Same bullet weight, different velocity?

[medeng8]

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?

[superdude]

This is common. Lead bullets (cast, swaged) generally go faster with the same charge as jacketed/plated bullets.

 

 

[chevrofreak]

Blues are the fastest bullets I've used.  They're also slightly undersized for my barrels. 

[4n2t0]

The short answer is that one's a plated bullet. If you want to see something cool try lead>coated>plated>FMJ.

Edited July 1, 2018 by 4n2t0

[medeng8]

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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[IDescribe]

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.   

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.
 

[medeng8]

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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[quietmike]

Another possible difference is bearing surface of the bullets.

[Banacek]

On 7/1/2018 at 12:51 PM, medeng8 said:

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?

I ran into something similar many years ago when testing round nose bullets to JHP (both MGBs).  Another shooter and I wanted to see which fed better in our race guns.  We found that the JHP was almost 100 fps faster.  This test was done over 10 years ago.  

[Racinready300ex]

37 minutes ago, Banacek said:

I ran into something similar many years ago when testing round nose bullets to JHP (both MGBs).  Another shooter and I wanted to see which fed better in our race guns.  We found that the JHP was almost 100 fps faster.  This test was done over 10 years ago.  

 

If you use different shape bullets with the same OAL you can end up with different case volume which will change the velocity.

[Banacek]

2 minutes ago, Racinready300ex said:

 

If you use different shape bullets with the same OAL you can end up with different case volume which will change the velocity.

Could be, but I have not seen that translate to my 223 reloading and that is with a 20 inch barrel. 

[Racinready300ex]

You're not going to see drastic differences in the shape of 223 bullets of the same weight.

 

It has to do with case volume. If one bullets profile is longer even though it's the same weight it will need to be seated deeper to have the same OAL. If it's seated deeper you have less room for powder and this will raise the pressure and change the velocity.

 

Idescribe explained it pretty well above.

[Banacek]

1 hour ago, Racinready300ex said:

You're not going to see drastic differences in the shape of 223 bullets of the same weight.

 

It has to do with case volume. If one bullets profile is longer even though it's the same weight it will need to be seated deeper to have the same OAL. If it's seated deeper you have less room for powder and this will raise the pressure and change the velocity.

 

Idescribe explained it pretty well above.

I agree with the premise just surprised to see that much difference from a 4.75 inch barrel.  I stuck with the JHP since it helped power factor.  

[Racinready300ex]

 

Just now, Banacek said:

I agree with the premise just surprised to see that much difference from a 4.75 inch barrel.  I stuck with the JHP since it helped power factor.  

 

I don't think it's so much related to barrel length as % of effected case volume. But, that's just a guess.

[IDescribe]

10 hours ago, Banacek said:

I agree with the premise just surprised to see that much difference from a 4.75 inch barrel.  I stuck with the JHP since it helped power factor.  

 

You're talking about rifle powders and bottleneck cartridges..  Even fast burning rifle powders are much slower than slow pistol/shotgun powders.  The slower the powder, the less dramatic the effects of shifting variables.  And since the interior diameter of the case below the neck is much larger in a bottleneck rifle cartridge than a straight wall pistol cartridge, seating the bullet deeper into the case doesn't reduce the case volume as dramatically, again, meaning less dramatic results.    

 

Edited July 4, 2018 by IDescribe