Cant get velocity to match bc and come up.

[chuckbuster243]

This past weekend i shot again at AIM rifle club in Blakely, GA on electronic targets. These targets show speed on arrival at the target. I shot 2 loads from my 308 one being loaded with 175 Nosler custom comps and the other with 185 Hybrid Bergers. The funny thing is with the velocity at the target used with bc and come ups the berger matched perfect with a couple different calculators that I used. But I cant seem to find a combination that matched even close to come up speed at target, MV or bc with the 175's. The last match in the fall I shot my 284 with 180 Hybrids and was able to match things up as well. This load with the 175's has me at a loss.

175ncc FC case 44.5 Varget CCI 200 last time I checked mv I was around 2725. On this day it took 34.5 min@1000 and target velocity was 1200 avg.

Somebody explain this. Thanks, Matt

 

[chkunz]

You did not give enough information to properly answer your question (for example what ballistics program and what BC, G1, G7?). I just read the Brian Litz book, "Applied Ballistics for Long Range Shooting". I would recommend that you buy this book if you do not have it. Read it and use the proper BC and the ballistics program provided and you may find that your have the answer to your question. Also, muzzle velocity is important and folks seem to get varying results with chronographs since we have no convenient way to calibrate them.

 

[chuckbuster243]

You did not give enough information to properly answer your question (for example what ballistics program and what BC, G1, G7?). I just read the Brian Litz book, "Applied Ballistics for Long Range Shooting". I would recommend that you buy this book if you do not have it. Read it and use the proper BC and the ballistics program provided and you may find that your have the answer to your question. Also, muzzle velocity is important and folks seem to get varying results with chronographs since we have no convenient way to calibrate them.

I follow...
I used both the Berger, strelok and Hornady Ballistics app and they all were close to the same numbers at the end. I used the G1 of .505 Muzzle velocity of 2725 and standard everything else. My actual come up was 34.5 min from a 100 yard zero. The the electronic target showed that the bullet averaged 1200 fps on pass through at 1k. According to the apps the speed should have been in the 1280+ range. I dont think it was that off at the target that much just because it was dead on for the other load that I shot with the 185 hybrids. I know that I could have some error in my mv but I dont think it would be that much. I just cant seem to figure why my muzzle velocity and come up are so close but velocity was so slow at the target. What was the G7 bc shown for this bullet? Thanks again, Matt

 

[chkunz]

You did not give enough information to properly answer your question (for example what ballistics program and what BC, G1, G7?). I just read the Brian Litz book, "Applied Ballistics for Long Range Shooting". I would recommend that you buy this book if you do not have it. Read it and use the proper BC and the ballistics program provided and you may find that your have the answer to your question. Also, muzzle velocity is important and folks seem to get varying results with chronographs since we have no convenient way to calibrate them.

I follow...
I used both the Berger, strelok and Hornady Ballistics app and they all were close to the same numbers at the end. I used the G1 of .505 Muzzle velocity of 2725 and standard everything else. My actual come up was 34.5 min from a 100 yard zero. The the electronic target showed that the bullet averaged 1200 fps on pass through at 1k. According to the apps the speed should have been in the 1280+ range. I dont think it was that off at the target that much just because it was dead on for the other load that I shot with the 185 hybrids. I know that I could have some error in my mv but I dont think it would be that much. I just cant seem to figure why my muzzle velocity and come up are so close but velocity was so slow at the target. What was the G7 bc shown for this bullet? Thanks again, Matt

The book lists information for 175 bullets but does not list data for the 175 Nosler Custom Comp. I understand that Bryan now has data for more bullets that may include this bullet or another bullet like the 175 Sierra Match King may be a close match. Either way you need to use a ballistics program that properly uses the G7 standard projectile curve. The Applied Ballistic program that comes with the book will do this and Bryan also says the program is available free on the web. I have not personally used this data and the program; as of now my interest is purely academic but I did stay in a Holiday Inn once.

