Chrony question

[Dr Killdeer]

I bought a Chrony from Midway and I tested some 06 rounds. My hunting loads are very accurate out to 200 yards, but they’re slow compared to what I could kick them out at. They’re one half grain over the accuracy load spec as stated in the manual for that bullet/powder combo. They shot so well that I decided to leave well enough alone.

But, what I’m curious about is the variation in bullet speeds.
How much of a deviation in FPS between the fastest and slowest bullets is normal for an accurate load?

Since the load I tested is accurate, is it safe to assume that I can use the speed spread from this load as baseline data when testing other 06 loads?

The Chrony is a fun gadget, but I’m not sure how to apply the data it provides to future load development or even existing load refinement.

[Brian Pfleuger]

Some people obsess about standard deviation but it has been shown that loads with a low SD are no always accuracy loads.

The chrony is a nice tool. By itself, it's mainly only useful for telling you if you're "in the ballpark" with a certain load velocity. Even then, the ballpark is a big place so it's not that useful.

The chrony is good for a few other things though, like verifying velocity increases with load increase. An incremental powder increase that does not result in an expected velocity increase, or even a decrease, can be a sign of high-pressure. I don't know if it's across the board true, but my rifles (204 and 7mm-08) show a very linear velocity increase with charge increase but the graph begins to flatten out at higher charges. The chrony is good for finding that point of diminishing returns.

[COSteve]

I bought a Chrony from Midway and I tested some 06 rounds. My hunting loads are very accurate out to 200 yards, but they’re slow compared to what I could kick them out at. They’re one half grain over the accuracy load spec as stated in the manual for that bullet/powder combo. They shot so well that I decided to leave well enough alone.

When reloading for accuracy, you are looking for what works best in your particular rifle, not one that works OK in everyone's. If you're happy with your groups at that charge weight, then you are good to go.

Generally, the barrel harmonics are such that there are two different velocities (charge weights) that give you the best groups. They will be different for each load, may vary with temperature, and are somewhat unique for each weapon.

But, what I’m curious about is the variation in bullet speeds. How much of a deviation in FPS between the fastest and slowest bullets is normal for an accurate load?

You are asking about Standard Deviation (SD) and the lower the variation, the more accurate your ammo will be for that velocity, however, you may need to adjust the velocity to 'tune' the load to your barrel harmonics to get the most accurate load. I usually work to get my rifle loads to under a SD of 10.0 as calculated by my chrono.

Since the load I tested is accurate, is it safe to assume that I can use the speed spread from this load as baseline data when testing other 06 loads?

Likely not as each case, primer, powder, bullet, OAL, crimp, etc. combination is unique and simply changing a single component can drastically affect consistency and accuracy. That's why you need to start low and work up each load to see what works best in your rifle.

The Chrony is a fun gadget, but I’m not sure how to apply the data it provides to future load development or even existing load refinement.

Generally, you're using the chrono to help you refine your loads to give the smallest SD and ES you can get for your accurate loads. You may note differences in SD from something as small as increasing/decreasing the crimp on your ammo. Noting the velocity spread changes and give you an indication on which way works better. That's why it's best to change a single variable at a time and note the results.

[oldscot3]

Extreme Spread, I believe, is what you're referring to, the difference between your fastest and slowest round in a string. It's one of those topics that can generate an an extreme spread of opinions as to what it indicates.

Conventional wisdom suggests that a given recipe which delivers a low extreme spread is more consistent in ignition and therefore should be near the sweet spot for a given combination of powder, primer and bullet. A low ES should, theoretically, vibrate your barrel closer to the same for each shot and therefore be more accurate.

As is usually the case though, it's not quite that simple. To discern the effects, on a group, between two rounds with say a 70 fps ES, would be hard to do since so many other factors come into play regarding accuracy.

I like to see a low ES, but if a load shoots well, as far as I'm concerned, thats the bottom line.

