From what I have found with low load density situations is that you will in many cases get velocity spread increases from this.
The reason, position of the powder in the case in relation to the primer flash. I have actually tested this in my 270 Allen Magnum with a 140 gr bullet and WC872 powder. The load I was using was about a about an 85% load density.
TO start the test I took five rounds and fired then normally and collected the velocity data. They averaged 3210 fps with an Extreme spread of 38 fps. To be honest, this is not bad with this large of a case capacity and this powder.
Next test was to position each cartridge bullet up and carefully chamber the round making sure the bullet never got below level. This made sure the powder was back against the flash hole.
Averge velocity for five shot was 3225 fps with an extreme spread of 18 fps. Pretty noticable drop in extreme spread. Keep in mind, these were the same exact loads as the first test all loaded in the same batch of ammo.
Next test was to position the cartridges bullet down and make sure that the powder was against the bullet when fired and the air space at the flash hole.
Interestingly enough, the average velocity was 3268 fps, quite a bit higher then the other loads of the same lot. Extreme spread also increased to 77 fps.
You can take this for what you will, only firing five shots does not make for completely convincing data but it did show a pattern.
also remember this was with very slow burning ball powders. I have done similiar tests using stick powder and did not see hardly any difference in velocity or extreme spread with the easier to ignite stick powders.
That said, it is my opinion that if you get the correct powder for a certain combination of bullet weight, bore diameter and case volume, you will be in the +90% load density range.
Obviously lower load densities can be very accurate and consistant but they can be more finicky compared to higher load densities in my opinion from what I am seeing with my Allen Magnums. Now these are more extreme then most rounds so that may well be the case. Everything is magnified using these rounds to some degree so its easier to see things that may only be slight in a conventional chambering.
One other point, in extreme cold temps, you are much more likely to see a hangfire with a low load density then you would with a +90% load density, even if the powder burn rate is compatable with the load you are using. By extreme cold I mean below 0 degrees F.
Good question.
Kirby Allen(50)