.223 POI change with bullet

[PKRobbins]

Loaded Speer 55gr TNT over 25.5gr of 4064 Remington .223 brass, CCI small rifle primer, avg velocity 12" from barrel is 2882fps, same load with a 55gr Vmax in Lake City 5.56 brass getting 2945fps. But the VMax POI is 1.5" lower. BC for speer is 0.233 Vmax is 0.255. Shooting from a 16" 1/9 bushmaster. Can someone explain the physics to me.

ETA: shooting at 100 yards

 

[gcoulsey]

Re: .223 POI change with bullet

What physics do you require explaining?
Lower impacts??
Your using 2 different types of brass for starters, so internal volumes will be different, giving different pressures.
So there's a strong possibility the two loads are going to behave differently regardless of matching bullet weight.
That's without factoring in all the other differences that can shift your POI.

The BC's of the bullets aren't going to come into play at 100yrds.. Not measurably under normal circumstances anyhow.
You need to put your speers into the LC brass for a more comparable result.
(or vmax's into the remmy brass)

 

[pjparker]

Re: .223 POI change with bullet

Any time you change any component you can expect to see a difference in performance.

Change one component at a time and you can attribute the change in performance to <span style="text-decoration: underline">that</span> component.

Change multiple components at one time and you have just wasted your time.

Your chronograph is 12" from the muzzle ???


Paul

 

[PKRobbins]

Re: .223 POI change with bullet

I assumed that the higher velocity (higher pressure from smaller volume) from the LC brass would make the POI higher for the Vmax. Back to the loading bench.