[Same bullet weight, different velocity?]

There are a number of differences.

OAL is the distance from headstamp to nose.  That distance has no direct ballistic effect.  What DOES have a ballistic effect is the depth the bullet base is seated into the case.  Two different bullets at the same OAL will not have the same seating depth.  The deeper the base is seated into the case, the smaller the initial size of the combustion chamber, and the faster the powder burns and pressure climbs.

There is also bullet diameter.

There is also the fact that lead obturates better to seal in gases.

There is also the difference in friction co-efficient between coated lead and copper plating.

Basically, there's a lot more going on than powder charge and bullet weight, and what produces faster or slower muzzle velocities isn't always intuitive.  [emoji6] 

As a general rule, cast and coated lead takes less powder to achieve the same velocities as plated or jacketed bullets of the same weight because the obturate (fill in the rifling grooves) better, and thus contain the gases better, and quite often need to be seated more deeply, and thus reduce the initial size of the combustion chamber, increasing pressures for the same powder charge weight.
 

Now that makes perfect sense. Thanks for the insight.

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[Same bullet weight, different velocity?]

Thanks for the answers. I figured it was something to do with the difference between coated and plated.

All I know is the blues run great in my mpx.

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[Same bullet weight, different velocity?]

Maybe some others have observed this, so I was curious. I have been working up some different 9mm minor loads for my mpx. I decided to try some VV n320. The bullets are blue bullets 135gr TC and Berrys 135gr HBFP. I used the same primers, brass, charge weight and OAL.  The chrono shows on average about 51 fps faster for the blue bullets. They are pretty much the same shape and weight. The Berrys are slightly larger in dia. 

 

I tried this with another powder, CFE pistol and the results were the same.

 

Any ideas why the Blue Bullets would be faster?