Ballistics Coefficient Ratings

[Rooster59]

This is specifically about a couple cast bullets in particular but I thought it suited reloading overall than bullet casting methods.

Just wondering what others think, believe, know about ballistic coefficient ratings that bullet and mold manufacturers assign to their projectiles. My casting and reloading buddy and I were discussing our current pet projects for 45LC and 45-70 this afternoon. In doing so we were comparing BC of two bullets he recently cast, the Lee 452-300 and 457-340. Both of these are casting about .002"-.003" larger than the design numbers which makes us happy but Lee's BC for the two don't jive for two amateur ballisticians.

If you look at the bullet profiles and their listed BC in the mold chart link below I would argue that those two bullets' BC is not right. The 452-300 is virtually a slightly tapered wadcutter the meplat is so wide and the BC of .233 is better than the 457-340 BC of .211 which has a significantly smaller meplat and longer, tapered nose.

http://leeprecision.com/bullet-casting/ ... ble-cavity

Does this make sense to anybody else? It's important to us in that we expect to extend our shooting yardages with these two bullets and the BC number will play a significant part in calculating trajectories and vernier tang sight MOA of adjustments to get there.

[Model 52]

Velocity makes a big difference with three major areas in the subsonic range, the transonic range and the supersonic range with the BC generally increasing as the velocity decreases through those regions.

My initial impression is that they may have made different assumptions on BC with different velocity ranges in mind for those bullets.

And, if they measured the BC by drop in velocity over measured distance, then the observation is what it is, and is of course dependent on the muzzle velocity, the range and the end velocity creating a situation where the Bullet may demonstrate distinctly different BCs that are however averaged to one figure by the way the data is collected.

[Rooster59]

Thanks for the explanation. I was afraid of exactly what you described. The BC numbers aren't necessarily what you will experience.

Sierra's BC information makes reference to different velocities and their BC for each of those speeds. That's when I began to wonder if this too is less than 100% factual/accurate when a single BC number is provided.

[Missionary]

Good morning
I used to get excited about B.C. with cast bullets (boolits) . It does of course have influence with shooting at metal critter shaped targets at long distances which I enjoy doing.
But when it comes to my lever actions abd the real usage they get B.C. really has little importance.
I live in East ILLinois when up north in the U.S. of A. Our local range has 300 yards of usable berm placed shooting area. I seldom fire my lever flippers that far. 150 yards is about my normal practice limit. Yea someties I fire a few at the 200 yard gong just to be sure I can still hit it. But actually I do the bulk of my shooting at 100 yards and less. Why ?
I hunt river bottoms. I like river bottoms. I enjoy slinking along through the trees and weeds seeing how close I can get to critters before they realise I am there. My longest corn cruncher shot to date is 33 yards or 35 paces. Most are plopped at half that distance. Actually I use a recurve more than a firearm anymore just simply because I do not need a firearm at those ranges. When I do use a firearm it is normally a flintlock.
So B.C. came to mean so little to me in actual hunting and daily shooting life I seldon think much about it.
I realise I am not the basic hunter. I dought I ever will take a long shot as across a field as some must. But even if I do have a change in hunting zones B.C. will mean so little. I will practice, learn how that bullet flies and drops and adjust accordinly. It is nice to have a reference point to begin with, an idea how the bullet may fly, but that nice little "point" number really will not change one bit how that lead boolit really zips through changing atmospheric conditions.
Mike in Peru

[Rooster59]

I hear ya on the practical shooting Mike. This excursion is to develop our long distance silhouette skills. My only concern on these cast bullets is initially calculating trajectories. Our range has berms to 600 yards and metal targets available at all intervals. We have had issues with members who were incapable of hitting the berms with their whizbang magnums and have skipped bullets off the property into dangerous situations. Therefore all members can shoot at up to 200 yards. Beyond that each member who wants to shoot over 200 must qualify they have the knowledge and skills, and can apply them, in order to shoot at 300 to 600 yards.

The first qualification step is at 300 yards that must have 5 shots placed within 3 moa of the target cold, with no practice shots and direct supervision during the qualification session. No margin of error. Not really a problem for an intelligent, responsible shooter with a .30-06 but a little more difficult for a guy like me with a Marlin 1895CB or a 45LC 92 trapper and heavy bullets travelling at 1200-1300 fps.

The trapper will only take on the chickens at 200 yards but the 45-70.........

