Is 500yd enough to true BC's

[Monday shooter40]

Pretty basic. Is 500 yds enough to true BC's? I ran a test today. 500yd. Berger140 LRBT. 2783FPS. I entered all my environmentals. Data called for 2.71mils of elevation. I dialed. Gave it my best attempt (mirage was brutal) And my group center was 1.25" under my aim point. Group displayed 1.5" of vertical with a 10mph wind from 5 o'clock

If I was to adjust the BC to correct my impact up, I would have to correct it from the advertised .304 G7 to ~.280G7 That seems like a lot to me, or is it not?

 

[Full Chisel]

I hate to even ask this, but have you actually lasered the distance? My 600 range is 605 from prone, 608 from bench. Our 200 and 300 benches are both 6 yards short of that because the old target hangers are where they originally measured from.

 

[Monday shooter40]

499.7 yds.

 

[Full Chisel]

Like I said, I hate to ask.

 

[stevenpoplin]

I did that with the new Sierra 169CN MKs. I dino't know if 500 is enough. I did 500, 700, 920 to try to back into a G7 BC. I came up with around that much lee from others doing the same as you did so I don't think that's too much. Also the manufacturers addvertised BC is often embeished some how. I would suggest try to get it to match a curv on multiple distances and don't foget to includ everything you can think of. like barrel twist.
Good Luck

 

[SKWERLZ]

I typically use speed to adjust out to 600 and BC for further.

that’s a huge change at that short of distance, I’d suspect that result.

 

[CharlieNC]

Given your bad mirage condition. I would not rely on that single result as the absolute truth to use for truing, and it's only one factor which could change next time! I keep a dope log, and average the results from several range sessions for truing.

 

[Monday shooter40]

I typically use speed to adjust out to 600 and BC for further.

that’s a huge change at that short of distance, I’d suspect that result.

I shot it across my magneto speed during the test, so im not questioning my speed. Magneto is chassis mounted so it does not change my POI. Stats were: SD of 5.3 ES 13 over 5 shots if anyone is wondering.

 

[SKWERLZ]

I shot it across my magneto speed during the test, so im not questioning my speed. Magneto is chassis mounted so it does not change my POI. Stats were: SD of 5.3 ES 13 over 5 shots if anyone is wondering.

I have a magnetospeed as well and I still adjust speed if needed.

unless you get absolutely epic conditions truing usually takes a few trips to get data.

 

[FeMan]

I am with SLWERLZ and true with velocity out to 600 and then 800 and beyond for BC. Magnus effect from a 5-oclock wind will raise your bullets above what the POI would otherwise be. That would make the disparity even worse.

Are you 100% sure of your zero? 1.25" is only a quarter moa. If you are a bullet diameter low on your zero that would explain it all. I have found Berger BCs to be close to advertised for the most part. Not true with all manufacturers.

 

[Monday shooter40]

I am with SLWERLZ and true with velocity out to 600 and then 800 and beyond for BC. Magnus effect from a 5-oclock wind will raise your bullets above what the POI would otherwise be. That would make the disparity even worse.

Are you 100% sure of your zero? 1.25" is only a quarter moa. If you are a bullet diameter low on your zero that would explain it all. I have found Berger BCs to be close to advertised for the most part. Not true with all manufacturers.

I too have found Berger's advertised BC's to work very well, which is why i am skeptical to adjust mine down so far in this instance. The advertised BC of .304 has kept me on plate out to 1K but i have noticed that my impacts and misses at extended ranges, tended to be on the low side. However, i can not stop a match to go check my plate impacts. This is the first time i am attempting to check data on a paper target. IF i was doing my normal data check on a steel target at 500 yards(which i often do) i would call this test good and run with it.

I am as close to Zero as my system will allow. I am running the Bushnell XRS3 which only allows for .1 mil adjustments. which as we all know is ~.36". My zero group was 5/8" and was covering my aim point both above and below, on my zero target.

