Velocity differences with different brass?

[rcol317]

I've been reloading for a number of years but have never understood why different brass gives different velocity with all else identical. I've been playing with an new 20-223 AI and am getting consistent 80 to 85 feet per second faster speed with 223 Lapua Match brass than I get with IMI or Rem. brass. I've checked with both of my chrony's and both show the same thing. Any explanation would be appreciated.

 

[Tempest]

Internal case volume.

 

[gotcha]

As Donovan has indicated there can be small inconsistencies in even the best brass. Stick with Lapua. Their alloy is better/stronger for continued reloading and will yield best all round results in the long run. I've worn out .223 bbl.'s with just 300 rounds of Lapua brass. (5000+ down the tube.)......... Some may argue but this has been my experience. Good luck!.............. I should add that I anneal the .223 brass every 10 firings.

 

[Patch700]

as mentioned case capacity most likely... Something to consider as well is the overall condition of the brass... Depending on the cartridge if brass gets work hardened with no set annealing schedule you will most likely come to a point where you have a problem with uniform case expansion and neck tension , both will show up on the chronograph.

 

[rcol317]

Thanks for the feed back. I was skeptical so I sized all the brass, turned the IMI brass to be as close as equal to the Lapua and then shot multiple shots with each load with two chronographs and still the result was the same, the Lapua brass is 80-90 FPS faster than the IMI. It has to be the volume of the case and therefore the pressure is a little different although the brass shows no sign what so ever. I have found that the Lapua brass can take pressure better than other brass.

 

[jlow]

If you are not up to measuring case volume, weigh the two and you will find the Lapua is significantly heavier. More metal in the same fix volume means less empty volume or case capacity = higher chamber pressure = higher velocity.

 

[waldo1979]

I've been working loads with Lapua and Lake City brass. Here's a couple case capacity measurements i took of my .223 brass:

- Unfired new Lake City 2013 29.8gr
- Fire formed Lake City 2013 30.8gr
- Fireformed once-fired Lake City 2005 30.7gr
- Lapua 30.5gr

I plugged this data into Quickload to get a good estimate when moving between brass and adjusting seating depth. I look at the pressure and velocity numbers; it gets me pretty close.