Grouping of bullet weights

[petepeterson]

I know that typically the bullet weight will affect the impact point (up and down) on the target, but has anyone experienced a different impact left to right using different ammo? I sighted in my 3" 60-15 today with 148 gr wadcutters, and was dialed in, but when I fired 158 +P lswchp's, teh impact point moved about 4" to the left, at the same elevation (still in a nice, tight group). This was done off of sandbags, so it wasn't a flinch.

 

[OFT II]

Yes, I've experienced it also. Can't explain it but have seen it.

 

[chief38]

When I have switched form 38 Special 148 grain Wadcutters over to 158 grain RNL the only difference I saw was that the 158's shot POA/POI and the 148 WC's shot 1" - 1 1/2" low.

When I have switched to other brands of ammo, different weights and bullet styles I have found that they also affect the windage as well.

 

[Bkreutz]

I'd suspect bullet shape over weight. Are these reloads or factory ammo. If reloads maybe the bullet isn't centered correctly.

 

[petepeterson]

They were factory Federal 158's, and Remington 148's. That was the first time I had shot anything other than 148 thru it. I originally had it sighted for 135 gr magnums, but decided that i wouldn't be shooting enough of them.

 

[shawn mccarver]

I have not noticed it, but rarely change bullet weights within respective calibers.

 

[Boge]

Different bullet lengths are affected differently by the rate of the rifling. As well, different powders, primers, bullet shape, etc., cause differences. Remember always that a gun's barrel does not know how much a bullet weighs. What is important is the bullet's length. Mass is a measurement of the amount of matter something contains, while weight is the measurement of the pull of gravity on an object. A good example of this are the solid copper bullets by Barnes that are longer than lead bullets of the same weight as lead is denser than copper.

Another factor is if you steadied the barrel on the sandbags as this can affect the barrel harmonics moving the impact.

 

[HKSmith]

I believe the difference you've seen with these 2 loads is due to the much greater recoil of the 158 grain +P load compared to the wadcutter load. When your revolver is aimed at the target, the barrel is actually pointing down; by the time the bullet leaves the barrel, the barrel is pointing up (due to recoil) and the bullet is launched on an upward trajectory. The barrel also twists under recoil, so the position of the barrel in the horizontal plane at the time the bullet exits will be different with different loads. Even though your gun is initially aimed at the target with both loads, it's ending up aimed 4 inches left with the heavier load.

At what distance were these groups fired? The greater the distance, the more pronounced this effect will be. In general, bullets don't have a curved trajectory in the horizontal plane except for wind effects and rotational drift (only significant in long range rifle shooting). Unless they affect the horizontal position of the barrel when the bullet exits, things like bullet length, primer type, powder et cetera won't affect the horizontal position of the bullet on the target at normal handgun distances.

 

[30-30remchester]

HKSmith has a firm grasp on the science of bullet travel. To clarify some, the barrel actually move in an oblong oval as the bullet travels down the barrel. I have know this as "yaw". Right hand twist will yaw in a different oval pattern than will a left hand twist. Different bullet weights will exit the barrel at differ points of the barrels oval travel. At least this is what I have read over the years.

 

[scooter123]

I've seen groupings move horizontally with nothing more than a change in the powder charge. Not totally sure why that is the case but with both my 625 and SR1911 either gun will shoot right if I have a 45 ACP loaded at 750 fps or less. As a result I now tend to ignore the starting load recommendation and charge my initial loads more towards the midrange and tune my charge weight so the POI coincides with how the sights are set on both pistols.

Now as for my theory about why this happens, I suspect that it's a matter of ergonomics coming into play it terms of recoil management. Because I'm right handed my suspicion is that I may be "breaking" my wrist slightly to the left during recoil. Since a 750 fps load in the 45 ACP is noticeably light I may not be reacting the same as I do with a more normal loading.

 

[gboling]

Boge brought up a good point. Were both bullet weights fired from a sandbag or just the 158's?

 

[petepeterson]

Both from sandbags, range was 60 ft.

 

[Boge]

I've seen groupings move horizontally with nothing more than a change in the powder charge. Not totally sure why that is the case...

Centrifugal Force vs Centripetal Force - Difference and Comparison | Diffen


OP, how were you resting the barrel off the sandbags? Was it touching?

 

[HKSmith]

Centrifugal Force vs Centripetal Force - Difference and Comparison | Diffen


OP, how were you resting the barrel off the sandbags? Was it touching?

I'm not sure what your point is about centripetal and centrifugal forces. The only forces acting on a bullet once it leaves the barrel are gravity, air resistance and wind. Any motion of the gun prior to the bullet leaving the barrel cannot impart a curved trajectory in the horizontal plane.

 

[petepeterson]

I feel extremely undergunned in this battle of science...

If it helps, I rested the grip and triggerguard. The barrel was free-floating (but not intentionally).

 

[rwsmith]

This is a theory

So please don't beat my brains out.

Could it be that the heavier faster bullet creates more torque in the barrel that adds a little twist to the recoil????

Update: I see that has been covered already but our wording is a little different.

 

[HKSmith]

The gun starts moving in recoil as soon as the bullet starts moving, so anything that affects that movement (like resting part of the gun on a sandbag) may affect the point of impact of the bullet. If you want to find out where your gun will hit "in the field", hold it in your hands when you fire it. Your groups won't be as tight, but I've found that 7 or 8 out of 10 shots fired offhand group the same as 10 out of 10 from a rest.

The best way to improve your groups is to spend more time firing your gun than reading my posts!

 

[Boge]

I'm not sure what your point is about centripetal and centrifugal forces. The only forces acting on a bullet once it leaves the barrel are gravity, air resistance and wind. Any motion of the gun prior to the bullet leaving the barrel cannot impart a curved trajectory in the horizontal plane.

Not so. Rifling imparts a rotation not unlike a curve ball. A good example of this was the tang sight used on the old Trapdoor Springfield for long range shooting whereas it calculated for this. Those today who shoot long range blackpowder ctg. guns, such as myself, know this all too well. Also, if a person wants to get really technical, the Coriolis effect comes into play as well, albeit in small increments at long range.

 

[mistahd]

Damn. My brain hurts now.

 

[Reddog]

I test fire all my trial loads from the bench with the butt of the stocks resting on a sandbag so as not to mar them. Also I find that this way the gun won't recoil away from the rest. Any change in the bullet, powder charge or support is liable to move the point of impact. Another thing to consider is sunlight on the front sight. That will move your point of impact, too.
Dick