CJW Posted October 12, 2014 Share Posted October 12, 2014 I have been trying out some different ammunition in my 3-Gun rifle. Something odd came up and I am in search of an explanation or an appropriate clue-by-four upside the head. Basic setup: rifle gas 18" Carbon-Wilson barrel; Super Sniper 1-6x scope; reticle height 2.75". I am zeroed at 50 yards with Black Hills 69gr. I have run several different types of ammo (various 55gr FMJ in .223 and 5.56, several different 69gr BTHP) and they all run to within +/- 1 inch at 200 yards and to within an inch or so of -1mil at 300 yards. Additionally, the ballistic chart I use is right on for -4.75 mil at 600 yards (as verified with both BH 69gr and Atlanta Arms 55gr on an electronic 600 yard F-Class target). So far, so good. The Prvi 75gr is on at 50 yards, on at 200 yards...and hits at -1.5mil at 300 yards. Furthermore, I shot it at the 600 yard F-Class target and put most of a 10-round group in the 9 ring using a 5 mil holdover. I do not have chrono data for this ammo. The problem is that there is no version of a ballistics chart I can run (varying velocity and G1) that shows close to zero at 200 yards, right on 5 mil at 600 yards, and anything other than 1 mil (like the rest of the ammo I have tried) at 300 yards. I have verified the -1.5mil @ 300yds on two different days (both very good weather with minimal wind) with multiple groups. It does not appear to me to be shooter error. What I have observed does not seem possible, so I have to be missing something. Help! Link to comment Share on other sites More sharing options...
StravIs09 Posted October 12, 2014 Share Posted October 12, 2014 It must being going very very slow for 75gr ammo. like 2350? Idk i am probably wrong but just a crazy idea. do you have lots of berms at varying ranges like a small berm at 50, 100, 200, 300, 500, 600? that could (maybe) have something to do with the wind and air pressure flowing over the peaks of the berms. ? OR your ammo is doing 2350fps. or your scope is not perfectly tracking mils or the latter is probably a more accurate answer.. Sam Link to comment Share on other sites More sharing options...
TonytheTiger Posted October 12, 2014 Share Posted October 12, 2014 Prvi 75 is known for being slow. Like 2500 or so from a 20" barrel slow. It very well could be under 2400 fps. Link to comment Share on other sites More sharing options...
CJW Posted October 12, 2014 Author Share Posted October 12, 2014 Scope matches drop table for other ammos I have tried. The 600 yard range is a different place than the 200/300 yard, but the other rounds don't indicate any massive atmospheric differences between the two. When I diddle the velocity or BC down enough to match the 300yd data, then I get charts that tell me I should be -5.5 to -6mil at 600 yards. So, that's the anomaly--I can fit two out of three points of observed data, just not all three. Link to comment Share on other sites More sharing options...
ku1185 Posted October 14, 2014 Share Posted October 14, 2014 I'm sorry if this post is completely off track or if I'm missing something, but could the twist rate of your barrel have something to do with it? So as to cause the bullet to lose stability once it dips below a certain velocity? I'm no ballistics expert but twist rates and bullet weights are something I've been looking into and thought maybe that could explain your anomaly. Link to comment Share on other sites More sharing options...
CJW Posted October 14, 2014 Author Share Posted October 14, 2014 It's a 1:8 twist, which should be fine for a 75gr bullet, and I am not seeing other indications of instability—that is, my groups at each range tested (200, 300, and 600 yards) are both good and consistent. Link to comment Share on other sites More sharing options...
Cap Posted October 14, 2014 Share Posted October 14, 2014 I shot a fair amount of that ammo before I sold the rest of it off. I wouldn't be surprised if it really was only doing 2300fps. I am surprised its grouping that well for you Link to comment Share on other sites More sharing options...
DWFAN Posted October 14, 2014 Share Posted October 14, 2014 Can you play with the BC instead of velocity and get close to the 3 ? Link to comment Share on other sites More sharing options...
Rob Tompkins Posted October 14, 2014 Share Posted October 14, 2014 (edited) While we often treat BC a fixed constant number it really is non-linear and velocity dependent. http://en.wikipedia.org/wiki/Ballistic_coefficient#Differing_mathematical_models_and_bullet_ballistic_coefficients The AVERAGE BC you're using maybe quite different for parts of the bullets flight. EDIT TO ADD: Twist rate may also have an effect. Depending on how effectivly the bullet is stabilized (also a function of how close the bullet CG is relative to the center line and there aerodynamic center of pressure) the more or less yaw there is and that also effects the drag. This is also very dynamic. Edited October 14, 2014 by Rob Tompkins Link to comment Share on other sites More sharing options...