And i understand the magnus effect, but if my ballistic calculator is adjusting for that, its sort of irrelevant, aside from wind gusts that would raise and lower my vertical.

I do not understand why someone would "true" their velocity, if in fact, i know exactly what my velocity was during the given test. That would make it "not true", and during a match, i am not going to use 2 or more different velocities for one set of ranges and then 2 different BC's for 2 different ranges. That would be a data nightmare.

Im not saying that changing the velocity to reach the desired data correction does not work, but thats not how i will be doing my data corrections.

 

[Nealm66]

I’d double check my poi at 200 if conditions allow. I bet it’s a tiny bit low. I find 200 tracks better for me, 100 has fooled me both in groups and tracking. I know there are guys with the skills to get it done at 100 but 200 sure helps me

 

[dmoran]

@Monday shooter40

Dr. Arthur Pejsa (Pejsa Ballistics) advised to test at multiple distances, ending at a distance near transonic region.
Myself use at least 3 distances for obtaining true BC values per barrel/bullet combination.

 

[CharlieNC]

So your group was 1.5" vertical spread and 1.25" low, meaning some shots were at the calculated zero. You stated the mirage was strong, and you can bet this affected the point of impact. Your rest, rear bag, recoil control, and a host of other factors also suggest 1.25" is not THE EXACT AMOUNT you were off center. To be on the low side of the steel plate at 1000yd does not require much of a velocity adjustment to account for the few tenths of a mil you are experiencing, which may also be within the measured SD from your magnetospeed.

 

[Jarhead6870]

I do not understand why someone would "true" their velocity, if in fact, i know exactly what my velocity was during the given test.

Chronographs, just like scales, can have a margin of error.

This said, there are other factors in play when verifying data/dope. The basics in particular order: all inputs are absolutely correct (this includes scope height, zero distance and zero height), exact distance to the target is known and scope has been tested to insure mechanical deviations in the turrets are considered and a correction factor entered. The checking of the scope's ability to track to the exact value of the turrets can be done using a tall target test, although there are jigs that one can use to accomplish the same thing. When doing a tall target test, I do mine at 100 yards, the distance from the turrets to the target MUST BE EXACT. If you are off a yard, the information will be tainted and be "just another bad input".

After I develop a load and have decided on a charge weight, I will do a tall target test. This consists of 9-10 shots over a chronograph while doing the test. This lets me check, or double check, the scope's tracking and obtain my MV at the same time. I've been fortunate that all of my scopes have tracked well but I've had students with some that were pathetic and had to be sent back to the factory for calibration. Some amount of error can be compensated for in your ballistic program but if grossly off, send the scope in and have it checked.

After getting a MV and checking the scope, I head to the range. As mentioned above, many shooters, myself included, true their MV at 600-800 yards (it needs to be a distance where the bullet is STILL supersonic) and then true their BC at 1000 yards or farther. BCs change as velocity changes and is more critical farther downrange as the bullet slows down. Guys that are really into checking their BCs sometimes use the Shot Marker system downrange (1200-1500 yards) and get their MV at the target and adjust their BC accordingly. Most of use don't have that luxury so we rely on POI on the target which usually works well enough.

REMEMBER, the target (or bullet) doesn't lie.

 

[46and2]

Pretty basic. Is 500 yds enough to true BC's? I ran a test today. 500yd. Berger140 LRBT. 2783FPS. I entered all my environmentals. Data called for 2.71mils of elevation. I dialed. Gave it my best attempt (mirage was brutal) And my group center was 1.25" under my aim point. Group displayed 1.5" of vertical with a 10mph wind from 5 o'clock

If I was to adjust the BC to correct my impact up, I would have to correct it from the advertised .304 G7 to ~.280G7 That seems like a lot to me, or is it not?

Can you borrow a Labradar? You can get your actual BC for your actual bullet from your barrel.
I do this frequently, make sure to actually measure your lot of bullets OAL. Try running the onfo from Labradar, your current atmospheric readings into JBM.
I find some of the manufacturers listed BC is wrong for the velocity and twist I'm running.

