What range to calibrate velocity and BC

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ssatt68

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Here we go.

Please read and attempt this TTP before bruning me at the stake for being a heretic

Next time you guys are out on the range and wonder why your long range data is solid and your intermediate range data is slightly off, give this technique a try.

I ran a clinic last summer and two of my shooter was running minimally invasive (MI) rifling and what I noticed was pretty wild. The clinic was a team train up and we were all running the same 105 6mm berger hybrid at about the same speed, 3100-3120. We chornographed all rifles before confirming at distance. All of our BC lined up perfectly out to 1220 except for the two rifles with MI rifling. Traditional wisdom says we should calibrate velocity at distance according to the known BC. When we did that with the MI rifles we got numbers that was un-useable. The MI rifles were hitting a full mil high at 1220 so when we calibrated for velocity we got 3280 fps which is not possible, a full 160 FPS off of what the magneto speed was telling us, and would throw off our mid-range trajectory.

So we are solving for two variables. For most shooters calibrating for velocity at distance is fine. For shooters that point bullets or have nontraditional rifling or using a bullet that Litz hasn’t tested, we will have to solve for one variable before we tackle the other. Modern chronographs are accurate enough to give a solid starting point so let’s take that number and confirm it. After that we need to solve for the BC. Most of the time I have to adjust the BC down from what the published BC is unless it has been tested by Litz.

So I started messing with the BC on AB and ended up going from .278 G7 to .309 G7 on the 105 hybrid to get the entire trajectory track to line up for these two rifles. Since then I have run a two more clinics were shooter were using box BC for bullets that Litz hasn't done testing for yet. Litz tests the BC of fired bullets which will give you a very solid number. Here is the rub all bores are different. The rifling cuts very slightly into the Ogive at the ogive bearing surface joint especially in the more aggressive shaped bullets.

After talking with quite a few PR shooters, my new technique for calibrating velocity and BC is:
1. I will shoot 5-10 over my magneto speed and take the average plug that into my ballistic app
2. Ensure that we have all atmospherics taken care of to include enabling powder temp and taking that into account. All powder is temp sensitive. I use .6 FPS per degree of temp even for the extreme powders. I live in Western WA where it is 20-50 degrees cooler than the rest of the US in the summer time and need to be able to take my data to much warmer areas and still make hits. .6 fps/degree has been working for me for the last few years.
2. Confirm on targets 200, 300, 400, 500, 600. These distances are minimally affected by bullet BC but still allow for a measurable difference in velocity. It is very important that you have your optic height over bore measured for this step.
3. Take the rifle to a distance that is close to transonic zone and begin changing the BC up or down to reflect what you observe down range. If you are hitting high you will need to adjust the BC up. If you are hitting low you will have to adjust the BC down.
4. Re shoot and confirm at as many un even distances you can. Continue to refine.

This technique has increased my accuracy without a doubt. The typical prone target in a PRS match ranges from < 1 to 2 MOA and a positional target ranges from 1.5-3 MOA with a wide variety of ranges. About half of the PRS matches are field fire events held on private land that is not set up like a traditional KD range so you will see very few even number yardages. You data has to be perfect.
Give this a try and see if it works for you.
 
Arctic,
My experience with velocity calibration is the distance required for 7 mils is too much. BC begins to be a factor past 600 unless you are shooting 375 Cheytac. I can shoot 6mm 105 berger or 105 amax and have nearly the same data at 600 but 7 mils at 970 with the amax and 1020 with the 105 hybrid launch at the same speed and that is a .040 change in BC. Additionally, I'm attempting to be less than .1 mil off at every distance I could possible shoot at from 50-1760. 75%-80% of match points in a PR style match will occur 300-900 at irregular distances. A common stage would be a cold bore shot on a 3” shoot and see at 360 the inner ring is 100 point the outer ring is 50 and a miss is 0. BC has little to no effect at this range but velocity does. Artificially lowing the velocity by 50 FPS because your impact at distance and velocity calibration tool told you to would change your data by .1 mils which is about 1” at 360 yards.
DA 500, Temp 55
105 amax at 3100 fps = 1.1
105 amax at 3050 fps = 1.2
 
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Since many reading this will only have experience shooting KD matches that have sighting shots, it should probably be pointed out that you only get a few shots at most at each target in these PRS matches, not enough to know where the center of your group is to be able to center it. Also, there are no markers showing where your shots went. Good ballistic data will make or break you since all of your shots are essentially sighting shots.