 

[chuckbuster243]

You did not give enough information to properly answer your question (for example what ballistics program and what BC, G1, G7?). I just read the Brian Litz book, "Applied Ballistics for Long Range Shooting". I would recommend that you buy this book if you do not have it. Read it and use the proper BC and the ballistics program provided and you may find that your have the answer to your question. Also, muzzle velocity is important and folks seem to get varying results with chronographs since we have no convenient way to calibrate them.

This bullet matches the 175 smk pretty close. I guess i could plug in the 175 smk g7 and see if its closer. I just wanna make as accurate of a drop chart as possible. Thanks, Matt

I follow...
I used both the Berger, strelok and Hornady Ballistics app and they all were close to the same numbers at the end. I used the G1 of .505 Muzzle velocity of 2725 and standard everything else. My actual come up was 34.5 min from a 100 yard zero. The the electronic target showed that the bullet averaged 1200 fps on pass through at 1k. According to the apps the speed should have been in the 1280+ range. I dont think it was that off at the target that much just because it was dead on for the other load that I shot with the 185 hybrids. I know that I could have some error in my mv but I dont think it would be that much. I just cant seem to figure why my muzzle velocity and come up are so close but velocity was so slow at the target. What was the G7 bc shown for this bullet? Thanks again, Matt

The book lists information for 175 bullets but does not list data for the 175 Nosler Custom Comp. I understand that Bryan now has data for more bullets that may include this bullet or another bullet like the 175 Sierra Match King may be a close match. Either way you need to use a ballistics program that properly uses the G7 standard projectile curve. The Applied Ballistic program that comes with the book will do this and Bryan also says the program is available free on the web. I have not personally used this data and the program; as of now my interest is purely academic but I did stay in a Holiday Inn once.

 

[jrm850]

I plugged your info into the applied ballistics app and came up with 35.1 minutes. My come up Sunday at Blakely ended up at 28.6m which this app just nailed.

 

[chuckbuster243]

I plugged your info into the applied ballistics app and came up with 35.1 minutes. My come up Sunday at Blakely ended up at 28.6m which this app just nailed.

Did the velocities match up?

 

[Laurie]

You are using a G1 BC which doesn't match the bullet form (shape). G1 dates from flat-base / round-nose days (the reference projectile it's based on has a very similar shape to that of a 40gn .22 Long Rifle LRN model).

The main problem with using a G1 BC with a modern streamlined HPBT type is that the BC varies according to speed with an up to 15% variation as the speed falls from ~3,000 fps to the speed of sound, the drag increasing and therefore the BC decreasing as velocities decrease. Sierra uses speed-banded BCs to get around this downside, but even that is not as good as using the G7 version, the G7 'reference projectile' being similar to a match boattailed rifle bullet.

Assuming that the Nosler 175 isn't too different a shape from that of the Sierra MK, Sierra gives its bullet the following three G1 BCs:

0.505 .... 2,800 fps and above
0.496 .... 1,800-2,800 fps
0.485 .... 1,800 and below

Even that will likely be optimistic for 1,000 yards as turbulence and hence drag increase as the speed drops below 1.2 MACH, and US Army tests back in .30-06 days with the old Frankford 173gn FMJBT showed a dramatic slowdown and poorer performance (precision plus wind drift) coming in ~100 fps above the speed of sound, 1,225 fps.

Run the 175gn Sierra MK and its G1 BCs through the Sierra Infinity program and it calculates 35.8-MOA come-up needed on a 100 yard zero and 1,238 fps retained velocity at 1K.

Using Bryan Litz's G7 BC of 0.243 and your 2,725 ps MV, his Ballistic Solver 2.0 program is more pessimistic, at 36.76 MOA and 1,159 fps.

Having shot the 175 SMK at 1,000 in the early days of FTR, I'd be inclined to take the poorer result!

All of these results are for standard ballistic conditions (29.92 inches Hg air pressure, 0% humidity and 59-deg F temperature). Variations from these can and will affect the predicted results.

