[pvdw]

Hi guys.

I am playing with some loads in my 30-30. The one thing I can clearly see is a change in POI as the mv change.
A change of 30ft/s or more WILL print 20-25mm higher at 50m. I can almost plot the shots according to mv.

1950ft/s will typically print 1.5- 2" lower than 2000-2050. Is this normal for the shorter/lighter guns? I ask this as my .303 Brit does not show the same amount of shifts as mv go up or down 30-40ft/s.

Thanks.

Peet in SA

 

[FredT]

Is your Brit shooting the same weight and style projectile as your .30-30? If not, then you changed the equation.

 

[pvdw]

Thanks FT

Tha Marlin is shooting 170gr FN Sierra around 2050ft/s and the Brit 174rg SP at 2350ft/s.

Peet in SA

 

[Halwg]

You know that isn't always the case. Ive seen slower moving projectiles actually print higher. That has happened to me on several occasions.

 

[burk]

PVDW,
Speaking ONLY of velocity you would expect the slower bullet to strike lower, especially with the exact same sight setting and a relatively long enough range to target to see the effect of velocity on the bullet arc. BUT there are a ton of other influences like recoil energy, how firm/soft is the forearm rest, bullet time in the barrel, etc etc. If the recoil energy increases and makes the muzzle 'flip' slightly as the bullet travels the bore the poi could be higher. A slower bullet with more bore contact time and the same recoil might strike higher. You can't really put an accurate rule of thumb on this that would generally always be true for all rifles, velocities, bullet weights and shooters.

 

[Plumber]

An other factor is ballistic coefficient.

 

[pvdw]

Goodness never realized that there are so many things that can shift the POI ??? ??? ???

Thanks for the reply's.

Peet in SA

 

[Ben Dover]

If you'd like to have some fun, use the program called www.jbmballistics.com
You can select a bullet from the library and select a range distance. Set wind factors to zero. Plug in various mv's and have it show you the differences in trajectory. The bullet doesnt care about the cartridge, it only knows how fast you started it and its bc. Everything else follows. Ben

 

[RaySendero]

Hi guys.

I am playing with some loads in my 30-30. The one thing I can clearly see is a change in POI as the mv change.
A change of 30ft/s or more WILL print 20-25mm higher at 50m. I can almost plot the shots according to mv.

1950ft/s will typically print 1.5- 2" lower than 2000-2050. Is this normal for the shorter/lighter guns? I ask this as my .303 Brit does not show the same amount of shifts as mv go up or down 30-40ft/s.

Thanks.

Peet in SA

Peet,

As a general rule – Most rifles I have will shoot higher off a bench rest to 100 yards as:
1) the bullet weight is increased with the same velocity or
2) as velocity is increased with the same bullet weight.

However, the degree to which this POI will happen varies a lot from one rifle to another.

 

[Tatersoup]

All the answers I see above are right. But to muddy the waters, there's another factor.

Barrel harmonics change as velocity and bullet weight combinations change. This could also make the POI shift left or right. May or may not be a concern and is gun specific.

 

[Woodchuck]

Forget everything except when the bullet leaves the barrel at some known velocity. With the same bullet and muzzle velocity the poi will always be the same. It is simple physics and the rules don't change with each gun. Lower the mv and the distance traveled will drop, increase the mv and the distance traveled will increase assuming that the barrel is horizontal when firing... IMHO, if you use sights zeroed at some yardage, all bets are off because the barrel will not be level and you introduce several new variables into the equation. The rules don't change, just the complexity of calculating the results once defined....