I've done similar testing to determine BC's but it was not near the transonic range. That's very hard to get to up here unless you go north and shoot on public land. Few ranges go past 1000 yards or 900 meters. Articles describing the process here:
http://reloading.jimdo.com/downloads-1/shooting-related-articles/

The one thing with the MagnetoSpeed is that is can affect accuracy. My shooting partner developed a load with it on the end of the barrel that shot bugholes, but could never get the load to shoot without the MagnetoSpeed on the end. I found it advantageous to shoot through a chrono and record the actual velocities of the shots that were actually going down range. Doing so would allows you to hold one of the variables constant since it is known. That LabRadar unit looks like the ticket for doing this type of calibration.

Have you considered doing an actual test to determine the temperature sensitivity of your load? I used to post a lot of that data on the Hide.

What people notice based on air temperature isn't the best indicator of temperature sensitivity. Rounds brought from home in a plastic case, transported in a heated/cooled vehicle will often experience very little change in temperature internally. Unless the rounds are soaked in an exposed manner, at temperature, for at least two hours, you don't really know the powder is at that temperate. The first time I ran this type of test, I put a thermocouple inside a loaded round (without primer) through the primer hole to see how long it took for the internal temperature to be the same as external. The way the tests works is that you soak 3 batches of ammo at hot, cold and ambient temperature for at least two hours (5 rounds of each). I use a cooler with a heating pad for hot, the freezer in the clubhouse for cold (an icebox works too), and just sit the rounds out in the open for ambient. Temperature is measured with thermocouples and/or mercury thermometers.

You then fire them one round at a time in a round robin sequence (ambient, hot, cold, ambient, hot, cold...). You let the barrel cool in between each shot. When you fire each round, you just transfer that round from the hot/room/cold storage to the chamber and fire it through the chrono as quickly as possible. You don't want to give the rounds any time to change temperature internally due to contact with the chamber. I set the scope on the lowest magnification and just make sure I'm sending the round through the sweet spot of the chrono (no groups here). You basically want to the Jerry Miculek of loading a boltgun and firing it through the chrono. This is best done on a short pistol range, close to the berm.

Here is some data from a test I ran with it in 308 Win.:

175_IMR3031_2nd.jpg


Here is a Varget load for comparison:

190SMK_Varget_Temp.jpg


The number in front of the 'x' is the slope in fps/F. So, IMR3031 changed 0.588 fps/F, while Varget only changed 0.149 fps/F.
 
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ssatt68 said:
Arctic,
My experience with velocity calibration is the distance required for 7 mils is too much. BC begins to be a factor past 600 unless you are shooting 375 Cheytac. I can shoot 6mm 105 berger or 105 amax and have nearly the same data at 600 but 7 mils at 970 with the amax and 1020 with the 105 hybrid launch at the same speed and that is a .040 change in BC. Additionally, I'm attempting to be less than .1 mil off at every distance I could possible shoot at from 50-1760. 75%-80% of match points in a PR style match will occur 300-900 at irregular distances. A common stage would be a cold bore shot on a 3” shoot and see at 360 the inner ring is 100 point the outer ring is 50 and a miss is 0. BC has little to no effect at this range but velocity does. Artificially lowing the velocity by 50 FPS because your impact at distance and velocity calibration tool told you to would change your data by .1 mils which is about 1” at 360 yards.
DA 500, Temp 55
105 amax at 3100 fps = 1.1
105 amax at 3050 fps = 1.2
Click to expand...

