Put on your nerd hat: vertical spread and fps


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jr_roosa
April 28, 2013, 01:18 PM
Ok, rather than spend this beautiful morning building a spreadsheet, does anybody have a formula that will estimate vertical spread at 600yds for a given velocity and extreme spread?

I have a debate going with my gunsmith on my next rifle project and I want to see if the chambering I want to use will be acceptable, or if I should cave and follow his advice. If I do cave, I want it to be because of real data from my own loads.

If there isn't a formula, does anybody want a copy of the spreadsheet I build this afternoon?

-J.

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BullfrogKen
April 28, 2013, 01:26 PM
What do you want to chamber it in, and what does he want?


And what's the intended target? Prairie dogs? Steel gongs? The 10 ring of a Mid-range (600 yard) NRA target?

jr_roosa
April 28, 2013, 01:43 PM
I'm being intentionally vague. ;)

I'll post the details after I churn the data.

-J.

BullfrogKen
April 28, 2013, 01:48 PM
OK then . . .


Just take your preferred round and his preferred round and run them both through the ballistics calculator here -

http://www.jbmballistics.com/

What's the difference?

By the way, I find wind a whole lot more important a consideration than range. I can get a pretty accurate calculation of range, either on a KD course or with my laser rangefinder, and eliminate that variable.

Wind on the other hand . . . much trickier to manage than distance.

jr_roosa
April 28, 2013, 04:25 PM
Thanks Ken,

I have the JBM tool, which gets me most of the way there.

I don't think I asked the question right.

What I want to do is estimate how much higher my fastest bullet will hit than my slowest bullet out of a measured string of shots over my chrony.

I think I just need to calculate how fast the bullet is falling at the target and multiply that by the difference in arrival time.

J.

jr_roosa
April 28, 2013, 07:21 PM
No takers, I guess. Now that the lawn is mowed, I guess I can crunch some numbers.

The 10 ring of a Mid-range (600 yard) NRA target?

You guessed it, BTW. Actually the X ring.

Turns out my gunsmith and I are both right.

I'm having him build me something goofy, and I wanted to avoid the comments on the goofiness. I am having him put together a Garand for highpower. There are a bunch of negatives to that, but the one we are debating is .308 vs .30-06. His argument is that the air space in the case of a .30-06 will end up with a 100fps spread in velocities depending on powder position, which even with everything else being perfect will put me above or below the 10 ring at 600 yards. With .308 there is a full case, and less FPS spread, and that's what he suggests that I build. My argument is I never see that kind of spread with my loads, and I really want it in .30-06 because, well just because. He says he can make a sub-moa Garand in either chambering, but that the .30-06 will handicap me at longer range.

My question is, with my loads, how much actual vertical spread could I actually anticipate, because I'm not quite the hard holder enough to do the test at 600yds, and I would already need the rifle built to do the test anyway.

So based on his numbers, which I believe he gets from Clark's NRA article where the author carefully made sure the powder was all the way forward vs. all the way back, and there is a 100 fps difference in velocity between the fastest and slowest rounds.

What I get in my spreadsheet with a 2481 fps mean velocity (taken from my 600yd load) and one bullet going 50fps faster and the other 50 fps slower, the fastest bullet will hit 3.6" high, the slowest 4.2" lower than dead center. This is based on the time the bullet falls from it's highest point, the speed it falls, and the difference in arrival time at the target. This would put me outside of the 6" X ring with a perfect rifle, and with a 1 moa rifle, would throw shots outside the 12" 10 ring.

However, my 600yd load in .30-06 gives me a 42fps extreme spread last time I checked it, and these are straight out of the powder measure. This gives me a high shot of 1.6" and a low shot of 1.9" which are both in the X-ring, and with even a 1 moa rifle are 4.6" high and 4.9" low in the worst cases, which is still in the 10-ring. I think that I won't be too handicapped

Looking online, I find some testing that reveals Federal GMM with 168gr Match kings in .308 was found to have a 9 round extreme spread of 27fps. This calculates to 0.9" above and below the center for the fastest and slowest bullets. I could stand to shrink my group's vertical stringing by 1.5 inches at 600yds, or be .75" closer to the center by going with .308 versus .30-06.

The next step is that I have a box of loads with 10 shots each at my current load and +1.5 gr and +3.0 gr to test with hand-measured charges to see what my best possible spread is as I decrease the air space in the case. I expect that the spread will be a little bigger with 10 rounds, since my old data are from a 5 round set. If I get these close to the federal match ammo, I think I will cast my die with the .30-06. If the extreme spread opens way up, then .308 it is.

If anybody wants the spreadsheet, I can send it. It was actually easier to build than I though. It probably isn't perfect, but it should be within a few percent of reality.

Nerd hat off.

-J.

jr_roosa
April 28, 2013, 07:25 PM
Wind on the other hand . . . much trickier to manage than distance.

Oh yeah it is. But the 10 ring is widest right in the middle!

J.

JSmith
April 28, 2013, 07:57 PM
...as I decrease the air space in the case.

