere with barrel vibration on firing.
Others are more theoretical — or at least, do not necessarily work every time. One of these is chamber length and the distance between the bullet and the rifling on firing.
In theory, a bullet that just barely touches the rifling when the cartridge is chambered should deliver the best accuracy. That way, it has no freebore to jump to reach the rifling, with the chance of entering the bore slightly askew. For this reason, when beginning to develop a load with a particular bullet, purists measure the maximum allowable cartridge length (MOL) with that bullet in that chamber. This is done by pushing a bullet into the chamber as far as it will go, and then running a rod down the barrel until it reaches the nose of the bullet. You mark that, then remove the bullet, close the bolt, and run the rod in until it touches the bolt. The difference between the two marks is the MOL.
Obviously, factory ammunition cannot be loaded to such exacting standards because there is wide variation among rifles even of the same chambering.
This is one reason why a particular combination of rifle and ammunition may work very well, while the same ammunition in a different rifle can be downright poor. However, even rifles that are ostensibly exactly the same will not behave identically with the same ammunition. As has been written over and over, every rifle is a law unto itself.
If you are having a custom rifle made, you can specify the amount of freebore you want, and the gunmaker can chamber it accordingly. However, even that is no guarantee.
I recently acquired a gorgeous custom .270 Winchester made by Al Biesen on an FN Deluxe action, sometime in the 1970s. With what factory ammunition I had lying around, it shot groups ranging from ho-hum to excellent. It is, obviously, an accurate rifle.
Before I started load development, I measured the MOL with one of my favorite bullets, the Nosler 130-gr. Partition. SAAMI specs for the .270 call for an MOL of 3.34 inches. My maximum MOL with the Partition was exactly 3.34 inches. Wow, thought I: This should really shoot! Further measuring showed that, loaded to that length in the .270, the base of the bullet lines up exactly with the base of the case neck — another theoretically desirable ballistic virtue.
Having done all this, I put together some loads using a “guaranteed accurate” formula from a friend who’s a .270 aficionado. It’s 59.5 grains of H4831, for anyone who’s interested and, I was told, it delivers fine accuracy with any good 130-grain bullet, in any good .270 Winchester rifle.
As a test, I also loaded some rounds with 130-grain Sierra SBT GameKings, and some with 130-grain Swift Scirocco IIs. The latter were not as dimensionally exactly perfect as the Partitions, but pretty close.
At the range, I got the best velocity from the Sciroccos (3060 fps) with the Partitions second at 3020fps. Best accuracy was with the Sierras (five shots, 1.1 inches) with the Sciroccos second (five shots, 1.2 inches). The Partitions delivered a lamentable five-shot group of around three inches across!
Every other aspect of the rifle worked beautifully: There was no vertical stringing of any kind when the barrel heated up, and as velocities varied, the group centers edged up and down like clockwork. But the “theoretically perfect” load had fallen flat on its face.
My next step will be to vary the Scirocco and Sierra loads by a quarter grain up and down to see what effect that has. Since there were no adverse pressure signs at all, I’ll increase the Partition loads by half-grain steps to see if that helps. Obviously, with the Sciroccos and Sierras delivering such fine performance, I’m set for a hunting load regardless, but I am really curious.
With the Partitions, here was a rifle and load combination, both of impeccable credentials and theoretical perfection, using a bullet with which I have had superb accuracy going back 30 years. Yet, it did not work.
Here is further evidence, were any needed, that there are no guarantees with rifle accuracy — neither with factory combinations nor handloads — and you should not let anyone convince you differently.
Every rifle — every rifle — is a law unto itself.