Question for the guru's

[BattleAxe]

I recently made the leap to shooting 6.5 Creedmoor and I've noticed something odd. The MV from the Magnetospeed seems high. For example I might get 2840fps but end up shooting low at 1K. If I then do a MV cal based on bullet drop I get 2770fps and when I use that calibrated speed my dope is spot on. Is there a setting or adjustment on the crono that needs to be changed? I've double and triple checked my data input to applied ballistics and everything looks right. Yes I'm aware that cronos aren't spot-on...just trying to figure out if approx. 70fps difference is pretty normal or is something "off".
-I'm using a Kestral Elite with AB
-Factory 140gr ELDM
-G7 BC
-26"barrel 1/8T

 

[EXTREMEPREJUDICE]

http://www.magnetospeed.com/reviews-tests/ If you look at their published data you would expect less than 1% error. I don't have a Magnetospeed so no help there except it appears that alignment and sensor gap are important. Of course, there are lots of reasons besides MV that your downrange results might not line up to your ballistics engine but you already know that.

 

[BattleAxe]

http://www.magnetospeed.com/reviews-tests/ If you look at their published data you would expect less than 1% error. I don't have a Magnetospeed so no help there except it appears that alignment and sensor gap are important. Of course, there are lots of reasons besides MV that your downrange results might not line up to your ballistics engine but you already know that.

Yes I read that before posting. My crono seems to work flawlessly other than speed not matching ballistic performance. The environmental data and firing solution comes from the same instrument (kestrel) so there's really very little user input to screw up. I'm starting to think its a powder temp issue. The one thing the Kestrel doesn't know is the temperature of your ammo. I went out today and made sure I kept my ammo in the bed of my truck for the trip to ensure ammo temp matched OAT. It was hotter than my last trip out and as expected I was printing high @ 1K. Did a MV cal then checked dope at every distance in 100 yd increments from 300-1000 yards and it was spot-on.

I thought about optics but I have a hard time believing 2 different brand new S&B's would be off by the same amount, considering that both dialing or a hold-over provide the same result and once I have a valid speed they dial precisely where they're supposed to. In any event, I can work around it easily for now...its just irritating.

 

[Matches Malone]

if its consistent true your calculator...

 

[pell1203]

BattleAxe, your MV does seem a bit high out of the Magnetospeed for that ammo and rifle combination... unless you are at very high elevation.

Last time I checked velocity, my 6.5 Creedmoor Ruger RPR (24" barrel) spits out the Hornady 140 grain ELD-M factory ammo at approx 2645 fps on a 50 degree day at 750' elevation.

I would tend to believe your trued 2770 fps number out of your 26" barrel is correct before believing the 2840 fps number out of your Magnetospeed. If it were me. I would think its time to give Magnetospeed a ring to see what they say with regards to the issues you are having.

Edited to add: I've seen comments in other threads where a loosely mounted or misaligned bayonet has given incorrect velocities by as much as 100-150 fps. Make sure your alignment is good and bayonet is tightly attached to barrel.

 

[BattleAxe]

if its consistent true your calculator...

If you're referring to tweaking the drop scale factor I simply don't have the available distance to do it. The farthest I can get out is to 1K and its my understanding that you need to be in transonic range to do it. Correct?

 

[BattleAxe]

BattleAxe, your MV does seem a bit high out of the Magnetospeed for that ammo and rifle combination... unless you are at very high elevation.

Last time I checked velocity, my 6.5 Creedmoor Ruger RPR (24" barrel) spits out the Hornady 140 grain ELD-M factory ammo at approx 2645 fps on a 50 degree day at 750' elevation.

I would tend to believe your trued 2770 fps number out of your 26" barrel is correct before believing the 2840 fps number out of your Magnetospeed. If it were me. I would think its time to give Magnetospeed a ring to see what they say with regards to the issues you are having.

Edited to add: I've seen comments in other threads where a loosely mounted or misaligned bayonet has given incorrect velocities by as much as 100-150 fps. Make sure your alignment is good and bayonet is tightly attached to barrel.

My thoughts as well. Yesterday I went out and at 92 degrees with a DA of +2400, my calibrated velocity was 2846. Using that speed I was center punching everything out to 1K. Several weeks earlier I was getting almost that exact same speed from the magnetospeed at 72 degrees which doesn't make sense. When I calibrated that speed it came in at 2771 @ 72 degrees and again...the calibrated speed was spot-on at distance.

That said, I've been cautious to make sure the MS bayonet is tight and I run it as close as I can without scorching the top and I'm pretty certain it's not moving and installed correctly. The rifle started life as a GAP HRT that was rebarreled to 6.5 wearing a S&B 5-25 in a Axe chassis and it runs like a Swiss watch so I know its not a rifle issue.