That makes me wonder. I load for handgun calibers, not high=performance rifle, but even so my powder charges never fill the case more than about 1/3. Is there any sort of inert filler/wadding/separator available that would keep a powder charge more or less together at the base of the case instead of allowing it to flow around loosely in a volume that's much larger than the charge occupies?

I'd thought about using a case mouth to cut disks of paper or thin card stock and push those down on top of the powder but I decided to ask if anyone else had tried anything like that and what the result was.

jr_roosa
April 28, 2013, 08:20 PM
I don't think it matters as much with fast pistol powders. There is a ton of air space in my 45 bullseye loads and they get single digit standard deviations sometimes.

J.

35 Whelen
April 28, 2013, 11:03 PM
It irritates me when I (or anyone) asks a 'smith to build something, and they try to talk you in to something else. :cuss:

I'd suggest you build exactly what YOU want, as you'll be much more comfortable and much more happy with your rifle. I really like your idea of shooting a Garand. I never got really deep into High Power and only shoot our local reduced course matches, but when I do shoot, I get so tired of seeing cookie-cutter AR's (I shoot a K-31).

Occassionally, we have a middle-aged gentleman show up for our matches who shoots a Garand. He is a very pleasant, humble man and amazing to watch. He shoots his sighters offhand then proceeds to mop the floor with the hind ends of the AR shooters.

I wish I could remember who said it and exactly how they phrased it, but it was something like: "If riflemen (HP competitors)would spend as much time practicing with their rifles as they did scrutinizing the minutiae of handloads, they'd all be High Masters."

I built a 600 yd. range here at the house and erected a steel target that is an exact replica of a 600 yd. repair center right down to the scoring rings. I shot it a few times with my K-31 and GP-11 ammunition. Even with the relatively low B.C. 174 gr. bullet loafing along at an MV of 2540 fps I have no problem shooting scores in the high 80's/low 90's once I figured out my elevation. But like Bullfrog Ken said, the wind is the real issue. Misjudged wind speed by 7-8 mph, and your bullet is liable to completely miss the target. The best competitors I know could read the wind very well.

Good luck!

35W

jr_roosa
April 29, 2013, 12:17 AM
It irritates me when I (or anyone) asks a 'smith to build something, and they try to talk you in to something else.

Cuts both ways. It's nice to get feedback to tailor the project to the goals.

-J.

35 Whelen
April 29, 2013, 12:27 AM
Good point!

BullfrogKen
April 29, 2013, 12:38 AM
You'll be able to take advantage of heavier bullets in that .30-06.

Beyond that I'm not the guy to speak to about running a Garand in High Power.

Edarnold
April 29, 2013, 01:20 AM
Just to throw in another factor, your assumption that vertical spread depends only on differences in drop due to velocity variation assumes an infinitely rigid barrel. The barrel plus assorted hardware on a Garand is anything but; the vertical vibration of the barrel is going to depend on barrel time, which may either increase or decrease dispersion with velocity variation depending on which way it whips as the bullet is exiting.

One of the great things about the wide-spread availability of digital high-speed cameras is that we finally get to see the movement of rifle barrels before the bullet exits. Many of the TV programs on History channel and others use this technology, and the images sometimes make you wonder how rifles manage to hit anything! Best case in point is watching an AK flex from receiver to muzzle like a garden hose when fired.

IMHO

jr_roosa
April 29, 2013, 01:20 AM
You'll be able to take advantage of heavier bullets in that .30-06.

I picked up a Garand Gear gas plug for one of my current rifles for that very reason. It gives me a little more latitude in powder speed and volume, I can get closer to bolt gun loads, and the heavier bullets worry me a little less.

Just to throw in another factor, your assumption that vertical spread depends only on differences in drop due to velocity variation assumes an infinitely rigid barrel.

Oh boy, I used a lot of other assumptions in there to get my numbers. I really wanted an idea of the magnitude of the difference, but it's really a back-of-the-envelope estimation of what's going on.

-J.

jr_roosa
May 3, 2013, 03:58 AM
Update:

So I took my very carefully assembled handloads to the 100 yard underground tube (very cool setup, by the way) and my usual load still runs an extreme spread of 43fps and an SD of 16 fps even with measured loads and carefully getting the powder to the base of the case in a 10 shot string. Interestingly, when I start filling the case the SD and spread go way down.

+1.5gr gives me a 39 FPS spread and SD of 15
+3 gr gives me a 27 FPS spread and an SD of 9, and this brought me to another accuracy node about 100fps higher than my old best load.

This was just using the powder I have on hand.

Also, it occurs to me that I made a few squirrely assumptions with my calculations on drop above. I'm going to run my numbers with a different approach and see what I get.

-J.

jr_roosa
May 3, 2013, 04:22 AM
I re-ran my drop numbers with a different approach. I assumed that the barrel was level and that the bullet would drop to the target a few feet lower than the firing line.

100FPS spread (worst case) will result in about a 15" top to bottom group at 600yds, all else being perfect.

44FPS (my old load) spread will give a 6.8" group top to bottom.

27FPS (Match .308 ammo and my current best load) will give about a 3.4" group top to bottom.

That was an interesting thought experiment.

The actual experiment showing that I could make loads competitive with the specs of .308 commercial match ammo seals the deal. This one will be a .30-06 match garand.

-J.

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