OBT to MV Speadsheet

[alwie]

I did a bit of research and came up with a spreadsheet that will calculate optimal MV for a given Barrel Length, COAL, Bullet Len, CBTO and OBT node.  Put in the values you want for those parameters and the spreadsheet(here) will calculate your target MV.   You will have to create your own copy to modify it.

Feedback would be appreciated 

Alex

[grayfox]

I wrote a whole paper on doing this, several years ago. If what you have is like that, then sounds like you came up with a similar thing.

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Only thing is, under my approach you don't need to know which "node" you're looking for, to a shooter that number doesn't matter a whole lot.

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The initial entries I use are your OAL, known barrel length, and MV (chrony or a what-if). The xls then shows whether you are "close" or not, to a node. This way you can vary your powder load in order to get close to the node, and not waste a lot of shots in the in-between "no-mans' land"...

[BD1]

I did a bit of research and came up with a spreadsheet that will calculate optimal MV for a given Barrel Length, COAL, Bullet Len, CBTO and OBT node.  Put in the values you want for those parameters and the spreadsheet(here) will calculate your target MV.   You will have to create your own copy to modify it.

Feedback would be appreciated 

Alex

happy to give you feedback, but you will need to explain to this luddite how it works.  I can copy it, but I still cannot modify any of the values

[alwie]

happy to give you feedback, but you will need to explain to this luddite how it works.  I can copy it, but I still cannot modify any of the values

Well, first of all, I,m not sure what a luddite is, and may be one myself.  Im assuming that you can open the file and that you see the table I generated ( see attached image).

At the very top of each column are letters … A throo J that has stuff below them.  In the sheet you should be able to modify any cell(box) in columns A throo E and the rest of the numbers will be calculated.

Each row is a barrel and cartridge parameters comb that you plan on using. 

First enter your barrel length by clicking in the …. A2 box and change the number to reflect the barrel length of your rifle. B2 … enter your COAL you just loaded; C2 bullet length you loaded and D2 …. your CBTO.  In E2 you start with a guess as to which node will work … so guess node 7 to start

Go to the range and shoot 3-5 rounds of the ammo you just loaded and get the average MV.

Compare your average MV to the value in H2.  When you are close to a node, measured MV will be close to the H2 value.  If your MV is far from the node MV, change the node number to the nodes closest to your initial measured MV.

Then create a ladder test that will cover the MV from min I2 to max J2. Your ladder test you should hit a load recipe that produces statistical consistent results.

[BD1]

The link in your post takes me to a "view only" spread sheet. So I cannot enter different values

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Do you include the muzzle brake in the barrel length? Or just the actual length from bolt face to end of rifled barrel?

[alwie]

The link in your post takes me to a "view only" spread sheet. So I cannot enter different values

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Do you include the muzzle brake in the barrel length? Or just the actual  length from bolt face to end of rifled barrel?

You have to make a copy of the spreadsheet.  The copy should be owned by you. Give yourself edit permissions if you still cant edit it.

Per my research …. barrel length only. Do not include anything attached to the barrel as the shockwave does not propagates throo the connection (my understanding).

[grayfox]

In actuality the longitudinal wave propagating in the barrel will go to the end of whatever the "pipe" is, meaning in reality it will go to the end of the brake before reflecting back, taking some minute amount of time to traverse that brake in both directions. That is the nature of wave propagation. So it will affect the timing of wave travel, and the timing of peaks that occur at the actual muzzle tip... but it is an effect that I haven't been able to figure out how to account for. I also think (but have not proved) that a brake will cause some distortion and/or dampening of the wave due to its irregular surface (holes and gaps around the the circumference of the brake).

OP did you develop this all on your own, or is/are there others to whom credit is due.

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'Way back in my Navy days they had just come up with a time domain reflectometry ("TDR") device that could measure how far down a cable or waveguide a break or short was, so you could identify and repair it. I got to use it for some troubleshooting of our nuclear instrumentation, since the cables went all over creation in our ship... and we were having some cable issues.
This longitudinal wave theory of the rifle barrel is the same thing, only its waves go at the speed of sound in steel (roughly 18,000 fps), while in my TDR case back then, they went at speed of light (well, close to it).

[BD1]

OK, I copied the spreadsheet and I can enter values. How are you determining the CBTO, (and is this cartridge base to ogive?)

[Old Bob]

How do you arrive at a Node # & what are Fxn length & OBT?

Copied your spreadsheet & transferred it to Microsoft Excel. Seems to work OK. Changing any of the input numbers changed MV results.

[BD1]

It occurs to me that if CBTO is cartridge base to ogive, that I do not know if this refers to the full bullet diameter, and if so, is it the first full diameter going forward from the base of the bullet, or the last full bullet diameter before the ogive at the nose of the bullet.

[BD1]

So I looked up CBTO and read the definition on the Berger website. I determined the ogive by setting one caliper to .243 and sliding it down the nose until it just started to bind a bit and then measure that distance to the nose with a second caliper using the depth rod. I subtracted that measurement from the COAL to get CBTO. Using those numbers in the OBT spread sheet gave me mixed results. The best groups using LVR under the 105 grain Bergers fell in the predicted node 7, But the best groups using Varget under the 107 SMKs fell outside of any node, (between 7 and 8). So at this point I would say that my feedback is "indeterminate". My rifle has a Little Bastard brake and a Little Tuna barrel tuner on it. These things are large and it seems like they must have an effect.

[alwie]

How do you arrive at a Node # & what are Fxn length & OBT?

Copied your spreadsheet & transferred it to Microsoft Excel. Seems to work OK. Changing any of the input numbers changed MV results.

I change the node number in order to get an achievable range for the mv goal based on reloading data. 

Fxn length is the function barrel length after you take into account the cartridge … the bullet does not travel 18” in an 18” barrel … COAL for 6mm ARC is 2.260? I try calculating from the back of the bullet to the end of the barrel.

OBT  optimal barrel time  …. time the bullet will be in the barrel

[alwie]

But the best groups using Varget under the 107 SMKs fell outside of any node, (between 7 and 8). So at this point I would say that my feedback is "indeterminate".  My rifle has a Little Bastard brake and a Little Tuna barrel tuner on it. These things are large and it seems like they must have an effect.

Where was the best node between 7 & 8?   If you read the info ..… I believe that half nodes can produce good repeatability too. I get an accuracy 1/2 node on my Savage 110. I found accuracy nodes @ 7 and 6.5 …. but the 7 node looked to be more repeatable (smaller STD Dev)

[BD1]

All of my most accurate loads with Varget are 20 -50 fps above node 8, but well below 7. With LVR I have one good load in the node 7 range with the 105 Berger, but I can't get there with the 107 SMK or the 109 Berger without going well over 52,000 psi. I don't see much point in using LVR in the node 8 range as the results are not as good as Varget, probably due to being well below 100% case fill. The issue I see with both Varget and LVR is that they don't really shine until you get near 100% case capacity, or a little over. After messing with the spread sheet a good bit I don't see much advantage to it relative to my usual course of action, (as close to 100% case capacity as possible and .01 to .02 off the lands using the projectile of choice for the intended application). With this rifle the OBT theory likely would have led me astray. The good news is that It looks like I have a load using Varget under the 107 SMKs at 2.260 COAL: 2531fps, +/- 50,000 psi, that seems capable of 1/4 MOA. I have a couple other choices that show good promise, but QL predicts both of those are a bit above 52,000 psi, so I'm going with the lower pressure load for now.