 

[delfuego]

You guys keep talking about the Magnetospeed. That device is less likely to be giving you erroneous info than the kestrel. Velocity is relatively consistent, and Magnetospeed just flat out works. Run you data through another ballistic calculator and see what you get. Eliminate one thing at a time. If JBM or Shooter give you proper data, you will know for sure which device is not working. I have seen more guys fidle-fucking with AB Kestrels than I care to remember. AB Kestrels work great, right up until they don't.

 

[BattleAxe]

You guys keep talking about the Magnetospeed. That device is less likely to be giving you erroneous info than the kestrel. Velocity is relatively consistent, and Magnetospeed just flat out works. Run you data through another ballistic calculator and see what you get. Eliminate one thing at a time. If JBM or Shooter give you proper data, you will know for sure which device is not working. I have seen more guys fidle-fucking with AB Kestrels than I care to remember. AB Kestrels work great, right up until they don't.

Yes you would think but bullet drop is a function of speed/weight/BC/gravity. It's is purely a math equation. When weight, BC, and gravity are known and verifiable what does that leave you?

 

[delfuego]

When weight, BC, and gravity are known and verifiable what does that leave you?

The Kestrel and software.

Speed is one of the easiest things to confirm. Do that and eliminate that first. Either trust the Magnetospeed (we trust ours). Or borrow someone's chrono and eliminate that variable. Then run your data through another calculator. Process of elimination.

 

[CMH]

Yes you would think but bullet drop is a function of speed/weight/BC/gravity. It's is purely a math equation. When weight, BC, and gravity are known and verifiable what does that leave you?

Just out of curiosity, how have you verified your BC?

 

[BattleAxe]

The Kestrel and software.

Speed is one of the easiest things to confirm. Do that and eliminate that first. Either trust the Magnetospeed (we trust ours). Or borrow someone's chrono and eliminate that variable. Then run your data through another calculator. Process of elimination.

I've already done that. It's what you're doing by basing speed on bullet drop...you're taking the crono data out of the equation. When I do that my dope is spot on.

 

[delfuego]

Just trying to help you.

Delfuego out

 

[BattleAxe]

Just out of curiosity, how have you verified your BC?

LOL, I read it on the box. Seriously though that's one of those things I assume to be accurate based on manufacturers data. I'm shooting factory ammo so I just select the bullet from the Kestral bullet library.

 

[BattleAxe]

Just trying to help you.

Delfuego out

And believe me I appreciate it. I'm all ears on this one. Truth is that if calibrated MV works, I essentially have no need for crono data but I'm trying to figure out why. For no other reason than gathering data during seasonal changes I could do it at short range with a crono, trust the data, and avoid the long drive into the swamp to calibrate it.

 

[BattleAxe]

And to add...I just punched my "calibrated" MV into the shooter app and the results are the same as Kestral AB...dope is good using calibrated MV.

 

[Matches Malone]

If you're referring to tweaking the drop scale factor I simply don't have the available distance to do it. The farthest I can get out is to 1K and its my understanding that you need to be in transonic range to do it. Correct?

I guess, it's been awhile since using a kestrel or an software. Technically you can true from any ranges, but they could have an effect on ranges not trued. I would shoot aroun 500-600, get your true drop, then at 1000. Then verify 300 to make sure it doesn't move (if it's good, or just get the true drop for that too) then bend the muzzle vel, to match your real world data. If you get close but no cigar, then do the same with the bc. Also verify your zero, a zero offset will through off your dope so much as far as the computer display.

 

[Steel+Killer]

I am so glad the op brought this topic up I have been wondering the same thing. I just bought the Sig Kilo 2400ABS and it has been spot on except the calibrated MV never seems to match what my chrono (Caldwell) was saying. So my big question is if the MV is known to be correct then the BC must be off am I correct in assuming that?

 

[_Windrider_]

I am so glad the op brought this topic up I have been wondering the same thing. I just bought the Sig Kilo 2400ABS and it has been spot on except the calibrated MV never seems to match what my chrono (Caldwell) was saying. So my big question is if the MV is known to be correct then the BC must be off am I correct in assuming that?

Well remember the BC is only correct at a specific MV right?

 

[BattleAxe]

I think there are a lot of takeaways from this. Primarily that crono data should only be used as a guide. From the testing I've done I'm not convinced that crono data can be assumed to be correct and may exceed the published error rate. And to be clear I'm NOT blaming this on the crono. I have at least 6 precision rifles. They all have different recoil pulses (particularly the gassers) and all have to be ridden differently for accuracy sake. It makes sense that regardless of how accurate the crono is...it does not know how far or fast the rifle is moving rearward when a projectile is passing over the sensors and I have a hunch that this fact can magnify the error rate, particularly on a unit like the magnetospeed where the sensors are essentially moving in the opposite direction of the projectile. I don't know if this even matters and admittedly I'm guessing here but there's no doubt that my calibrated MV is dead on. Throw in BC variations with speed, actual powder temp verses "assumed" powder temp, wind, and its easy to see why the data wouldn't always line up.