[ZeSpectre]

But, what I’m curious about is the variation in bullet speeds.
How much of a deviation in FPS between the fastest and slowest bullets is normal for an accurate load?

I agree with Pizzakilla, don't get too hung up on the connection between deviation and accuracy. For example I have a couple of target pistol loads that demonstrate an awfully wide (almost 80fps) deviation that are still wonderfully accurate.

Some powders (Blue Dot leaps to mind) have a pretty narrow "sweet spot" in terms of performance. Below or above and the deviation gets pretty wild but using a chrony to find that sweet spot (with MY pistol) gave me a decent SD of around 20 fps.

[Unclenick]

Many guns have bands of barrel times over which they are accurate because the barrel doesn't move much in this dead zone. At short ranges, like 100 yards, the trajectory difference is so small it doesn't open groups up much. At long range it is another story. A velocity difference that alters point of impact a tenth of an inch at 100 yards can make a foot or two of difference at 1000 yards, depending on the bullet and load.

There are usually multiple accuracy sweet spots in a barrel. More than just two if you don't restrict yourself to near-full power loads. Take a look at Dan Newberry's site for a systematic method of finding wide charge tolerance sweet spots.

If you decide to shoot at long range, then you do need to narrow ES. That usually is approached by doing anything that improves ignition. How much you need to do depends on the powder you choose and how well it fills the case. In general, stick powders are less ignition sensitive than spherical propellants. If the case is not well-filled, sometimes going to a magnum primer can help. Just seating the primers hard can improve velocity variation. Deburring case flash holes helped me substantially with a partial case full of Accurate 2520 in .308, but did nothing for me with stick powders using the same bullet in the same gun.

So, you just have to try some things to see how they work out in your gun. Be aware that changing primers means you need to back the charge off 5% and work back up to your accuracy spot while watching for pressure signs.

[SomeGuyInMidwest]

As your post states, if you are mostly using your loads for hunting at 200 yards or so, the SD will not matter much so long as you are happy with the accuracy of you loads. Personally, for hunting loads, I find the load that groups the best in my rifle before I ever shoot it through a chrony. When I am satisfied with my load, I shoot it through a chrony to learn the velocity so I can calculate the drop off and wind drift for hunting ranges.

I also use the charts along with data from the bullet manufacturer to determine the maximum range for the gun/load. For example, a 180 gr Nosler Partition requires a velocity of 1800 fps to expand properly. I can chart that my particular load for my 300 win mag will expand properly to 525-550 yards. Beyond that, the bullet will not expand properly (assuming I would ever shoot something that far).

I also see higher SD when the light conditions are not perfect. Seems that a few random clouds that cast a shadow or moving shadow impact results. I have had the best consistency when I take a black sharpie and color the bullets all black before shooting.

[RWNielsen]

I also see higher SD when the light conditions are not perfect. Seems that a few random clouds that cast a shadow or moving shadow impact results. I have had the best consistency when I take a black sharpie and color the bullets all black before shooting.

I'm going to try that out on some .308s. Thanks for the tip

[Dr Killdeer]

The info you guys provided is enlightening, to say the least. I can live with a 46 ft/sec variation form high to low speed. The load is accurate and I don’t know if narrowing the gap would prove anything. The real proof is already on the paper.

Here, we usually take deer at short ranges, under 75 yards. But as SomeGuyInMidwest mentioned, I was curious about down range speed, because I’m using a 180 grain Nosler Accu Bond. So, I thought if the Chrony can provide me with baseline data, maybe I could use it to save myself some bench time with other loads. But, it doesn’t work like that.

In a way it’s a no brainer, because I’ll never shoot at anything that far away around here, but it's fun to experiment and I learned a few things from the feedback I got on the forum. Things that I need to consider, besides speed.
The Chrony is a nifty device, but as you guys mentioned, don’t get wrapped around the axle over it. I Copy that!

Thanks,
Joe