What I said is that following velocity calibration, AB is able to calibrate your drag model (BC) as a function of stability. In no way am I saying fudge your velocity numbers, that is totally incorrect. I think what you saw with your two shooters is difference in bullet stability manifest in differences in BC compared to the rest of the 6mm guys. Your BC changes continuously throughout the flight.

Cool to be able to observe that!
 
Didn't Litz do an experiment lately that showed that under-stabilizing the bullet affected the BC by a significant amount? I'm sure I read that, but I can't find the test. I wonder how the MI rifling is formed? Button rifled barrels are often slower than the advertized twist because the button often drags instead of turning.
 
kombayotch said:
Since many reading this will only have experience shooting KD matches that have sighting shots, it should probably be pointed out that you only get a few shots at most at each target in these PRS matches, not enough to know where the center of your group is to be able to center it. Also, there are no markers showing where your shots went. Good ballistic data will make or break you since all of your shots are essentially sighting shots.

I've done similar testing to determine BC's but it was not near the transonic range. That's very hard to get to up here unless you go north and shoot on public land. Few ranges go past 1000 yards or 900 meters. Articles describing the process here:
http://reloading.jimdo.com/downloads-1/shooting-related-articles/

The one thing with the MagnetoSpeed is that is can affect accuracy. My shooting partner developed a load with it on the end of the barrel that shot bugholes, but could never get the load to shoot without the MagnetoSpeed on the end. I found it advantageous to shoot through a chrono and record the actual velocities of the shots that were actually going down range. Doing so would allows you to hold one of the variables constant since it is known. That LabRadar unit looks like the ticket for doing this type of calibration.

Have you considered doing an actual test to determine the temperature sensitivity of your load? I used to post a lot of that data on the Hide.

What people notice based on air temperature isn't the best indicator of temperature sensitivity. Rounds brought from home in a plastic case, transported in a heated/cooled vehicle will often experience very little change in temperature internally. Unless the rounds are soaked in an exposed manner, at temperature, for at least two hours, you don't really know the powder is at that temperate. The first time I ran this type of test, I put a thermocouple inside a loaded round (without primer) through the primer hole to see how long it took for the internal temperature to be the same as external. The way the tests works is that you soak 3 batches of ammo at hot, cold and ambient temperature for at least two hours (5 rounds of each). I use a cooler with a heating pad for hot, the freezer in the clubhouse for cold (an icebox works too), and just sit the rounds out in the open for ambient. Temperature is measured with thermocouples and/or mercury thermometers.

You then fire them one round at a time in a round robin sequence (ambient, hot, cold, ambient, hot, cold...). You let the barrel cool in between each shot. When you fire each round, you just transfer that round from the hot/room/cold storage to the chamber and fire it through the chrono as quickly as possible. You don't want to give the rounds any time to change temperature internally due to contact with the chamber. I set the scope on the lowest magnification and just make sure I'm sending the round through the sweet spot of the chrono (no groups here). You basically want to the Jerry Miculek of loading a boltgun and firing it through the chrono. This is best done on a short pistol range, close to the berm.

Here is some data from a test I ran with it in 308 Win.:

175_IMR3031_2nd.jpg


Here is a Varget load for comparison:

190SMK_Varget_Temp.jpg


The number in front of the 'x' is the slope in fps/F. So, IMR3031 changed 0.588 fps/F, while Varget only changed 0.149 fps/F.
Click to expand...

Very cool stuff! I guess the trouble like you mentioned is the warm/hot chamber imparting heat on your powder.

Even if you do fit a regression line to the rapid fire points as you plotted, you would have an alternate set of points for leaving the loaded rounds in a chamber for 5 seconds, 10 seconds.......
 
Great info but what is discussed has been known for a very long time... well, before the advent of a smartphone.

Always, verify your drop chart in real world shooting. Error from Scope, bore, bullet, chronie, rangefinder and personal weather stations can all conspire to offer varied firing solutions vs the "norm".

As long as the system provides the correct output for the bullet to land on target, the absolute values are irrelevant.

From real world shooting, the ballistics drop chart is varied to the specific system being used... then modified as each part is change - typically, that is with a new barrel and bullet lot.