[Missionary]

Howdy.... Initially that BC stuff all has some merit.
What I did was to make a portable target stand that is 6 feet tall. Simple lath wood frame 3 feet wide , 6 feet tall (rectangular) with two hinged legs (hinges at top) that fold out to the rear covered with stappeled on refrigerato box .
I start at 100 yards with the target near the top and get group centered. Then move to 200 yards and repeat. Write down sight setting in my little book for that rifle. Move to 300 and repeat. Then on out as far as needed.
Our club Danville Rifle Pistol (ILLinois) used to have the old National guarde range till some new neighbor lawyer bought a tract and got a court injunction that closed the local NG range forever.
So now we have our own registered property on low,wet non-tillable farm land but only 300 yards. But the 6 foot target stand still works. Actually 4 foot tall would be enough for 300 yards. But once you start throwing 480 grainers to 500 yards those extra 2 feet really help catch the low hits.
Mike in Peru

[Ranch Dog]

Not much to add. I believe the only way to know for sure is to shoot the bullet across two chronographs and then use a BC calculator to arrive at the actual BC. The closer the far distance used to your shooting requirements the closer the calculated BC will be to the actual BC experienced.

I have done this work with all my bullet designs. The first chronograph is set at an appropriate distance from the end of the barrel and the second is used out at the target berms. My poor Chrony F1 has seen its share of mishaps out at the lonely, far berms but the information it has supplied has been useful. With my pistol bullets it is placed at 25 yards, the short cartridge rifle bullets at 100 yards and the long cartridge bullets 200 yards. Over a period of a decade this has involved a lot of shooting and a lot of shot up screens!

All my bullets share a common feature, the round flat nose that is 72% of caliber. When it was all said and done, the actual results with my bullets, there was one fact that really stood out. The BC is usually very close to the Sectional Density of the bullet.

That is what I would recommend on using with the bullets identified in the opening post, the SD as BC until you can shoot it. I think it will be closer to what you will experience in real life across a wider range of velocities than a computer calculated BC.

[Ranch Dog]

If you look at the bullet profiles and their listed BC in the mold chart link below I would argue that those two bullets' BC is not right. The 452-300 is virtually a slightly tapered wadcutter the meplat is so wide and the BC of .233 is better than the 457-340 BC of .211 which has a significantly smaller meplat and longer, tapered nose.

I agree with you Rooster. Based on my experience detailed in my post above, I would expect the BC of the 452-300 to be closer to .210 rather than .233. I would also plan on the 457-340 being .226 rather than .211.

[Model 52]

I figured out awhile ago that overcast days are ideal for figuring BC with a downrange chronograph as no sky screen is needed. I set the chrony behind a forward angled AR500 plate to deflect any low rounds into the ground and protect the chronograph.

----

Lately though, when I've had a cast bullet of totally unknown BC, I've taken to just establishing the actual sight corrections over a known distance with known atmospheric conditions, and with the known velocity of the bullet (shooting over a chronograph). Then, I'll solve for any differences between the trajectory predicted with a wild ass guessed BC, and the actual trajectory. With the original estimated BC re-adjusted accordingly to get the program to replicate the observed results, the resulting BC will be flawed by any errors in the G1 drag model used in the ballistic program, but on the other hand, it will work well with that ballistic program.

-----

All that said, at a practical level the difference due to the BC in those low BC ranges is minimal. For example, the difference in 200 yard trajectory for a .452 diameter bullet at 1,300 fps with a .210 BC and a .235 BC is minimal. With a 100 yd zero and a 0.7 height over bore, the drop at 200 yards is 13.1 MOA with the .210 BC and 13.0 MOA with the .235 BC. That's a 0.22" difference - some thing that will be lost in the noise of normal shot dispersion.

[Rooster59]

Well guys, your comments are most appreciated. As in any "science" I've been exposed to, this one is as clear as mud also. Actually, I should say it is clear but never exact.

The trajectories results from using Hornady's online ballistics calculator did show that neither bullet has much of an advantage as far out as 500-600 yards. Both seem to hit near the other so closely as to make my shooting skills the primary limiting factor in hitting the intended target.

Once I get more experience under my belt with both I'll have to come back and revisit this to find out how you are arriving at a calculated BC. I don't have any ballistics software. I'm using online calculators like Hornady's to give me some perspective. That has enabled me to determine what length MVA vernier tang sight staff to purchase for the 45-70. It has also assisted me in determining if my 45 trapper can serve as a 200 yard chicken-dinger at our club.

Just for the record, I'll have to redo my trajectory numbers on the 452-300 sized to .454". My load with 20.5gr of Lil Gun averaged 1,469fps and was pretty accurate even with the semi-buckhorn ladder rear sight. And yes, after 20 rounds even today I can still tell exactly where I placed the trapper's butt stock in my shoulder.