What is considered great standard deviation in .223 precision load and how to get it?

[IMtheNRA]

I have a few terrific loads for my .223 precision AR that print really, really great groups at just about any range up to 1,000 yards. The SD of 20-shot samples is usually around 13 to 17. Despite this high number, downrange performance has been satisfactory out to 800 yards.

However, now that I'm shooting more often at 1,000 yards, I see that inconsistent velocity turns into huge vertical spreads out at that range.

I'm accustomed to single digit SDs in my precision .308 loads, but my SD is almost double in .223 even though I use the same high quality components and exactly the same technique when loading .223.

Both shot in ARs, both use Lapua brass, single stage loading, carefully consistent resizing, Vihtavouri powders dispensed from an RCBS Charge Master... There is no difference in my procedures between the two cartridges.

If I can consistently get SD of 6 and 7 in my .308 using this technique, why doesn't it work in my .223 loads?

[Metal god]

Do you shoot them in the same position . Prone , off a bench , bi-pod etc . If you are shooting the 308 in any way different you could see that in your ES/SD

I'm assuming your 308 is a bolt gun while your AR has a gas system ???? The gas bleeding off during dwell time could cause some slight inconsistencies .

I'm sure you are using different primers , That could easily cause the difference .

Combine all three or just two of the above and I'd say that could be the issue . FWIW I'm pretty happy with my AR having an SD under 20fps

I load very similarly to you and my ES/SD in my 308 target rifle has always been better then my best ES/SD in my NM service rifle . I never gave it much thought . Oh and thanks for that . Now I'm going to be freaking out about it

[IMtheNRA]

Both the .223 and .308 are AR rifles, configured almost the same, and both with gas systems adjusted "just so" for their favorite loads. I shoot from prone and bench, and the chrono results are consistent for the cartridge regardless of my shooting position.

Both are high quality bullets - Berger in .223 and Lapua in .308.

All I can think of is that even a tiny variation in charge, lets say 0.05 grains, represents a significantly larger percentage of the whole charge in .223 than it does in .308, so my SD is going to be higher.

If that's the cause of my larger SDs in .223, then I'll have to start finishing the Charge Master charges with a pair of tweezers, a granule at a time to drive down the SD for my 1,000 yard .223 loads. This is a technique that I'd like to avoid...

[overthere]

You might want to consider a better scale than the charge master, such as a sartorious or ad FX. You would be surprised at how much deviation there is in the charge master. A readout of 23.5 could be anywhere from 23.4 to 23.6.

What projectile are you shooting out of your AR and at what velocity (for your 1000 yard load)?

[IMtheNRA]

For the 1,000 yard .223 AR-15 load, I use a Berger 80gr VLD Target PN:22422 with an average muzzle velocity of 2,537 at 62*F and my usual altitude of 5200 feet and higher.

[Jimro]

The performance of 223 beyond 800 is one of the reasons why it has never done well in Palma, despite having a lot going for it.

The biggest reason: Primer inconsistency and case volume interaction. The much smaller case volume of a 223 magnifies the effect on SD of the initial explosion of the primer. Even though it's a small rifle primer, it's still a much larger part of the initial pressure than a large rifle primer in 308.

The 308 case volume is really helps smooth out initial pressure inconsistencies which leads to better velocity SD. Even the small rifle primer 308 Palma cases don't do any better on SD than standard 308 Win cases, but the small rifle primer pocket lets them take higher pressures before case head expansion.

If you can find a particularly uniform lot of primers, then your SD can go down. Maybe you could try lubricating the case necks with some graphite for a few FPS less SD, after all that should uniform the release pressure needed to get the bullet out of the case neck, which might help.

Jimro

[IMtheNRA]

Thank you Jimro. I'm already planning to lube the bullets with wet lube prior to seating a test batch this weekend. I saw that another member here had posted some great SD results after lubing bullets, but I don't think it was in .223 Nonetheless, it was an interesting thread, and I'm going to try a couple of things I read about there.

I agree with you that the relatively small case volume of .223 results in magnified effects of primer variation, case inconsistency, and charge variation. That's why I asked what is considered to be great SD in .223, because if low teens are all that I can achieve on a regular basis, I'll recalibrate my expectations for this cartridge's performance.