As will .... scope height (above the bore centreline) being different from the default 1.5 inches used in nearly all ballistic programs.

As will .... your scope adjustment values not being accurate. (A 5-10% error in MOA is not at all unusual even in quality match scopes.)

As will .... the 1,000 yards firing line being more or less than 1,000 yards actual from the target. Many ranges adopted compromises arising from fitting in with terrain features, and many older ranges are none too precise in their distances for a variety of reasons.

As will .... even a relatively small error in readings from either your chronograph and/or that at the target. The usual two optical unit type set a mere 24 inches apart only need a small tilt away from being parallel to the bullet's path to give an erroneous reading even if the equipment is otherwise 100.000% accurate and most aren't that close. Plus a conventional chronograph set ahead of the muzzle should have its distance from the muzzle measured and the resulting velocity value corrected to a true at the muzzle value. (Not applicable to the Magnetospeed of course!)

 

[chuckbuster243]

You are using a G1 BC which doesn't match the bullet form (shape). G1 dates from flat-base / round-nose days (the reference projectile it's based on has a very similar shape to that of a 40gn .22 Long Rifle LRN model).

The main problem with using a G1 BC with a modern streamlined HPBT type is that the BC varies according to speed with an up to 15% variation as the speed falls from ~3,000 fps to the speed of sound, the drag increasing and therefore the BC decreasing as velocities decrease. Sierra uses speed-banded BCs to get around this downside, but even that is not as good as using the G7 version, the G7 'reference projectile' being similar to a match boattailed rifle bullet.

Assuming that the Nosler 175 isn't too different a shape from that of the Sierra MK, Sierra gives its bullet the following three G1 BCs:

0.505 .... 2,800 fps and above
0.496 .... 1,800-2,800 fps
0.485 .... 1,800 and below

Even that will likely be optimistic for 1,000 yards as turbulence and hence drag increase as the speed drops below 1.2 MACH, and US Army tests back in .30-06 days with the old Frankford 173gn FMJBT showed a dramatic slowdown and poorer performance (precision plus wind drift) coming in ~100 fps above the speed of sound, 1,225 fps.

Run the 175gn Sierra MK and its G1 BCs through the Sierra Infinity program and it calculates 35.8-MOA come-up needed on a 100 yard zero and 1,238 fps retained velocity at 1K.

Using Bryan Litz's G7 BC of 0.243 and your 2,725 ps MV, his Ballistic Solver 2.0 program is more pessimistic, at 36.76 MOA and 1,159 fps.

Having shot the 175 SMK at 1,000 in the early days of FTR, I'd be inclined to take the poorer result!

All of these results are for standard ballistic conditions (29.92 inches Hg air pressure, 0% humidity and 59-deg F temperature). Variations from these can and will affect the predicted results.

As will .... scope height (above the bore centreline) being different from the default 1.5 inches used in nearly all ballistic programs.

As will .... your scope adjustment values not being accurate. (A 5-10% error in MOA is not at all unusual even in quality match scopes.)

As will .... the 1,000 yards firing line being more or less than 1,000 yards actual from the target. Many ranges adopted compromises arising from fitting in with terrain features, and many older ranges are none too precise in their distances for a variety of reasons.

As will .... even a relatively small error in readings from either your chronograph and/or that at the target. The usual two optical unit type set a mere 24 inches apart only need a small tilt away from being parallel to the bullet's path to give an erroneous reading even if the equipment is otherwise 100.000% accurate and most aren't that close. Plus a conventional chronograph set ahead of the muzzle should have its distance from the muzzle measured and the resulting velocity value corrected to a true at the muzzle value. (Not applicable to the Magnetospeed of course!)

Thanks Laurie,
The sierra numbers seem the closet to what I saw on the target. 1238 fps not to far from 1200. I guess thats the reason I mainly shoot the berger 185's at 1k and keep the 175's for the closer in stuff. I guess when it gets close the 1.2 mach it just kills the bc. Thanks, Matt