What makes bullets vary in velocity?

[FLChinook]

I also just got a new chronograph; my first one. I used it yesterday and was gob-smacked at how much I liked it. Why didn't I get one about 20 years ago!!!

I got the very inexpensive (on Amazon) Competition Electronics Prochrono Digital. It says it has an accuracy of +/- 1%. That's +/- 30 fps at 3000 fps (I wonder if these things can be more accurate than that. The readings come out to 4 digits... ).

Using this for the first time yesterday, I was shooting three shot groups of different loads for my .257 Wby Mag. Groups where the chronograph gave very similar readings were nice and tight. If one of the readings was different by 150 fps, that shot was always 1" or so outside the group.

AMAZING... rounds having the same velocity group nicely and rounds with velocities different from the group are fliers. Physics works!!!

But why the different velocities??? I weigh and group bullets. My RCBS digital scale gives me powder weights to probably 0.05 grain. My primers come from the same box. There seem to be random processes at work here and yet there are some consistencies. For example: For this gun, in every case but one, the minimum loads gave the most velocity consistency and the greatest accuracy.

How can you know if a gun has inherent accuracy when possibly the greatest source of inaccuracy comes from spurious bullet velocities? How can I get a handle on this...??

Thanks

[Bart B.]

How hard/soft you hold the rifle to your shoulder will cause anywhere from 20 to 100 fps spread in muzzle velocity. It's the old Newton's Law.

Powder doesn't make exactly the same pressure curve each time a charge of it burns, so that's one of the variables effecting muzzle velocity.

Primers do not put out exactly the same amount of fire for the same length of time for each one in a box. That determines how the powder starts buring and inturn how the pressure curve's shaped.

Bullets gripped by the case neck with different amounts of neck tension will have a different amount of release force pushing on them to move them into the rifling. Just another thing that determines the pressure curve shape and therefore, muzzle velocity.

And minimum, or loads below the absolute maximum limit for pressure, often give better accuracy. Their pressure curves from the primer firing and powder burning is more repeatable from shot to shot. Bullets leave not only at about the same muzzle velocity but also at the same place in the barrel's vertical whip at the muzzle.

And all this is bullet-powder-primer dependent. A different powder and charge weight with a different primer or bullet may need near maximum charge weights to get best accuracy.

[FLChinook]

How hard/soft you hold the rifle to your shoulder will cause anywhere from 20 to 100 fps spread in muzzle velocity. It's the old Newton's Law.

Thanks, Bart. I hadn't thought about that. I shoot from a very solid bench rest but I do use my shoulder and not a "lead sled". 100 fps is about 3% and I guess a very sloppy shoulder could cause that. I'll focus on this next time...

I also hadn't thought about the grip of the brass neck to the bullet but if I use the same lot of brass and resize the same way, that should be pretty minimal.

I know that variables like size of flash hole can, in theory, make a difference but that should also be small; again, if brass is kept in matched sets.

I'd like to think that primers, while certainly different between brands/types, would be a very small factor so long as consistent brands/types/lots are used.

Until I had my chronograph, I was attributing all bad groups to my gun but it's now clear that bullet velocity variability is a big culprit and must be controlled as much as possible.

[swmike]

The largest reasons for speed variations have to do with the following:

Proper case preparation. If you have varying neck tensions you'll have speed variations.

Powder volume to case volume ratio. Powders will burn more consistently when they are "squeezed just right" Every powder has a sweet spot when it fills the right percentage of the case. This is often revealed when shooting all load development over a chronograph. You'll see some charge wieghts with huge speed variations (SD, ES), and others that suddenly tighten up then spread out as you pass the sweet spot. You will also reach a point where each increase in charge weight will yield less and less of a speed increase.

Primer consistency also yields variations in speed. If you light off a charge differently with each subsequent shot, what would you expect. Some standard primers are fairly consistent and others are atrocious. If consistency is important then consider the Bench Rest primers although I've found that in my .308, Wolf and Tula primers are just about as consistent (single digit ES, SD)

Variations in seating depth. Just like with having the right powder charge you need the same amount of "jump" or "jam" on a bullet when the round is chambered. If you merely measure the finished round from tip to case head you haven't taken into consideration any ogive to tip variations. Better to measure the finished length of your round with a comparator attached to your caliper. This will allow you to measure the lengths of your rounds from case head to the point that will come in contact with the rifling first.

If you want the speeds to all be as close as possible then you have to control ALL the variables you can.