From there, the user has to understand how ambient changes will affect this system and adjust accordingly. I think with so much info at ones fingertips, modern shooters tend to think in absolutes.

Back when scope clicks didn't always match up even with scopes of the same brand, you learnt real quick to work with a system of 1...YOURS

But it is nice to see shooters questioning and testing for best results.

They might still have it, but you can definitely find it in older catalogs.... Sierra listed BCs for their bullets over various operating velocities. It has always been known that BC vary with velocity and some bullets had quite dramatic changes. never found software that allowed you to adjust all inflight parameters along the distance traveled .... maybe military or NASA... certainly nothing "off the net".

So even today, I take the generic printout .. mostly from JBM, and scribble down what my real world results end up being. More, less, the same... doesn't bother me. Sure, I have played with the input data to try and fudge the program to print out what my scribbles told me.

Then there was the Spreadsheet program (I have moved on from Lotus. Open office for me now) and now I can make my own fancy cards at home.... But most still have scribbles on them.

Jerry
 
arcticcathonda said:
Very cool stuff! I guess the trouble like you mentioned is the warm/hot chamber imparting heat on your powder.

Even if you do fit a regression line to the rapid fire points as you plotted, you would have an alternate set of points for leaving the loaded rounds in a chamber for 5 seconds, 10 seconds.......
Click to expand...

I didn't really do the experiments with rapid firing in mind. My goad was to quantify the velocity change with temperature so that I could make accurate cold bore shots, taking the velocity change into consideration. But yes, your velocity will be increasing as the barrel heats up if your rounds are sitting in the chamber long enough for the powder temperature to change.
 
I have a spreadsheet that does all of the ballistic calculations and puts them in various charts (elevation, wind, slopes, movers, etc...) for my databook and wrist coach. I've been working on modifying it to automatically produce sets of cards for different steps in density altitude.

Right now, it generates wind wheels for every 100 yards.



Not very useful for PRS, but handy for KD deliberate shooting.
 
Mystic Precision said:
K, why not tell your friend to add a weight equal to the magnetospeed to his barrel.

Call it a barrel tuner if you want to. some very positive results can occur.

Jerry
Click to expand...

I'm not sure his muzzle brake would like that. Plus, added weight on the barrel isn't very helpful in positional shooting.
 
If he already had a brake, then this will be so much easier.

Compare weight of brake vs magnetospeed bayonet. Whatever the difference, add to the brake... done.

To prove the point, duct tape wheel weight to the barrel and see if tuning returns to w/bayonet testing - assume he did his testing with brake AND bayonet?

Yes, I have found the same thing and sometimes that extra weight just makes the barrel sing.....

To stir up the pot a little more, repeat all testing with varying degrees of bore fouling... Yes, it will make a difference... to a point.

Here is a good one, leave a bore dirty, come back the next day and shoot your cold bore... at distance, did it land in the same spot? How many shots until it did? Or did it never return?

F class shooters are very aware of the cold bore shots and changes due to working temp and ambient temps. When you shoot Bisley style (3 to a target) with max 45secs per shooter, you can be waiting a good long time before you shoot again. Given how wonky air can change, many shots are also "sighters" per se. Alot of the time, the systems are stable enough, wind is the only issue. Sometimes, it isn't and you better know the difference.

I know for a fact, bore condition has a profound affect on ontarget results. We all find that "sweet range" of dirty to shoot our matches in... then work as best we can to maintain that through some very long weekends.

At some point, a shooter can worry about too much and simply stop being effective. All that is being discussed IS important and great to be aware of BUT what I feel is important is knowing how to "center the group".... to know where to hedge your bets for a hit given the vagueries of any rifle system... and the body of air you will be shooting through.

As much as it can help, there is always the risk of paralysis by analysis.

Or taking data as absolute instead of letting the hole in your target determine your next course of action.

Jerry
 
How do you "center the group" when you only get two shots per target and the only thing you have to read is swirl and splash if you miss and the splatter mark if you hit the steel?
 
