Shorter barrels, slower speeds = more deviation from BC/CDM models?

[Graves14]

I haven’t seen this discussed specifically anywhere but was wondering if I missed it, or if anyone has meaningful insights.

I’ve been helping a pile of friends get their hunting rifles ready for this season and with the popularity of shorter barrels on hunting rifles and thus slower than “average” speeds, I have noticed we’ve had to adjust MV quite a bit more to get the data to line up with what we are seeing at 400yds.

This made me wonder if the further you move away from the speed of bullet used to establish either a BC or CDM, will you see a greater deviation from that drag profile? AB’s calibration video has scared me off adjusting the BC or drag model so I am back to fudging MV.

Just one example. I was switching powder lots and testing both velocity and accuracy last night. I shot the same round out of an 18” barrel at a measured 2586fps and from a 24” barrel at a measured 2702fps. To get the data to line up at 400yds I had to increase the 18” MV by 38fps but the 24” by only 13fps.

Does this match what you’ve observed as well? Thanks!

147 ELDM
40.5 H4350
FGMM205 SRP
Federal Brass

Garmin Xero Chrono
Using AB Quantum CDM for both.

 

[ARShooter91]

Absolutely positive zeros are good? 400yds is a little close to do velocity trueing. 400yds BC variance doesn't matter yet.

Twist rate, starting velocity and velocity drop are all variables that would effect you drop profile.

 

[Scopes]

Could also be a scope tracking issue.

 

[Graves14]

I agree that 400 is too short to really verify but if it’s going to be correct at 800 it should also be correct at 400 no?

Scope is a possibility but it’s a NF NX8 so I don’t think that is it.

I’ll pop out to 800 and see where everything lands