Low ES vs. large groups and vice versus

[1shot]

I could use any advice offered on this one.
I’m doing load development on 6.5mm cartridge that will be used for 1K shooting only, and I’m running into a very perplexing problem. I will give you all the stats that I have thus far. I’m using fully prepped Lapua brass. Fully prepped as in, I have 200 pieces of brass that are with-in .8 of a grain. All primer pickets have been uniformed, as have the flash holes and burrs removed from the inside. (They were very few, but did exist.) Necks were expanded then turned to a uniform thickness to allow for .0025-neck clearance with a loaded round and bullet grip is .0028, case mouths are chamfered and deburred. I have repeated this test a second time keeping all other variables the same but switching bullets. Bullets used for the test are 139 Scenars and 142 SMK’s. Please keep in mind that this is all new, unfired brass and I am fireforming as I go.
During my testing I have two loads that show promise, but at opposite ends of the spectrum so to speak. For the purpose of trying not to muddy the waters, loads #1 and #3 will be identical with only the bullet change, as will be loads #2 and #4.
Both loads are identical with the exception of the make of powder and the charge weight for that specific powder.
I have omitted the actual cartridge and charge weights for liability reasons, as well as they are not relevant to the problem at hand.
All shooting was done at 100 yards from a rest. Given the fact that this is a dedicated 1K rifle, my questions are two fold.
#1) Why am I getting large groups with small extreme spreads?
#2) If you were faced with this problem, which load would you take to a match? The load with the small group, or the one with the low ES?

Load #1 with 142 SMK/ N-160 powder
Ave. of 2953 fps
ES 4 fps.
Group size .840 – 5 rounds

Load #2 with 142 SMK/ H-4831sc powder
Ave. of 2878 fps
ES 52 fps.
Group size .333 – 5 rounds

Load #3 with 139 Scenar/ N-160 powder
Ave. of 3,005 fps
ES 30 fps.
Group size .436 – 5 rounds

Load #4 with 139 Scenar/ H-4831sc powder
Ave. of 2,906 fps
ES 47 fps.
Group size .244 – 5 rounds

Thank you all in advance for any advice,
Lloyd

 

[Laurie]

Lloyd,

there is rarely any direct relationship between ES spread and short-range group size. Grouping is about finding the action-barrel-stock combination's 'sweet spot', that is where its harmonics are such that it points the muzzle to the same place when the bullet exits the muzzle, and is not affected by pressure and pressure build-up changes as the powder charge burns. The Audette Ladder test method is all about finding such spots, and doing so at a potentially more meaningful range than 100yd. (I won't get into the arguments about whether you need to do testing at a minimum range of 300 or whatever. Personally, I reckon that you can learn a lot at 100yd, providing the test batches are chronographed.)

In an ideal world, you find a load that gives both tiny groups and tiny spreads, but we often don't unfortunately. Get hold of a copy of the book "Rifle Accuracy Facts" by Harold R. Vaughn to get an understanding of some of the mechanisms in play for both features. Putting accuracy aside, why do some loads give larger spreads and what can you do about it?

Unfortunately, there are multiple reasons for large ES values, and they almost certainly interract, so tuning it down may not be an easy or quick task. Brass and neck tension issues provide one set of causes. Look at each shot's MV, not just the average, ES, SD for the string. Does a string that has an ES of say 45 fps actually have a widely spread distribution, or are there 10 in less than 10 fps say, plus one 30 fps + away from the rest? If so, there may be an issue with that case. Mark it and see if it is out of the pattern with the next test batch you try.

Some powder-bullet combinations simply never give small spreads in a particular rifle, no matter how hard you try to overcome the problem. Find another powder that performs better. Iginition is often an issue, so try different primers with small powder charge variations around that which gave the original small group - that often reduces (or increases) the spread. Incidentally, it's a mistake to think a particular powder always performs in a similar way with bullets of a particular weight and type. I've seen huge difference in spreads between combinations that were identical apart from the bullet make. If you talk 0.308" 168s for instance, a particular rifle might produce small spreads with the Sierra MK, and large spreads with the apparently Hornady Match or Nosler Custom Competition despite the bullets being similar designs and loaded to have identical jump into the rifling - or vice versa, so it works with the Hornady, but not the Sierra. Minor differences in shape, bullet diameter, jacket thickness hence pressure build-ups? Search me!

Vary the neck tension you're using; likewise the COAL to increase / reduce bullet jump into the leade.

Turning to the group sizes, beware of the single good group out of a series with powder charges rising in small steps. The ideal situation is for a string of combinations to give reasonable or good groups, then to fine tune the load to obtain the smallest group size. If you get 1", 0.25", 1" as you go along, the small group may be an unrepresentative fluke, as is sometimes seen when shooting several groups from one load. I've only just looked at a 100yd test target from my .308W F/TR rifle firing five groups with 155gn factory match ammo that ranged from 0.3" to 1.2" with an overall average around 0.75". That 0.3" could have been very misleading taken in isolation. A load that does suddenly come together may be genuine, but the risk there is that it is very load and pressure-specific. That is, you get a tiny group with exactly that powder charge weight, and it becomes large if anything at all changes - new lot of powder, different temperature when you shoot in competition, your charge weights varying by 0.1-0.2gn.

