Accuracy - Velocity Relationship

[harro]

I’m a reloader who has never really dived deeper than producing accurate hunting ammo .

I have a question to ask about the velocity - accuracy relationship of loads .

For example
You have a chosen load you have developed that is accurate and safe with brand X powder/projectile/primer/brass
You chronograph that load and get it’s average speed , lets say 2700fps for argument’s sake.

You then change powders to brand Y but change nothing else ( same seating depth ) and find a powder charge weight with that new powder that duplicates the 2700fps average velocity.

Will that load match the first load in regards to accuracy/ performance or will there be a difference?
Does the burn characteristics of a powder / case fill percentage / pressure characteristics etc effect the accuracy / barrel harmonics of the load even if the projectile is leaving the muzzle at the exact same velocity ?

Just thought I’d ask those who are way more knowledgeable than me on these subjects.

 

[Fast14riot]

Pressure curves and burn rates all affect barrel harmonics. Generally we change between close burn rate powders in rifle cartridges, so thing don't seem too different between different powders. But, when I've tried H4350 in my 260AI vs H4831 (all else being equal) which is about a 3gr difference to get same velocity, I don't end up at the same velocity. I do, however end up around the same barrel time (OBT tuning) which gives approximately the same exit during muzzle oscillation. I mostly tune by what the paper tells me rather than the data, but use the data to support my decisions later in development.

 

[dthomas]

A different powder will usually tune at a different velocity.

 

[Ned Ludd]

What we're really after when we carry out load development is to "tune" the load to a particular rifle's barrel harmonics. In other words, we want the bullet leaving the bore at a specific point or window of time within the barrel's harmonic cycle.

As such, it is equally important to consider the acceleration of the bullet in the barrel as it is to determine its ultimate muzzle velocity. The concept here is referred to "barrel occupancy time", or simply "barrel time". It is the amount of time the bullet spends in the bore, usually measured in milliseconds. The key is that powders with different burn rates and/or different gas expansion rates can produce the exact same muzzle velocity, but with very different barrel times. This is because the bullet's acceleration can be quite different due to the different properties of the powder burn. What it means in practical terms is that the even though the muzzle velocity may be identical, the bullet may not be leaving the bore at the same point in the barrel harmonic cycle, thereby causing noticeable differences in precision.

One would expect that the closer the burn/gas expansion rates of two different powders are, the closer the loads would end up by matching muzzle velocity. But it is important to be aware that two loads based on different powders might end up having markedly different precision, even though they ended up at the same muzzle velocity. One can only test and do proper load development to know with certainty. Likewise, using different primers, brands of brass, and bullets, even those having the same weight, can also significantly alter acceleration and barrel time. This is really why the safest approach is to carry out a full load development procedure any time a reloading component has been changed.