CJW Posted October 14, 2014 Author Share Posted October 14, 2014 While we often treat BC a fixed constant number it really is non-linear and velocity dependent. http://en.wikipedia.org/wiki/Ballistic_coefficient#Differing_mathematical_models_and_bullet_ballistic_coefficients The AVERAGE BC you're using maybe quite different for parts of the bullets flight. Rob, thanks, that is a good point. I would wonder, though (purely out of ignorance rather than argumentativeness), if that is a real world issue at the ranges we are talking about (with the anomaly being at 300 yards). Do people really see that much variance in BC in .223 bullets in those ranges...? By the way, as for BC, I have seen everything from .31 to .404 claimed for the Prvi 75gr bullet. (The .404 is from Prvi's website, the .31–.33 range was thrown around on AR15.com as a reverse engineered G1 to get a table to fit actual observed performance out to a couple hundred yards, and other sources gave values in the .34–.36 range.) So, using a single value fit, I can get from dead-on at 200 yards to -1.5mil at 300 yards by lowering either the BC or the velocity (of course); I just then don't get a chart that is within a mil of observed performance at 600 yards. And, of course, if the BC or velocity gives me the observed -5mil at 600 yards, well, then, it insists in -1mil at 300! I wish I had access to a range with other distance berms so I could get more data points... Link to comment Share on other sites More sharing options...
DWFAN Posted October 14, 2014 Share Posted October 14, 2014 (edited) I can get your 50 to 300 and 300 to 600 to match up. Where I find the difference is the 200 yard zero. In most cases, the program wants to put your 2nd 0 around 150-160 yards. The 9 ring on the 600 yd f-class target is .5 mil if my calcs are correct, so you have some variance on that end to play with as well. Meaning your 600 yd zero could really be somewhere around 4.75 to 5.25 or even farther out from 5 mil. Especially if your scope holdover isnt quite a perfect 5 mil. Edited October 14, 2014 by DWFAN Link to comment Share on other sites More sharing options...
Rob Tompkins Posted October 14, 2014 Share Posted October 14, 2014 While we often treat BC a fixed constant number it really is non-linear and velocity dependent. http://en.wikipedia.org/wiki/Ballistic_coefficient#Differing_mathematical_models_and_bullet_ballistic_coefficients The AVERAGE BC you're using maybe quite different for parts of the bullets flight. Rob, thanks, that is a good point. I would wonder, though (purely out of ignorance rather than argumentativeness), if that is a real world issue at the ranges we are talking about (with the anomaly being at 300 yards). Do people really see that much variance in BC in .223 bullets in those ranges...? By the way, as for BC, I have seen everything from .31 to .404 claimed for the Prvi 75gr bullet. (The .404 is from Prvi's website, the .31–.33 range was thrown around on AR15.com as a reverse engineered G1 to get a table to fit actual observed performance out to a couple hundred yards, and other sources gave values in the .34–.36 range.) So, using a single value fit, I can get from dead-on at 200 yards to -1.5mil at 300 yards by lowering either the BC or the velocity (of course); I just then don't get a chart that is within a mil of observed performance at 600 yards. And, of course, if the BC or velocity gives me the observed -5mil at 600 yards, well, then, it insists in -1mil at 300! I wish I had access to a range with other distance berms so I could get more data points... In short, to anwswer your queston, I think yes there can be a lot of variation for 223 bullets. Out to 600 yards 223 bullets (as a general class) change velocity the most, as compared to larger bullets with the same BC's. Link to comment Share on other sites More sharing options...
Stlhead Posted October 14, 2014 Share Posted October 14, 2014 Without a crono this entire discussion is a waste of effort. Find out how fast they are actually going. Right now you are trying to determine the ballistic path with an estimated velocity AND an estimated BC. As you remove variables from the equation things will get far easier to sort out. Link to comment Share on other sites More sharing options...
dauntedfuture Posted October 14, 2014 Share Posted October 14, 2014 Its also entirely possible that your scope is not adjusting true. F class targets are also very small. If your group was only 1 MOA low at 600 yds, or in the 9 ring low, then I would say you were very very close. that F class target is very hard. The entire 10 ring is 1 MOA. I don't think you are as far off as you might think. From shooting highpower at a consistent 600 yds I know that my zero from range to range and day to day can vary .5 to 1 MIN or more. Link to comment Share on other sites More sharing options...
CJW Posted October 14, 2014 Author Share Posted October 14, 2014 Its also entirely possible that your scope is not adjusting true. F class targets are also very small. If your group was only 1 MOA low at 600 yds, or in the 9 ring low, then I would say you were very very close. that F class target is very hard. The entire 10 ring is 1 MOA. I don't think you are as far off as you might think. From shooting highpower at a consistent 600 yds I know that my zero from range to range and day to day can vary .5 to 1 MIN or more. I am not adjusting; I am holding over using the reticle. The reticle holds were exactly to chart at 600 yards for the other ammo types I tested, so I don't have any reason to think that the observed -5mil holdover for 9 ring hits at 600 yards is untrustworthy. I started by holding the same -4.75mil I had just verified on my BH 69gr, and the rounds were definitely low, so I moved to a -5mil holdover. At that range my hits were in a nice circular distribution around the "X", so it wasn't a biased offset. More importantly, the issue isn't really the 600 yard performance—it is the 300 yard performance. Twiddling the 600 yard data up or down a few inches doesn't help—for the extra half mil of drop I am seeing at 300 yards to carry through the rest of the projected track, I would be off by almost two feet at 600 yards. So, your 1 MOA of variance is about .3mil—I need to up that leeway to almost 4 MOA, while still holding a 9 ring group, to discount the 600 yard data. Link to comment Share on other sites More sharing options...