You get real intimate with your barrel and scope and understand what it does... this assumes you are able to release the shot with any degree of accuracy.

Also, having very accurate set ups.

At the current levels of top tier F class shooting, the equipment is more then capable of eating up a 1/2 MOA X ring at 1000yds. When you stop moving a 3 1/2" spotter (size of 1000yds marking disc) and just tape the impacts, you know both shooter and gear is working well. how about 8 consecutive shots in the wind inside 4" at 1000yds... I had the pleasure of scoring this shooter.. WOW!!!!

Now expand that to a 2 to 4 moa target. Based simply on mechanical accuracy, that target is HUGE (refer to first line)..... you have alot of shooter/position error and still hit the target (refer to first line)

With practise, you learn what the grouping capability of you, your ammo and your rifle is... now when the target is presented, you come up with a firing solution and "center the group or where you believe your shots will go" over the target accounting for ambient and send it. Start adjusting from the impacts.

I use a Sightron SIII scope with the LRMOA reticle. There is a central dot that is not far from my mechanical accuracy of my rifle. That dot gives me a feel of what my group center is relative to the target and with my gut on wind, make a calculated guess on where that shot is going to end up. The hope is to have the shot land where the dot was put (assumes you dial for elevation and wind on the turrets)

You have to understand that F class shooters depend on their sighters to help solve what is happening in the air. We don't just shoot a couple of "foulers" then start peddling.

If you chat with better F class shooters, they will tell you that they can adjust off that first sighter... yes, 1 single point on the target. The 2nd shot confirms and away they go. We work real hard to know what every shot is going to do wrt to changes in the barrel/bore.

Let's play pretend... I have a rifle that I know will shoot in the 2's mechanical accuracy out to 600yds. ie the rifle will group under 2" at 600yds under ideal conditions.

A target is placed at what is estimated as 550yds. The target is 2moa wide and 4 moa tall. It is windy but not crazy hard. I take a SWAG that the wind is somewhere in the 3 to 4 moa range (under 10mph)

Given how tall the target is, I can hedge my bets on the drop - 1 min of drop error falls into a 50yds distance mis read... more then good enough. What I don't really know is what the drift is so...

I would dial elevation for a ball park of 550yds and only dial in 2 or maybe 2.5 mins of wind. Hold on upwind edge of target, centered for elevation, send it... on the assumption all the above was correct, I will hit somewhere from center to downwind side... Somewhere within the middle for elevation. If lower or higher then "1/2 min", I know the target is quite a bit different in distance vs my estimate.

If I need to shoot again, I can go off the splatt to make corrections to center in the target. But likely given that I am under time constraint, rack and send another.... done. AFAIK, all impacts score the same.

Because I will be moving locations, presentations relative to target and winds, knowing the firing solution in absolute terms isn't really all that important. What I need to know is that my approximations are correct and that the guesstimates are within the size of the targets.

Jerry
 
Kom,
My testing wasn't nearly as sophisticated. The MI rifling is button pulled you can find out what it looks like by google searching black hole weaponry rifling imagery or click on this link.
https://www.google.com/search?q=bla...ygonal-ar15-sniper-barrel-p-1452.html;500;300
As you can see this type of rifling will not cut grooves into the bullet. Which is why the two rifles didn't degrade the bullets BC as much as traditional rifles will. I'm not sold on the barrels but Anette Watcher 30 Cal Gal seems to really like it so far.
 
I'm going to have to read up on that rifling and maybe get a blank to try.

How did the MI barrels track with the traditional barrels at 1k and under? At what range did they start to deviate? And also, did the traditional barrels include 5P/5R barrels? MI is just a 3P barrel from your pic...
 
Kom,
They started at about 600 and was dramatic by 1000 then unbelievable at 1220. Plug 3120 FPS on the 105 berger hybrids with the standard .278 G7 then up the G7 to .309. I'm thinking about getting one myself. Right now I'm shooting Hawk Hill Customs and they have been the most consistently accurate barrels I've shot so far.
 
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