Lastly, optical chronographs (most all with the exception of the Magnetospeed) rely on the bullet causing a change in light level in the sensor. Measured speeds can vary, depending on what part of the bullet is causing the sensor to be triggered. Varying lighting conditions are a huge issue for most. First, make sure that your chronograph has a steady source of light. May have to even tilt the sensors and screens to a different part of the sky (left or right) in order to have this. Second, some find that blackening your bullets with a marking pen helps create a bigger "target" for the sensor and thus a more consistent reading. You also want to make sure you bullet's line of travel over the sensors is consistent. If you're just "shooting the sand" and vary your aim point then the sensor distance in relationship to the bullet path is not consistent. A small fraction here can make a significant error in readings.

BTW, if you are holding your rifle different with every shot, then your larger problem will be accuracy, not the speed.

[Sierra280]

Looks like someone needs to mention significant digits. You can never be more accurate than your most accurate measure! If your chrono has a 1% margin of error (really good BTW) then that's 10fps at 1000fps, so if your chrono displays out to 4, 5 or even 10 decimal places, they are insignificant. For example if your chrono reads 2518.3835fps, recording that number is just silly. Now say 2518+\- 25fps, or 2500fps-2550fps. Just remember, your records cannot be more accurate than your most accurate measure.

Edit--why does the chrono go out to 4 digits? At 1fps that's still only accurate to .01

That being said, minimizing inconsistency is the most important thing you can do. Uniform flashholes, primer pockets, ALWAYS trim to length, weigh every brass (.2grs is a good variation to go with) weigh every powder charge, seat bullets consistantly (check OAL to ogive), etc.

Always record everything, everything is quantifiable, with clear and detailed records it makes it easier to identify and minimize inconsistency.

[Bart B.]

Then there are some rifles whose accuracy the longer ranges was improved with ammo having large spreads in muzzle velocity. When loaded with cartridges with small spreads, they were not nearly as accurate far, far away.

The British SMLE in .303, for example. Even the military M14NM's had a little bit of this helping them shoot accurate at the longer anges

[tangolima]

Using this for the first time yesterday, I was shooting three shot groups of different loads for my .257 Wby Mag. Groups where the chronograph gave very similar readings were nice and tight. If one of the readings was different by 150 fps, that shot was always 1" or so outside the group.

One factor that can possibly cause error in chrono readings is muzzle flash. I'd like to place the chronograph at least 5 yards away from the muzzle. That sometimes is a problem as I could hardly read the number off its LCD display.

You didn't mention the details of setting, but I assume you had range of 100 yards, muzzle velocity of 3000 fps, the group size of 1" was sole due to vertical stringing, and the worst case speed variation was 150 fps (5%). The muzzle velocity variation alone causes vertical stringing of about 0.5", the rest is due to other factors. You can work on your load to shrink the variation down to 1%, but your group will probably be no smaller than 0.75". I know it is whole bunch of hot air till you shoot your loads. If it works, it works.

-TL

[FLChinook]

Thanks to you all for some very helpful input. Here's my specific data for 8 three shot groups; the first 4 using IMR4831 and the second 4 using RL25; each group having loads increasing from min to max in 1 grain increments (more or less)

AvVel SD
2920 102 Min
2888 10
2990 68
3034 89 Max
2932 3 Min
3016 21
3056 28
3103 36 Max

Except for the first case, which had one very spurious velocity reading, the Standard Deviation of the velocity readings increased from min to max. I suspect when I redo the test of min-4831 I'll get better results.

Other comments:

1. Cloudy, windless day.
2. Chronometer sited 15-ft in front of muzzle.
3. As much as possible, all targets were fired through the middle of the "antennae" and at the same elevation above the box.
4. All at 100 yards.
5. Rounds with higher velocities always impacted above the group.
6. The one higher velocity in the first group was 200 fps above the average of the other two values and it impacted 2.5" above the other two (which were touching side-by-side).

This data, while very small, suggests my .257 Weatherby is shooting pretty well and variations in round velocity are the main culprit in "fliers".

I am doing most of the suggestions offered here. The only thing I haven't done is chose a powder that more fills my cartridge. According to Berger, that would be VIHT N170 (which I don't have).

Two last questions:

1) How much velocity variation can be expected from primer variability?

There's some data on the effects of different primers:

http://www.accuratereloading.com/primer.html

But this data is pretty hard to use.

2) Which primer do reloaders recommend for a .257 Weatherby? Do any of you use standard large rifle primers in lieu of large rifle magnum?

Thanks again

[tobnpr]

You can use magnum primers in non-magnum applications (they're hotter, so loads might need to be reduced), but not vice-verse, far as I know...

They're also useful with ball powders.

You need that hotter spark plug with magnums due to the large case capacity.

The chrono provides useful information, but I also did successful load development before I got one, just doing OCW (groups), looking for vertical, and never having a clue how fast the bullet was traveling. Vertical told me I had excessive SD.

