Does velocity affect ballistic coefficient?

[Blackdirt Cowboy]

I loaded up a load for my 7 rum using 145 grain hammers. Average velocity of this round is 3635 FPS over 12-15 rounds fired over a magnetospeed. I zeroed the load at 200 yards and entered all the data into my ballistic calculator and went out to shoot steel and verify drops. I used a bc of .230 as indicated on hammers website, which they said was verified by drops, not estimated.

At 415 yards, I was about 1 1/4" high. Not too bad, so I backed off to 525 yards and was 3" high. When I got back home, I started making adjustments to the bc in the calculator. When I entered it as .320, it's dead on with the results I saw on the steel plate.

I know my velocity numbers are good, as the SD of my load was single digits and the ES was around 13. I also measured the sight height, so that's not giving me any issues either. The only thing "unknown" is the bc. Is it common to have to adjust the bc that far from the advertised bc? I know this is a pretty fast load, so maybe that has something to do with it? What are y'all's thoughts?

 

[Mikecr]

Why is the bullet still climbing from 415 to 525? Are you adjusting the scope for these different ranges, and the disparity (highness of shots) is w/resp to what should be zero at those ranges? That's likely it, just want to be clear.

BC does change with velocity, and with air density, as these affect drag. If you knew what velocity and air density conditions Hammer tested at to determine a BC of .230 it could really help. Your ballistic software could use that info to correct for local situation.
It's more accurate to determine BC from measured velocity decay over distance (more accurate than drops), but Hammer's numbers might be pretty good. I'm sure they were validated many times by users.

With your low ES, and consistent info near muzzle, you could now setup the chrono at 100yds or 200yds and shoot through it. See what velocity is slowing to by that range. Then adjust BC in software to match that velocity decay & see what you get. Always measure & log your air density conditions for this kind of testing. That needs to be used in your software.

 

[Blackdirt Cowboy]

Yes. Sorry I didn't mention that. I am dialing my scope. The bullet isn't really rising over that distance. It's just the error in the calculation of my program. As I said, when I adjusted the bc in the calculator, it brought both solutions to exactly where they needed to be based on the results I saw on the steel.

 

[jraulsten]

The Hammer BC was most likely not determined at as high of velocity as you are shooting. BC does increase with higher velocities but I wouldn't think the difference would be that much since they use the G7 model........?

 

[Kmccord]

It will all depend on what velocity they set it to, same for any bullet manufacturer, it is a good measurement to get you very close to the target point, then it is up to you to adjust as necessary for spot on accuracy. As was mentioned, air density is another factor, higher elevations usually create higher velocity than lower elevations.

 

[Calvin45]

Not wanting to sound like too much of a simpleton here but you say hammer advertised .230 and you found .320 to be accurate. Could be as simple as a typo on their part. Punched in 23 instead of 32. Easy enough mistake to make….

 

[FEENIX]

Not wanting to sound like too much of a simpleton here but you say hammer advertised .230 and you found .320 to be accurate. Could be as simple as a typo on their part. Punched in 23 instead of 32. Easy enough mistake to make….

I do not think that is the case. All of their .284s are in the .2s except for the 177s at .313 estimated.

 

[Calvin45]

I do not think that is the case. All of their .284s are in the .2s except for the 177s at .313 estimated.

Okay, that makes it unlikely for sure.

 

[CMP70306]

Could also be a scope tracking issue as well or a multitude of different factors. All my hammer shooting has been close so far and I haven't gotten a chance to stretch its legs and check the BCs.

 

[ButterBean]

I loaded up a load for my 7 rum using 145 grain hammers. Average velocity of this round is 3635 FPS over 12-15 rounds fired over a magnetospeed. I zeroed the load at 200 yards and entered all the data into my ballistic calculator and went out to shoot steel and verify drops. I used a bc of .230 as indicated on hammers website, which they said was verified by drops, not estimated.

At 415 yards, I was about 1 1/4" high. Not too bad, so I backed off to 525 yards and was 3" high. When I got back home, I started making adjustments to the bc in the calculator. When I entered it as .320, it's dead on with the results I saw on the steel plate.

I know my velocity numbers are good, as the SD of my load was single digits and the ES was around 13. I also measured the sight height, so that's not giving me any issues either. The only thing "unknown" is the bc. Is it common to have to adjust the bc that far from the advertised bc? I know this is a pretty fast load, so maybe that has something to do with it? What are y'all's thoughts?

Simple answer is Yes, Especially with the Hammers, As with any bullet the faster they are the BC goes up but you are 635fps over where BC's are estimated so you're right on the money, FWIW this is what I see on all of my Hammer loads

 

[Blackdirt Cowboy]

Simple answer is Yes, Especially with the Hammers, As with any bullet the faster they are the BC goes up but you are 635fps over where BC's are estimated so you're right on the money, FWIW this is what I see on all of my Hammer loads

Thanks, Butter Bean. That makes me feel better. I thought that it might be too big of a difference and maybe had something else going on. I'm going to load more ammo and take it out to 800-1000 and see if my corrected BC holds.

 

[david g ranes]

I loaded up a load for my 7 rum using 145 grain hammers. Average velocity of this round is 3635 FPS over 12-15 rounds fired over a magnetospeed. I zeroed the load at 200 yards and entered all the data into my ballistic calculator and went out to shoot steel and verify drops. I used a bc of .230 as indicated on hammers website, which they said was verified by drops, not estimated.

At 415 yards, I was about 1 1/4" high. Not too bad, so I backed off to 525 yards and was 3" high. When I got back home, I started making adjustments to the bc in the calculator. When I entered it as .320, it's dead on with the results I saw on the steel plate.

I know my velocity numbers are good, as the SD of my load was single digits and the ES was around 13. I also measured the sight height, so that's not giving me any issues either. The only thing "unknown" is the bc. Is it common to have to adjust the bc that far from the advertised bc? I know this is a pretty fast load, so maybe that has something to do with it? What are y'all's thoughts?

I've asked the same question twice and haven't gotten an answer I quite understand your bc should get less at distance but mine was always high I wondered how much high a bc should jump either direction I decided heck with it shot out to 800 and used the data I collected and things are repeatable for me that way David

 

[ButterBean]

Thanks, Butter Bean. That makes me feel better. I thought that it might be too big of a difference and maybe had something else going on. I'm going to load more ammo and take it out to 800-1000 and see if my corrected BC holds.

I've asked the same question twice and haven't gotten an answer I quite understand your bc should get less at distance but mine was always high I wondered how much high a bc should jump either direction I decided heck with it shot out to 800 and used the data I collected and things are repeatable for me that way David

The biggest multiplier in ballistics is always speed

 

[waveslayer]

I loaded up a load for my 7 rum using 145 grain hammers. Average velocity of this round is 3635 FPS over 12-15 rounds fired over a magnetospeed. I zeroed the load at 200 yards and entered all the data into my ballistic calculator and went out to shoot steel and verify drops. I used a bc of .230 as indicated on hammers website, which they said was verified by drops, not estimated.

At 415 yards, I was about 1 1/4" high. Not too bad, so I backed off to 525 yards and was 3" high. When I got back home, I started making adjustments to the bc in the calculator. When I entered it as .320, it's dead on with the results I saw on the steel plate.

I know my velocity numbers are good, as the SD of my load was single digits and the ES was around 13. I also measured the sight height, so that's not giving me any issues either. The only thing "unknown" is the bc. Is it common to have to adjust the bc that far from the advertised bc? I know this is a pretty fast load, so maybe that has something to do with it? What are y'all's thoughts?

What scope are you using and ballistics calculator?

BC is velocity dependent and all ballistic calculators need to be trued