External Ballistics of cast bullets.

[popper]

Slower bullets invariably hit lower. Cheap solution is to aim higher. Mark your target 6-8" above desired point, aim there and look at groups lower.

[dondiego]

Slower bullets invariably hit lower. Cheap solution is to aim higher. Mark your target 6-8" above desired point, aim there and look at groups lower.

I have heard of low velocity pistol rounds that impacted higher due to barrel hang time and recoil.........just saying.

[Pirate69]

I think the observed difference is a combination of both internal and external ballistics. Based on information provided in the initial post, I created two loads and ran the two loads through QuickLoad.

Cast Load: NOE TL310-165 RF, BC 0.242, 1550 FPS
Speer Load: Speer FN 2011, 150g, BC 0.255, 2280 FPS

I first looked at the total drop, due to gravity at 100 yards. The cast load was predicted to drop 8.1". The Speer load was predicted to drop 3.7". A difference of 4.4". The time to 100-yard target for Cast/Speer was 0.2108 seconds/0.1416 seconds. Cast drop differences for 125 yards and 150 yards were 7.1" and 10.6", respectively. Gravity alone does not account for the observed difference between the points of impact at 100 yards.

I think the remaining difference is the internal ballistics and barrel harmonics. When the initial "explosion" occurs in the chamber, a pressure pulse (wave) is generated that moves from the chamber to the muzzle and back through the steel barrel. The pulse moves at approximately 19,000 fps and probably moves up and down the barrel 5 or 6 times before the bullet exits the barrel. The pulse makes the barrel vibrate much like a tuning fork. It vibrates in both a vertical and horizontal direction. In essence, the muzzle moves in a very small circle with respect to the static bore centerline. All things equal, a bullet that exits the barrel when the bore is in the 12 o'clock position will hit higher than a bullet that exits in the 6 o'clock position. Same effect in the horizontal plane. Don't think OP indicated a difference in impacts right or left of the other load.

The pulse also sets up a vibration across the barrel as well. This results in the bore diameter actually expanding and contracting as the pulse traveling up and down the barrel. If the bullet exits the bore when the bore is expanded, there may be some unequal gas blow by which may generate some yaw as well. Best to exit the bore which contracted (accuracy node) to ensure best bullet fit to the bore.

I do not think the barrel is free floated. If so, then the wood contact can interfere with the normal barrel harmonics and clock exit positions can change from shot to shot.

In this case, I think the recommendation to sight the rifle in for each desired load and record the settings is the best solution. Gravity alone is giving about half the observed differences in POI.

Just my thoughts. I know this is a lot of "what ifs" but these could be contributing factors.

[Larry Gibson]

"Is there an easy way of estimating this trajectory prior to going to the range and re-zeroing the rifle every time I change loads?"

Not really, because you won't know what range the "new" loads are zeroed at.

Ballistic tables would tell you that IF you knew the zero range.

Selecting a bullet with a similar BC [doesn't have to be the same weight nor caliber, just a close BC] from the ballistic tables in the back of Lyman's Cast Bullet Handbook #3 may help you some.

[john.k]

Actually,there is an easy way......simply compare published drop figures for similar velocities and bullet shapes .........You might also note that 1500f/s is a problematic velocity ,as the bullet is affected by the turbulent flight zone around the "sound barrier" for much of its flight.Air drag is as much as twice what it is with a 2000f/sec.muzzle velocity

[Recycled bullet]

I have heard of low velocity pistol rounds that impacted higher due to barrel hang time and recoil.........just saying.

I found this interesting correlation several years back between point of aim, and point of impact,

when tested under less than ideal conditions with my Smith and Wesson two and a quarter inch barrel fixed gutter trench sighted J frame

stainless steel magnum snub. Generally I found heavier bullets strike higher for a given distance and lighter bullets tended to strike lower all

other things being equal, trying to be as consistent as is reasonably possible.

It's just food for thought.

When tested in my right hand, fired offhand in single action at twenty yards these two loads both shoot to the exact same point:

1) old school graphite lubed 148 grain Remington hollow base wadcutter bullets seated flush with the case mouth and lightly crimped in r-p thirty eight brass over two grains of WW231.

2) 110 grain jacketed hollow points that I bought from everglades ammo company loaded over twenty two grains of H110 in star line 357 mag brass and heavily crimped.

Could those two loads almost make a display of what that revolver is ballistically capable of doing with the reloaders mind set??

[popper]

Ballistic calculators don't take recoil or barrel harmonics into account. That is our job. I waded through the code of an on-lie calculator, interesting the approach used. There is one, forget which, that allows a comparison between a known load and an unknown, at the known zero.

[Larry Gibson]

The "known zero" is the problem. With out that drop tables won't give the OP any information because they too, depend on a "known zero".

[braddock]

Just a final(for me) post, basically got it sorted. I had a long and involved conversation with a well experienced armorer and he echoed all the points that were raised and one that was nearly raised in the 3rd paragraph of post #23, in that the lower velocity bullet spends more time travelling down the bore of the gun and, given the inherent problems with attachment points in mag tube lever action rifles, the harmonics significantly affect the POI downrange in addition (or sometimes in opposition) to the drop caused by gravity on the lower velocity projectile.
It made sense to me and helped with my shooting.
I've also found that instead of experimenting with several different loads and bullet weights I should concentrate on fine tuning and zeroing a specific boolit and I did and it has paid dividends both in POI and group size.
Many thanks for all the responses sharing your knowledge, I really appreciate it.
FWIW I stopped worrying about BC and adjusted the rifle by the target results - instant karma!

[Rich/WIS]

IIRC the Lyman #3 cast bullet book has ballistic tables in the back for their designs.

[charlie b]

FWIW, lever guns also can have problems shot from a rest. I think it was Paco who said to grip the fore end with your hand and then rest your hand on the bag. I found that helped my groups with a Win94.

[MUSTANG]

I am surprised no one else has cited it; but the Cast Boolits site has a thread where the Ballistic Coefficient's for a wide variety of Cast Boolits from different Mold Manufacturers is provided. There has to be at least 100 or more listed:

https://castboolits.gunloads.com/sho...efficent-Table

In addition; for those using NOE Molds, their site has dimensional drawings for each (most?) of their Boolit Molds. These drawings include the Ballistic Coefficient for that particular Mold. This information can be found at:

https://noebulletmolds.com/site/bull...nsions2/308-2/

I personally appreciate the level of information and the design drawings that NOE provides us.

[Digital Dan]

Model 94 Trapper, 16" barrel, 50 yards w/Lyman 311041 BHN 10. Ballpark same velocity as Hornady 170 gr RN. Sights were not adjusted.

https://i.imgur.com/jmQScof.jpg

https://i.imgur.com/upvGZ9n.jpg

[mehavey]

At that range, we're looking at barrel dynamics (not BC).