[Metal god]

Another thing to consider is powder choice . I've read that some powders are more consistent at low to mid pressure range but when they're pushed hard they become a little less consistent. I've heard varget likes to be pushed hard . Not sure all the powders you've tested , I'm just thinking out loud here .

[Jimro]

IMtheNRA,

Like I said before, you might try looking for a more consistent lot of primers, I think that you should try CCI-BR4, CCI #41, Rem 7 1/2, and Wolf KVBM556. A tray of 100 primers won't set you back much, and you can do full load workups to see if you can't decrease your SD.

http://www.p3multigun.com/reloading-...ng-comparison/

http://precisionrifleblog.com/2012/0...ion-reloading/

Wolf brand primers get good reviews: http://www.savageshooters.com/archiv...php/t-426.html

If you can drop down to below 12 fps with a 223 Rem, I'd consider that really good.

Jimro

[Unclenick]

Not just the consistency of the primers, but how you seat them.

"There is some debate about how deeply primers should be seated. I don’t pretend to have all the answers about this, but I have experimented with seating primers to different depths and seeing what happens on the chronograph and target paper, and so far I’ve obtained my best results seating them hard, pushing them in past the point where the anvil can be felt hitting the bottom of the pocket. Doing this, I can almost always get velocity standard deviations of less than 10 feet per second, even with magnum cartridges and long-bodied standards on the ’06 case, and I haven’t been able to accomplish that seating primers to lesser depths."

Dan Hackett
Precision Shooting Reloading Guide, Precision Shooting Inc., Pub. (R.I.P.), Manchester, CT, 1995, p. 271.

Manufacturers typically suggest around 0.003" compression past the point the primer anvil can be felt touching the bottom of the primer pocket.

I'll also point out I've seen up to 80 fps MV difference between having the powder over the flash hole and having it froward over the bullet in the long .30-06 case. Something you can use enough of to get very slight compression, like IMR4064 in the .308 with 175 grain MatchKings, or H4895 in .223 with 77 grain bullets. Just slight compression tends to lock grains in place. This also prevents transportation vibration from changing the powder packing, which can change effective burn rate between home and the range.

I don't know what group size you normally shoot, but if it is 10 shots, you can figure SD is typically going to be 1/3 the value of the extreme spread, with exception on occasion. It is, however, the extreme spread you want to worry about when it comes to vertical string size at a 1000 yards.

[IMtheNRA]

In .223, I use CCI "Bench Rest" primers - they have a reputation for quality and I like their extra thickness to resist slamfires in my ARs.

Unclenick, do you load .223, and if so, what is your usual SD and ES among your precision.223 loads?

[JeepHammer]

No comment, you all are doing better than I am.

[Unclenick]

IMtheNRA,

Yes, I do load .223. I am, as you might guess, an experimenter, so I seldom settle on one load over the long haul. As a result, I can't tell you that something that gave me a certain result with cases, primers, bullets and powder at one time would give me that same result with different lots today.

The other thing is, when you collect a lot of data over time you get some unrepresentative results that are randomly better or worse than usual. On that point, I have had small groups (5) with SD's as low as 0 after rounding to the nearest whole foot per second. Do I expect that to be repeatable? No. Do I expect it to hold up with a more reasonable sample size? No. It just randomly happens once in a long while and may as easily be due to random chronograph errors that happen to neutralize some velocity variation as due to getting an actual number randomly that small.

I also have to point out that small SD doesn't automatically mean you get the smallest groups. You still need to be on a barrel time sweet spot for that, too.

In general, I don't look at SD's in fps, but in % of the mean (average) velocity for the load. You see a lot of folks chronographing rifles report SD's in the 0.5-1% range. For a 2700 fps load, that's 13.5-27 fps. I like to try try for 0.3% and figure there is room for improvement if I haven't got there yet. For a 2700 fps load, that's an ES of 8 fps. For a 3200 fps load, it is almost 10 fps.

I've achieved numbers below 0.3%, but I don't really trust optical chronographs to be accurate enough to make accurate SD determinations smaller than that without really large sample sizes. Really large sample sizes often take long enough to shoot that light and other ambient conditions have changed during the session, adding another variable into the mix.

