POI change due to weight/velocity

[ptf18]

Fellows. Excuse me as I should know the answer to my question but....

How will a heavy/light bullet weight effect the POI on a target?

Also. How will a faster/slower velocity effect the POI on a target given the same bullet weight?

Thanks.

 

[Kurusu]

With an heavier slower bullet expect some higher impact point(sometimes also a bit to the left or to the right depending on the direction of the rifling).

 

[Nevada Ed]

It comes down to time in the barrel (grs & fps )

plus the amount of recoil of the weapon and grip.

Change anything and POI can change.

 

[ptf18]

Thanks Fellows. Now a bit deeper.

Lets say the firearm is secured in a rest and bench mounted (to remove the recoil/grip issue).

So if a given weight bullet (A) has a POI of X and its pushed faster than the faster POI will be higher? Conversely, If its pushed slower the slower POI will be lower?

If you change to a heaver bullet (than A) how can you achieve the same POI that (A) originally had? ( I "assume" the POI would be lower due to its heavier weight and slower velocity) (Not concerned about "blowing up the gun).

I should know this stuff but have never paid much attention other than adjusting sights to center up my POI.

 

[Nevada Ed]

Load a few rounds and go shoot your weapon.

Best way to find out what will happen.

Good luck.

 

[Loyaljeeper]

I can only speak for handguns. My experience is velocity tends to matter a little bit more than weight. Generally, the slower the bullet the higher it will group. I am curious as to what others experiences are. This is a topic I have never been able to find good science proven answers.

 

[mikld]

If the recoil factor is removed, thus no barrel time/recoil difference, your theory is correct; a heavier bullet may print lower due to added weight/gravity. The major difference is how recoil raises the barrel and how long the bullet is in the barrel during the rise (the longer the bullet is in the barrel as it raises from recoil, the higher the barrel is when the bullet exits the barrel, the higher the bullet prints on target)...

 

[Kurusu]

Thanks Fellows. Now a bit deeper.

Lets say the firearm is secured in a rest and bench mounted (to remove the recoil/grip issue).

So if a given weight bullet (A) has a POI of X and its pushed faster than the faster POI will be higher? Conversely, If its pushed slower the slower POI will be lower?

If you change to a heaver bullet (than A) how can you achieve the same POI that (A) originally had? ( I "assume" the POI would be lower due to its heavier weight and slower velocity) (Not concerned about "blowing up the gun).

I should know this stuff but have never paid much attention other than adjusting sights to center up my POI.

That is only a theoretical question at best.

The best test rest I know(Ransom rest) lets the pistol move under recoil.

 

[OKFC05]

Thanks Fellows. Now a bit deeper.

Lets say the firearm is secured in a rest and bench mounted (to remove the recoil/grip issue).

So if a given weight bullet (A) has a POI of X and its pushed faster than the faster POI will be higher? Conversely, If its pushed slower the slower POI will be lower?

If you change to a heaver bullet (than A) how can you achieve the same POI that (A) originally had? ( I "assume" the POI would be lower due to its heavier weight and slower velocity) (Not concerned about "blowing up the gun).

I should know this stuff but have never paid much attention other than adjusting sights to center up my POI.

Unless you plan on welding the action to an anvil, the gun is going to move to impart rotational momentum (barrel moving up) and the longer the bullet stays in the barrel, the higher the departure angle of the bullet compared to your initial sight axis. I did the calculations once for a GI .45, and the barrel only moved 0.050" before the 230gr bullet departed the barrel.
Yes, the bullet has already left the barrel before the shooter is aware the gun has fired, but all the momentum transfer happens in that 0.050". That's why we know the flinches that affect the shot start BEFORE the gun fires.

 

[Twoboxer]

Thanks Fellows. Now a bit deeper.

Lets say the firearm is secured in a rest and bench mounted (to remove the recoil/grip issue).

So if a given weight bullet (A) has a POI of X and its pushed faster than the faster POI will be higher? Conversely, If its pushed slower the slower POI will be lower?

IMO, considering this alternative only confuses the practical issues. As pointed out previously the barrel will move anyhow.

If you change to a heaver bullet (than A) how can you achieve the same POI that (A) originally had? ( I "assume" the POI would be lower due to its heavier weight and slower velocity) (Not concerned about "blowing up the gun).

I would select the bullet and powder charge I wanted and test them together with the baseline load for the original bullet. If this were a handgun, and the heavy bullet printed lower, I'd lower the powder charge and test again. If it printed higher, I'd raise the powder charge. If it were a rifle, I'd re-zero the sights.

There are just too many variables.

 

[Rogeronimo]

My 9mm printed 124gn low, threw 147gn correctly. I slowed the powder down to 800X, and stuffed the charge to achieve 1250 fps ... problem solved.

 

[HKSmith]

The answer to this question depends on the distance at which you're shooting At close distances (50 ft), the slow heavy bullet will hit higher because it's launched at a steeper angle. However if you look at the trajectories out to 100 yds, the heavy bullet will cross the path of the light fast bullet and hit lower at the longer distances. There will be one distance at which the 2 bullets have the same point of impact.

Gravity has exactly the same effect on both bullets; they fall at 32 ft/sec/sec independent of their weight (Galileo showed this in the 16th century at the Leaning Tower of Pisa). The difference in trajectories comes about because the heavy slow bullet takes longer to get to a given distance and is subject to the downward force of gravity for a longer time. My Freshman Physics Professor demonstrated this in a way I haven't forgotten in 61 years - he had a 4 foot long glass tube with all the air pumped out of it and a silver dollar and a feather inside. He would hold the tube up vertically and quickly invert it and the feather and silver dollar would fall and hit the bottom together.

 

[buck460XVR]

If the recoil factor is removed, thus no barrel time/recoil difference, your theory is correct; a heavier bullet may print lower due to added weight/gravity. The major difference is how recoil raises the barrel and how long the bullet is in the barrel during the rise (the longer the bullet is in the barrel as it raises from recoil, the higher the barrel is when the bullet exits the barrel, the higher the bullet prints on target)...

^^^Best answer. But weight has no impact on drop due to gravity. Velocity does.

 

[Gamecock]

Gravitational acceleration of a bullet is unaffected by the bullet's weight.

It is unaffected by the bullet's velocity, too. It's just that a faster bullet will get there with less time for gravitational acceleration to operate on it.

 

[buck460XVR]

It is unaffected by the bullet's velocity, too. It's just that a faster bullet will get there with less time for gravitational acceleration to operate on it.

....but I didn't say velocity had anything to do with gravitational acceleration. I said velocity has an impact on bullet drop. Why does a 180 gr .44 mag bullet drop 6"@100 yards while a 180 gr 30-06 only drops less than 2". Velocity.

 

[Gamecock]

I said velocity has an impact on bullet drop.

Why does a 180 gr .44 mag bullet drop 6"@100 yards while a 180 gr 30-06 only drops less than 2". Velocity.

Time. The drop is the same. Gravity isn't just a good idea, it's the law.

 

[buck460XVR]

Time. The drop is the same. Gravity isn't just a good idea, it's the law.

yes, and that is why velocity affects POI(bullet drop at specific distances).....as was the question asked by the OP.

Also. How will a faster/slower velocity effect the POI on a target given the same bullet weight?

 

[Gamecock]

I think we are using two different definitions of drop.

I am using drop as a lowering of the trajectory of a bullet in flight due to gravity.

You are using drop as a lowering of where a bullet prints on a target.