BC trueing (StrelokPro)

[evetseel]

Hi folks

so I’ve been doing some range work out to 450 yds with a load that works.

I’m I have used the ballistics data for my bullet heads etc etc along with the BC data on the app. I’ve verified my MV using a magneto speed, and have used all the correct ambient sensitivities for zero and for the range work.

long story short I’m shooting 4” low on my first try. I thought about using the FPS trueing feature to see if that was the issue but stopped short as my theory is if I know my MV then that shouldn’t be needed.
That said what I did do is use the BC trueing feature which when corrected to the value needed I was shooting bang on centre.

My question is - why should my BC be different to that of the bullet head data. I mean it’s corrected the issues I was having but I’m confused why? Does the BC of a bullet change dependent on the barrel it is shot from??? For the record I’m using a .308 5R barrel on a 700 platform.

 

[75]

My understanding is a) BC changes with velocity so a single figure is quite meaningless for all ranges and b) there is no consistent way of measuring and reporting BC by different manufacturers and some of them may be a little “relaxed” about the numbers they report in the interests of marketing!

 

[GregorG]

Good explanation is here:

 

[Eric the Red]

Use G7 over G1. I'll take a guess that your true zero range isn't quite right. You may think it's 100m - but it can be quite a long way out for the purposes of Strelok - 50m can be as little as 0.2in. Scope-bore height also needs to be accurate.
These will have quite big impacts on that curve.

 

[User00040]

Always use a drop for trueing, don't use BC's and MV's...just look up Todd Hodnett.

 

[User00040]

Scope-bore height also needs to be accurate.

Wrong.

 

[evetseel]

Always use a drop for trueing, don't use BC's and MV's...just look up Todd Hodnett.

I would do but I keep getting distracted by how his dodgy goaty wobbles when he talks like a badger getting his end away.

 

[User00033]

450 yards is about half the range needed to start worrying about changing ballistic coefficient.

I would be very interested to hear by how much you changed your BC to get it to work.

Apart from the knowledge that you are using a 308 calibre barrel we don’t know anything about the bullet or MagnetoSpeed MV etc. This is really important information to have to be able to make a decent recommendation.

It is very important to understand that a lot of the BC truing that is described on YouTube is done by the ELR guys and not someone that is only shooting ordinary medium-range. Take a look at how ballistic coefficient changes downrange using for example the Sierra Bullets data. Very very small changes over that kind of short-ish medium range (450yds).

Pretty sure we debated this recently but the bottom line is I have never found a chrono that is absolutely true. The ELR guys wet their knickers at drop test trajectory validation but the simple fact is that sub-1,000m, it works.

 

[evetseel]

450 yards is about half the range needed to start worrying about changing ballistic coefficient.

I would be very interested to hear by how much you changed your BC to get it to work.

Apart from the knowledge that you are using a 308 calibre barrel we don’t know anything about the bullet or MagnetoSpeed MV etc. This is really important information to have to be able to make a decent recommendation.

It is very important to understand that a lot of the BC truing that is described on YouTube is done by the ELR guys and not someone that is only shooting ordinary medium-range. Take a look at how ballistic coefficient changes downrange using for example the Sierra Bullets data. Very very small changes over that kind of short-ish medium range (450yds).

Pretty sure we debated this recently but the bottom line is I have never found a chrono that is absolutely true. The ELR guys wet their knickers at drop test trajectory validation but the simple fact is that sub-1,000m, it works.

Thanks for the reply.
I essentially adjusted the BC to 0.2177 from 0.243 (which is the ballistic data from G7. The result was 2 clicks which brought me up a couple of inches at 450 yds. Photos attached of before and after the BC trueing

 

[75]

I would be very interested to hear by how much you changed your BC to get it to work.

I trued a 6.5x284 load out to about 900 yards. 130gn Accubond running at 3110fps and I changed BC from 0.488 (from Nosler) to 0.4342 and to get it spot on. At 750yds, for example, it's about 1 MOA different for the two different BCs

 

[User00033]

The amount that these two examples need to be changed in terms of either velocity or BC alone is unrealistic. There has to be other factors involved as well, including dare I say it, the shooter. It’s not a binary problem i.e. one or the other, it’s a mix of variables.

@evetseel your example requires a velocity drop of around about 100 ft./s to get the 4 inch difference so that doesn’t sound like it’s only velocity. That’s a bit of a guess and happy to be corrected, I’m just hacking the numbers sitting here in the hospital. The challenge is that you are using the Applied Ballistics G7 BC which means that if you want to change it then Mr Litz must be wrong. The 175gr MatchKing is one of the most understood bullets in history, so I doubt that he is wrong by anywhere near that much. So some of the other points raised earlier about your zero distance etc should be investigated.

It’s fun to try and work these problems out but at the end of the day you just need a working solution that will put you on the money at the distance you want to shoot. If that’s only 500 or 600 yards then you don’t need to get too technical.

 

[DRN]

Whats your zero distance?
I found with Strelock to get different calibres working a long way out the zero distance needed changing. My .308 zero distance happened to be 100yards and that worked out to 1200yards.
But my .243 zero was 250 yards before the data worked out at longer distance.
See how many clicks you are at 450yards and then try altering the zero distance on Strelock till the clicks match. Hopefully you will be on the money then.
Hope that makes sense.

 

[Sakoodin1]

Hi,I’ve used stretlokpro and I now use the kestrel elite,in my experience don’t get to wrapped up in what mv or bc you need to change as long as you’re hitting the target out to you’re desired distance that’s all that matters! What I do is change my bc out to 350 then 500 to make sure I’m on at that, then I go to 750 and change my bc to get the desired elevation then out to 1000(depending on you’re calibre as my .243 and .25.06 trues between 814-914 yards but my 7mm rem mag and 6.5-284 are at 1000) then do the same again,so it might give you a unrealistic mv as in to fast or slow or a low bc but at the end of the day who cares!! as long as you’re hitting you’re bull at you’re maximum range ie(800 or 1000 or beyond) it doesn’t matter what the numbers are!! As long as when you type in say 988 yards it then gives you say a figure of 24.57 moa(or the equivalent in mills) and you’re hitting the bull then that’s all you need to worry about.remeber you’re magneto or any other chrony just helps you get on paper then you true it yourself to you’re maximum distance with a kestrel or a stretlok and what that does is finesse every other distance from you’re maximum back to zero distance

 

[jcampbellsmith]

Use G7 over G1. I'll take a guess that your true zero range isn't quite right. You may think it's 100m - but it can be quite a long way out for the purposes of Strelok - 50m can be as little as 0.2in. Scope-bore height also needs to be accurate.
These will have quite big impacts on that curve.

I concur with this. I would add that I didn't get expected results from Strelok Pro first time out or second time out. Each time I go out, I take notes on what I did and review them after the event. To date I have not needed to fiddle with the Ballistic Coefficients. I have however found a number of data entry errors. I use a Magnetospeed, but keep in mind it's not a laboratory instrument. I suspect based on a number of outings/rifles that my Magnetospeed velocities are a few percent high. What I do find is that I don't use Strelok Pro often enough to become proficient with it.
Rome wasn't built in a day, practice, take notes, review and then try again.
Regards
JCS