This year I bought a LabRadar chronograph which claims a minimum of 0.1% precision, so I should be able to get some tighter SD's that are real using that. I just haven't had a lot of range time this year, and that hinderrf experimenting with it.

There are things you can do to cut SD down: controlling firing conditions and barrel temperature. Choosing the right primer. I've had some really low .223 SD's using the inexpensive Russian Tula KVB556 primer, which is a military sensitivity spec primer for floating firing pin guns like the AR. I bought a slip of 5,000 at one point, and that lot, at least, has proved very, very consistent. Tighter than Federal 205M's, even. The only problem with them is it takes some extra effort to seat them.

For seating, I've been using the K&M Primer Gauge tool whenever really tight results were what I was after. For all small primers except the 205M I go 0.003" below the point of anvil contact with the floor of the primer pocket. For the 205M, Federal recommends 0.002" below contact to set the bridge, so I've been using that, though I haven't notice real difference with that an -0.003" with it. The K&M tool is the only one that gives results individualized for the particular case and primer and that is measured. But man is it slow going priming that way. I'd never bother for my 200 and 300 yard service rifle match loads. Only the 600 yard slow fire loads get that kind of tender care.

I always deburr flash holes anymore. I've proven to my own satisfaction that it makes little difference with easily ignited stick powders, but with older ball powder formulations and loads that don't fill a case well, in particular, it can reduce SD measurably. So now I just make it a part of routine case prep so I don't have to wonder if I mismatched a case with a wrong powder.

For cases I anneal and either lightly brush the insides with a bore brush in my drill press at low speed after resizing and doing any trimming that may be needed. The I apply a little graphite and alcohol solution and let that dry before charging and seating. I skip the graphite step with moly-coated bullets.

For super tight loads of stick powders, I use a volumetric/weight hybrid powder dispensing technique. I put the primed, empty case on the scale, tare it, dispense the powder volumetrically, then look at the charge weight by putting the charged case back on the scale. If I hit the right charge weight, it's a go. Otherwise, I dump it, check there are no stray grains left behind in the case, then do it over again. I'm using the JDS QuickMeasure for this as it is the most consistent stick powder measure I've tried and gives me the fewest re-do's. Nonetheless, this is tedious. I could also charge small and trickle up, but that takes time, too. Nonetheless, I should try to find time to compare how the two approaches perform.

The rationale behind that dispensing approach is the number of reports from folks getting better accuracy from volumetrically dispensed loads than from weighed ones with some powders, and particularly with stick powders. Apparently a heavy charge from a volumetric dispenser tends to be more tightly packed powder, and more tightly packed powder tends to burn more slowly which, in some instances can compensate for the charge weight error nicely. In others, though, that doesn't work out, so this has to be tested with your load to see if it holds up or not. But anyway, I figure that if both the volume and the weight are correct, I've got both bases covered, and if I take a load into production for the season's shooting, I can figure out which is better to do then.

Obviously, if your barrel temperature, bolt closing to firing time and level of bore fouling are kept constant, that helps, too, but probably isn't practical for match shooting where you can't normally control all those things.

[tangolima]

I am with unclenick. Absolute SD has little meaning. I look at SD / velocity in % as well. I used to fixate on it, trying to achieve 0.3%, till I realized small SD didn't always mean small group. Now I only use it as a secondary figure of merit, mostly as indication of chrono readings' validity.

Variation in velocity translate into variant in poi, no doubt. But that translation process is function of other factors; velocity, bullet weight etc. That's why sometimes good SD in velocity gives lousy group.

It is statistics. Sample size has a lot of say. Shoot more before drawing any conclusion.

-TL

[Jeffm004]

We hired a guy that shoots competitively. Other than costing me money, he claims to have a SD of 7 with .223. I have no idea how but I guess it can be done.

After loading he has a gadget that measures the concentricity of the case to the neck. Those that are off are for under 600. Not SD related but maybe helpful.

[tangolima]

(SD / velocity) =0.3% is equivalent to +/-1% 6-sigma variation. That's excellent even for precision electronic systems. Chronograph needs to be good enough to measure it, before spending money and time to chase that sort of performance.

I highly doubt my cheapy chronograph can handle that, so I take (SD / velocity) < 1% with sample size of no less than 10.

-TL