Ballistic coefficient

[19Rams]

IDK if this is the correct forum to post this, but what are the experiences of others shooting longer distance with cast and trying to follow a ballistics chart? I'm shooting a Lyman 457132 and working on reaching out a bit. Looking at the charts in the Lyman 3rd edition cast manual, it shows a BC of .384 and a drop of about 96 inches from 300 to 400 yards with a MV of 1200 fps. I've seen more current publications saying that bullet has a BC of .402. Plugging that into a calculator shows a drop at the same distances of 70 inches. Finally getting sighted in had a true bullet drop of 116 inches, which is closer to the old data. I know the BC is not an exact science and other conditions can affect bullet drop too. I'm loading a starting load of 4755. The temp is 20F and humidity is low. The square root of 4 = 2. Am I over thinking things? The bottom line is the BC is probably closer to the older data. What are others experiences?

[mehavey]

My experience?
Since getting a LABRADAR that allows calculation of actual BCs based on velocity
measurements over extended ranges -- book BCs are waaaay over estimated.

[Win94ae]

IDK if this is the correct forum to post this, but what are the experiences of others shooting longer distance with cast and trying to follow a ballistics chart? I'm shooting a Lyman 457132 and working on reaching out a bit. Looking at the charts in the Lyman 3rd edition cast manual, it shows a BC of .384 and a drop of about 96 inches from 300 to 400 yards with a MV of 1200 fps. I've seen more current publications saying that bullet has a BC of .402. Plugging that into a calculator shows a drop at the same distances of 70 inches. Finally getting sighted in had a true bullet drop of 116 inches, which is closer to the old data. I know the BC is not an exact science and other conditions can affect bullet drop too. I'm loading a starting load of 4755. The temp is 20F and humidity is low. The square root of 4 = 2. Am I over thinking things? The bottom line is the BC is probably closer to the older data. What are others experiences?

I go by the BC to make my drop chart; then record the actual data on the other side of the card, after observing he POIs at 50 yard intervals.

[Bad Ass Wallace]

I don't rely on ballistic charts but actual shooting on range. I use a 545gn PGT boolit in my 45/90 and there is no other that is so accurate with less wind drift.

[Larry Gibson]

My experience?
Since getting a LABRADAR that allows calculation of actual BCs based on velocity
measurements over extended ranges -- book BCs are waaaay over estimated.

Quite correct; computer generated and estimated BCs of cast bullets are most often very optimistic. I measure the BC of bullets actually fired via an Oehler M43 PBL which can actually measure the time of flight to 100 yards for each bullet fired. Computer generated or guestimated BCs do not take into account the additional drag caused by the lube grooves or the degree of stability the bullet may have on launch. The more stable the bullet is on launch with a given load at a given velocity the higher the BC will be because there is less drag on the bullet and the time of flight will be less.

However, they can also be under estimated. For example Lyman lists the BC for the 311291 at .202 in the CBH #3. However, the actual measured BC of that bullet at 1761 fps/125,690 RPM ran .244. At 1912 fps/136,400 RPM the BC ran .235 . At 2049 fps/146,200 RPM the BC ran .228. at 2297 fps/163,800 RPM the BC dropped to .216. At 2515 fps/179,400 RPM the BC dropped to .204. Thus we see with the 10" twist barrel with the RPM below the RPM Threshold we had an excellent measured BC. As the RPM approached and went over the RPM Threshold the measured BC went down considerably indicating the the increasing instability. The increasing group size on target also indicated the decrease of stability as the RPM increased over and above the RPM Threshold.

[mehavey]

Correctomundo. Drag changes with velocity range.
Higher velocities generate higher drag proportional to the velocity squared.

So a BC is only "really" good in the measured velocity range.

[MOC031]

Am I over thinking things? The bottom line is the BC is probably closer to the older data. What are others experiences?

Perhaps, more accurately, not looking at the whole picture?

Exhibit one is Brian Litz(?) work has shown how inaccurate published BCs can be.

Exhibit two is BC's are usually published using the G1 model rather than a more accurate model like G7 or others.

Exhibit three is looking at Sierra reloading manuals and noticing that the BC changes as velocity changes.

I think most of us getting wrapped around the axle would be far, far more ahead if we spent the same amount of time just going to the range and measuring how the drop of our chosen load changes at each range increment out to as far as we intend to shoot.

I suggest the resulting dope card with come-ups you can make up for that cast bullet load is going to be far more accurate than a computer generated drop chart from a website like JBM, using whatever ballistic coefficient you decide to use. Go ahead, ask me how I know...

You can even use that dope card to take a shot at calculating your own BC.

[megasupermagnum]

Exhibit two is BC's are usually published using the G1 model rather than a more accurate model like G7 or others.

The G7 model is a better model for modern, very sleek boattail jacketed bulllets. The G1 model looks pretty close to most cast rifle bullets.

[dtknowles]

Using a predictive ballistics model can help you get on target for building your custom dope card. You will need to refine your BC estimates so that you can dope the wind too.

If you did not have some sort of drop estimate, how big a target would you need to put at 500 yards to make sure you hit the paper. Regarding wind drift, if you don't have a pretty good estimate of the BC you are just out of luck. It is not like you can test in a range of known wind conditions. Well I guess you could, maybe, I don't know, seems unlikely.

Tim

[fcvan]

Using a predictive ballistics model can help you get on target for building your custom dope card. You will need to refine your BC estimates so that you can dope the wind too.

If you did not have some sort of drop estimate, how big a target would you need to put at 500 yards to make sure you hit the paper. Regarding wind drift, if you don't have a pretty good estimate of the BC you are just out of luck. It is not like you can test in a range of known wind conditions. Well I guess you could, maybe, I don't know, seems unlikely.

Tim

You are absolutely correct. During the infancy of the Bench Rest Rifle shooting, a 300 yard range was set up in a large warehouse, controlling wind, temp, etc. as loads were developed. The handful of shooters using this range did so at night when nobody was there but them. An interesting read, but I would have to do some searching to find the article again. Point was, they could remove all of those variables to develop 'the load.' They would then adjust for actual range conditions like wind speed/direction, altitude, humidity and such. Pioneers of that level of precision shooting they surely were.

[charlie b]

Yes, the book BC will give you the data to get on paper, which you then adjust. It also gives you a gross estimate of windage corrections you may need, which also have to be refined by shooting. Yes, we do get a variety of wind conditions, some of which can be used to refine the dope card.

And, no, it does not take a really accurate starting number for the BC. I've used a BC that was 0.05 off (based on quite a few labradar measurements) and it still puts bullets on paper out to 600yd.