Chrony data and OAL

[berettaprofessor]

Got a little confusing chrony data that I need some expert eyes on. Shooting 6.5 Creedmore using a Hornady 147 ELD and 41.3 grains of H4350 (previous ladder indicated a node here). I was trying some varied OAL's to see if there was a best OAL for precision group, shooting 5 rounds at each OAL, and as the OAL increased, the velocity also increased. Shouldn't it be the other way around? I thought as OAL increases, pressure decreases and velocity decreases?

OAL Mean Velocity fps SD
2.795 2652.0 7.9
2.810 2659.5 15.0
2.830 2682.3 12.0

I did shoot the OALs in this order, so the only thing that I thought of was that the barrel/chamber might have been warming up? Shot these three strings over about 20 minutes.

 

[twistytorn]

With some powders there is some precedent for reduced velocity with compression. I don't know about H4350.

 

[berettaprofessor]

None of those loads are compressed, so I don't think that's causing the reduced velocity. Max fill of H4350 is 41.8 according to Hodgdon for that exact bullet, and it's not compressed even then at the 2.800 OAL.


Further info: I considered the bullet bearing surface being different, but even at 2.830, the base of that long bullet is below the neck so I didn't think the case-bullet contact probably changes. All were very mildly crimped with a LCD, all the same force. All cases were FC and trimmed to 1.790 before loading so if the difference was case volume variability, then this happened despite random selection of cases.

I should also note that all cases chambered without difficulty and bolt closed fully. No signs of overpressure on the brass, no split necks, etc.

 

[Slamfire]

I think you need to fire more rounds to establish more confidence in your means.

 

[Walkalong]

Yea, 5 rounds is fine, but a small sample and the next five can be different, even if you let the barrel cool.

 

[Dwb1957]

I have seen that when you bullet starts getting close to the lands you will get a pressure increase.

 

[denton]

When the bullet is seated deep, pressure is high because case capacity is low. When the bullet is seated long, pressure is high because the bullet is engaging the lands, or nearly so. In between, there is a happy low pressure region.

And yes, you probably need several samples to see the change through the random noise. Plus you have to carefully control barrel temperature if you want to make proper comparisons.

 

[murf]

less neck tension

murf