CJW Posted October 14, 2014 Author Share Posted October 14, 2014 I shot a fair amount of that ammo before I sold the rest of it off. I wouldn't be surprised if it really was only doing 2300fps. I am surprised its grouping that well for you That's the thing—it does group great. That's why I am banging my head to try and figure out the ballistics; a 1.5MOA group at range would be great, but not so much if I can't figure out the holdovers all the way out. *sigh* Link to comment Share on other sites More sharing options...
ToddKS Posted October 14, 2014 Share Posted October 14, 2014 Why not just use your observed data. As long as the result is consistent why does it matter what the ballistic calculator says? Link to comment Share on other sites More sharing options...
CJW Posted October 20, 2014 Author Share Posted October 20, 2014 Why not just use your observed data. As long as the result is consistent why does it matter what the ballistic calculator says? Because if I can't get a chart to fit it, I can not be sure what the round is doing at the ranges where I can't get observational data. I have no place where I can set up ranging targets anywhere between 300 and 600 yards, so with the weirdness right at 300 I would be in the dark on a 400yd target, Link to comment Share on other sites More sharing options...
dauntedfuture Posted October 21, 2014 Share Posted October 21, 2014 OK. I think you are using a MIL reticle. Just make your own ballistic charts from data at JBM. You can enter your 69g data or the privy 75g data. If you cant find the privy 75g bullet use the Hornady 75. You will need access to a chronograph. If your scope had a BDC reticle I could see your concern, and in truth all the reticles are going to be close but not dead on at range based on specific ammo etc, in general a BDC reticle will be close enough. If you have a MIL dot reticle you just need MIL holdover or come -ups at range. The only way to verify this is to shoot at each range; since this is usually tough, I zero at 200 and its been good enough to work for three gun. If you are serious about shooting some medium range stuff it makes sense to have a chronograph of your own. For reference from rom another site: Privi 75g velocity from 16" is 2468. I got this from a simple google search. Link to comment Share on other sites More sharing options...
PacMan Posted October 24, 2014 Share Posted October 24, 2014 (edited) Without a crono this entire discussion is a waste of effort. Find out how fast they are actually going. Right now you are trying to determine the ballistic path with an estimated velocity AND an estimated BC. As you remove variables from the equation things will get far easier to sort out. I was wondering where the chrono data is but you got to it first. Without knowing how fast the bullet is going out of the muzzle in a particular barrel, might as well not waste your ammo and time. Sorry but spend some $ on a chronograph or borrow one. Edited October 24, 2014 by PacMan Link to comment Share on other sites More sharing options...
CJW Posted October 24, 2014 Author Share Posted October 24, 2014 As for the question of chrono data... Look, we are talking about variables to fit a given (fixed) formula that only has so many degrees of freedom. Let's assume that scope height, zero distance, and bullet weight are already known; that's the easy part. At this point, to fit a ballistic curve, you have exactly two degrees of freedom. One way to solve that problem is (1) ballistic coefficient and (2) muzzle velocity. Well, the way math works, I can substitute two other known values in order to solve the problem, as well. In my case, that would be ballistic drop at two other known distances. The point that I think you are missing is that there is no possible value for muzzle velocity that makes the observed impact data at three observed ranges consistently fit a single ballistic curve. That is why it is an anomaly. I could tell you right now that "the bullets chrono'ed at 2500fps" or "2400fps" or "2673fps" and that still would not explain why I have impacts that are either (depending on how you look at it) 1 half mil too low at 300yds or a full mil too high at 600yds. Link to comment Share on other sites More sharing options...
DocMedic Posted October 24, 2014 Share Posted October 24, 2014 (edited) What scope? Edited October 24, 2014 by DocMedic Link to comment Share on other sites More sharing options...
shootsforfun Posted October 25, 2014 Share Posted October 25, 2014 Has anyone been able to find a good bc to use on the pp75 as that's what I load with and can't find and good bc info Link to comment Share on other sites More sharing options...
CJW Posted October 27, 2014 Author Share Posted October 27, 2014 My scope is a Super Sniper 1-6x FFP. All distance/zeroing shots taken at full zoom. shootsforfun, that is one of the interesting problems. The PP website claims (no kidding ) a BC of 0.404 for the 75gr match and I don't think that is even close. Their website apparently used to claim something more like 0.36. A shooter on AR15.com used chrono and range data and then tried to 'fit' a BC that would produce those results, and he turned up something in the 0.31–0.33 range. By comparison, a 69gr MatchKing is around 0.31 and a 77gr MatchKing is around 0.36. Link to comment Share on other sites More sharing options...
grouse Posted October 28, 2014 Share Posted October 28, 2014 Green or blue box? Link to comment Share on other sites More sharing options...
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