Suggest you don't spend too much time "hair-splitting". I've found that sending lots of rounds downrange and gaining experience to make accurate wind calls is far more important than trying to control every variable (which as Bart noted above, some are always variables).

[old roper]

If your 257 Wby is a factory rifle it will have the Wby Freebore

Here the Wby chart on their freebore

What is the throat length (free bore) on your Weatherby® rifles chambered for Weatherby® magnum calibers?
Cartridge Throat length (free bore)

.224 Weatherby Magnum .162
.240 Weatherby Magnum .169
.257 Weatherby Magnum .378
.270 Weatherby Magnum .378
7MM Weatherby Magnum .378
.300 Weatherby Magnum .361
.340 Weatherby Magnum .373
.375 Weatherby Magnum .373
.378 Weatherby Magnum .756
.416 Weatherby Magnum .239
.460 Weatherby Magnum .756
.30-378 Weatherby Magnum .361
.338-378 Weatherby Magnum .361

You might want to get a better understanding about the freebore and how to load for that before making too many change. Wby has a forum
http://www.weatherbynation.com/user/register You might be better off over their as poster are loading for the Wby not saying this is a bad site.

My experience is with Custom 257Wby I had it build without the freebore and my new 270Wby is build same way.

[Bart B.]

Tobnpr, I've used mild RWS standard primers in .30-.338's when temperatures were in the 30's. With 65 grains of 4350 under 190's and 200's, excellent accuracy at long range.

[FLChinook]

Here the Wby chart on their freebore

Can I ask where you got this table. I wonder why Weatherby gave the .257-7mm calibers such a large free bore?

I logged into the Weatherbynation site and searched on this topic. Only one hit came up and the comments all said to not worry about free bore... Can you discuss briefly how it would affect my reloading; faster powder to account for the larger volume?

Regarding primers, the volume of my .257 Wby Mag is casing is not much more than some of the larger non-mag calibers. Do you think Large Rifle primers would give a better consistency than LRM primers for the 257?

[jimbob86]

The very same things that make ballisticians grey!



http://www.leverguns.com/articles/ballisticians.htm

[FLChinook]

The very same things that make ballisticians grey!

This article describes tests of different bullets in different handguns. In my case, I'm using the same gun and, as much as possible, the same hand loads; I'm trying to define the parameters that give me velocity variation, to quantify them and decide which, if any, I can better control...

[FLChinook]

weigh every brass (.2grs is a good variation to go with)

OK posters, I think I've found my problem. It's my brain... or my lack of using it...

First, let me say that all my .257 Wby brass is from Weatherby. I use some Nosler, Federal and Lapua with my .300 but my .257 is all Weatherby. For that reason, I did not weigh the brass.

I was setting up to reload some more rounds using a different batch of Weatherby brass when I noticed (to my horror) that the stampings on the heads were different. It seems I have three different head stamps on my brass and they go with three different brass weights. One of the "lots" weighed circa 235 grains and another weighed circa 210 grains. That's a 10% difference!! The other "lot" was in the middle.

There's a bigger range of weights in the heavier brass (+/- 3 grains) than in the lighter brass (+/- 1 grain) but .2 grains is an impossible target for this stuff. Maybe I need to buy some Nosler or Lapua for the .257...

Anyway, I thought I'd better fess up...

[old roper]

FLChinook, One thing is missing and that is what bullet are you shooting? 2900fps is more or less start load looking at Hodgdon and Nolser data for the 257Wby.

I should mention case weight vs case volume big difference between the two for accuracy. H2o capacity for the 257Wby is 84gr and if you check the case capacity on the heavy cases see what that is.

[dahermit]

The variation of individual volume of the cases can be a factor also. I do not know about conventional jacketed bullet bench rest competitions, but in rifle cast lead bullet CBA (Cast Lead Bullet Association), competitions, there are some shooters who shoot the round with just a single case, just reloading it at the bench for each shot.
It may be an interesting experiment to do that in your quest for uniformity, just to see how much of a factor the case itself is in the matter of uniform velocity. If there is still variation, then you would know for sure it is not the result of case variation, but has to be something else.

[Bart B.]

dahermit's correct about extreme spreads in case weight effecting muzzle velocity for a given charge and bullet weight.

I've chronographed reduced loads in .308 Win. cases with weight spreads from 150 to 180 grains; a 20% increase. Those heavier cases shot the same bullet out about 70 fps faster.

As the heavier cases had thicker neck walls, I made sure they had the same mouth diameter and neck tension/grip on the bullets so only case capacity would be the difference. Otherwise, the heavier cases may well have gripped the bullets harder and that alone would cause muzzle velocity to be faster.