At the end of the day, you can only try so many things and have to decide what you can live with. A lot depends on the discipline, the target size used, and the level of competition. It is no good adopting a load that shoots into 1-MOA at short range in long-range F-Class for instance where you are shooting onto half-MOA ring targets and a 1-MOA bull, but it may be just OK for another discipline with a 2-MOA bull ring. Your group will be bigger at long-range than at short even before you get into wind effects. Range also tends to exaggerate the group-pattern, so a three-quarters inch vertical line at 100yd will be all over the place vertically at 1,000 even before you suffer any velocity induced effects.

For short-range (up to 600yd) competitions, go solely for group size and forget the spread. For 800yd and up, my personal inclination is to go for the smallest acceptable grouping allied to the smallest ES, so looking at your results, I'd initially go for Load #3 to get started then try and refine that a bit. But, that's for the competitions I enter - it may not be best for whatever you're intending to shoot in.

You don't say which 6.5mm cartridge this is. Is it by any chance 6.5-284 Norma? Your results pretty well mirror those I've obtained from this cartridge in a factory Savage 12 F-Class rifle. So far, I cannot get the combination of high enough MV and small enough group that I wanted to use the rifle in Factory Sporter class 1,000yd benchrest. Yes .... and the ES values are all over the place too!

Laurie,
York, England

 

[7x57]

have you shot more than this or is 5 shots per load combo your sample number per load? meaning 5 shots per load has very little statistic significance other than to weed out loads that are out in left field

 

[people]

This is kind of off topic. It took many rounds to figure out how to shoot my rifle so it would make smaller groups. Once I got that down I was beating my head against the wall to try to figure out why one day a group would shoot good and then the next it would be a pattern on the paper at 100yds. I was fighting different powder and how clean the barrel was. What I ended up doing was cleaning every time I changed any thing. Then when I found something that looked good I would load up 10 to 15 rds and then shoot groups with them. Then retest to confirm. The current load I am shooting does not shoot for beans untill there is about 10 shots down range. After that it is fine untill I let it sit for a month or two. One streach I got about 600 rds down range before I stopped shooting it. One was about 50 rds.

I sure hope your tube does not suck like this one. Currenty I am at 400 something I shoot 5 rds every other week and it is still holding .5 moa when I do my part out to 800yds. Granted most of the I can not do that due to the wind but it will hold 1moa all of the time.

 

[Rust]

#1) Why am I getting large groups with small extreme spreads?

You are getting small groups at 100 with larger ES/SD because the velocity is at a barrel null in the vibration and at 100 yards 40-50 fps difference in velocity doesn't matter that much to group size.

#2) If you were faced with this problem, which load would you take to a match? The load with the small group, or the one with the low ES?

At 1,000, the vertical dispersion from velocity spreads at 50 fps would be about 10" or a little bit under that. So 50 fps ES with a .333 moa group works out to 13", while 5 fps ES with a .840 moa group works out to about 10". Those are really rough estimates by the way.

It can be difficult to get to that optimum point where usable velocity, low ES/SD and accuracy all come into line. You'll want to find a load that gives you the best combination rather than just best accuracy at 100, or just best ES/SD. Realize that load may not be the best in any given particular, but rather the best overall combination.

ES and SD numbers are important at 1,000 as they tell you what sort of vertical dispersion you'll have. Things which affect ES/SD are seating depth, neck tension, precision of weighing the powder charges, primers, and work hardening of the neck which affects neck tension.

As far as velocity goes, most the time with a 6.5 X 284 or any other 6.5, the sweet spot is around 2950 fps give or take thirty fps or so depending on the rifle.

After using up about half the barrel life looking for that best load I settled on a 139 Scenar, CCI primer and H4350. Yes, H4350 is a little faster but I prefer a little shorter barrel than the 32" sticks that seem popular so the slightly faster H4350 works out just fine. The load runs right at 2945 fps or so which is about perfectly in the barrel node and the ES/SD numbers are single digit.

Keep in mind, 50 fps spread is about 1 moa at 1,000.

Another good combination was RL-22 with a Winchester primer, the RL-22 liked the hotter primer.

 

[BoydAllen]

As has been said by earlier posters the 100yd group size is a function of the timing of the exit of the bullet in relation to the vibration cycle of the barrel. The larger groups with smaller SD indicate poor timing of that sort. I would think that adding a tuner to the barrel would have the potential of changing the vibration pattern of the barrel so that you could have your cake and eat it too. In other words modify the vibration pattern so that you could have low SD and good short range accuracy. Just a thought....

 

[queen_stick]

Laurie...

That's a great write-up on the subject! I was just thinking about this stuff when I was at the range yesterday, and your post above pretty much answered every question I had about chrono numbers vs group sizes, and what to do with that data once it's obtained.

Thanks for posting that!