 

[Monday shooter40]

Thank you everyone for your info and input. I am gathering that 500yds is not quite enough to gather Accurate BC data for this bullet and speed. I didnt want to do this at 500 yds, but cows were blocking my access to 800yds. i ran the test anyways at 500. Data is always better to have than not. thanks again guys.

 

[Superjet]

Kestral recommends truing at 85-90% of your supersonic range. This applies whether or not you are using their product. That said, if you are trued up at 500 you should do fine at a match. You can always make an adjustment on match day if you have a long prone stage and can spot your hits.

 

[mac86951]

Question, what do you mean by true BC's? By their definition, they are coefficients and we adjust them to make the equation results more accurate; however, the model may be inefficient to begin with.

The longer the distance the better you're assessing "average" BCs of whatever your model is. Accurate BCs really need a custom model to include your specific shooting conditions.

Going back to "average" BCs, if you're shooting in similar conditions, and trying to make a door chart for long range, the next question is are you shooting far enough for transonic region to play a role, that will really mess with your averages based on your chosen construction of bullet, velocity, and shooting conditions. My general recommendation is to collect drop information as far out as you can while ensuring you're above the transonic velocity and averaging the BCs there for shorter range work; and then collecting data by target groups and drop measurements beyond that for your reference.

Once you have your average BC, you'll find that most bullets have a higher BC at higher velocity meaning impact higher than the calculated solution.

How much error? Well, let's look at a Sierra 107gr in 6mm; assuming published data is exactly how your rifle shoots, it'll start at .547, and filling in for an arbitrary cartridge that is 1700fps at 1000y, the BC is .529.

Collecting drop data at 1000y let's assume again that you reverse calculate a result of a BC value of .532. plug in .532 for a short range target vs. .547 and you'll see how off your POI calculation is.

My preference is still an as tested dope chart with a hot day and a cold day and from that "table" you can make an educated swag at the dope around those two tables and measured or ranged distance. That "table" is the actual solutions for all the averages and unknowns on those days. If you're trying to hit an IPSC at unknown distance this'll get you there, you make the wind call; trying to hit a grape at unknown distance; well, we're all trying to get that good, several of us will settle for all X at 600y+

 

[PracticalTactical]

Pretty basic. Is 500 yds enough to true BC's? I ran a test today. 500yd. Berger140 LRBT. 2783FPS. I entered all my environmentals. Data called for 2.71mils of elevation. I dialed. Gave it my best attempt (mirage was brutal) And my group center was 1.25" under my aim point. Group displayed 1.5" of vertical with a 10mph wind from 5 o'clock

If I was to adjust the BC to correct my impact up, I would have to correct it from the advertised .304 G7 to ~.280G7 That seems like a lot to me, or is it not?

The 10 mph 5 o'clock wind is a half value wind, so treat it like 5 mph full value... That will blow you high and left. If you hit below your aim point, then you need to add the 1.5 inches to the vertical effect of the 5 mph cross wind. So you are probably more like 2.5 to 3 inches low.

I have seen multiple videos on YouTube where the advice provided for truing ignores the effect of wind and that is simply bad advice. We absolutely need to consider the vertical affect of wind in order to accurately tune your velocity and BC.

A wind from 3 o'clock will blow the bullet along a line toward about 9:30 or so. The exact angle may vary slightly based upon distance, bullet length and spin rate. At the very least you should calculate what angle that line is calculated to be at your test distance using your Kestrel or ballistics solver. (Assuming it factors for aerodynamic jump). You should know that the angle is closer to 45 degrees at 100 yards and closer to 15 degrees at 600 yards.

So when you hit low and right along a 3:30 line, because of wind, you may very well have everything perfectly configured. If you hit high and left along a 9:30 line because of wind, again, you probably have velocity and BC properly configured.

As for trueing out to only 500 yards, well you would be true to 500 yards. That is at least a good place to start. I would still try and get out farther if the opportunity arises.