What is an acceptable Extreme Spread?

[MVW]

I have read on some of the forums that for long range shooting, reloads should have an nominal extreme spread of ~15 fps. For my .308 reloads, I end up in the 25 fps range, which is approximately an 8" difference at 1,000. If 15 fps is indeed the goal, I wonder if someone could help me trouble shoot things.

I am using Lapua Brass, SMK 175 GN bullets, and 43.5 GN Varget
I have a Redding turret press with Redding Comp Dies, a Sartorius milligram analytical balance and a Mitutoyo 0.0005" dial indicator - plus it seems just about every other gadget you can imaging to measure things - and I do measure almost everything.

I prepared the brass according Jacob Gottfredson's article "Preparing Cases for Long-Range-Accuracy" and on the first firing I had an extreme spread of 20 for the 13 pieces of brass in my test. I then fire formed the brass 5 times and carefully reloaded the cartridges for the 7th time. I bumped the shoulder back a few thousandths, trimmed to a length of 2.005", sized roughly half the neck. In resizing the neck, I have tried a single pass and also tried resizing once, turning the case 90 degrees and sizing a second time. In seating the bullet, I just get it seated, turn it 90 degrees, seat it until it breaks and then turn it 90 degrees again and finish the stroke. I measure it and then make a final pass to get the final OAL (to the ogive). I also have also tried cleaning each bullet with alcohol. None of these things seems to make a difference. ES was 25 fps.

I weigh each component including the primers and compare the final cartridge weight to the sum of the components. The difference is less than .1 GN. I sort the bullets by length to the nearest thousandth and then by weigh to +/- 0.05 GN

Bullets are seated to on OAL of 2.210, which is about 0.020 off the lands on my gun. Runout is less than a thousandth.

I have kept the stats on each case and it is not a single case that is giving me issues, but rather a couple of random rounds from each firing.

Given that it is not s specific case and the weights of the powder and bullets appear uniform, it seems like it must be variations in neck tension or bullet bearing surface. But, I'd be interested in any thoughts folks might have.

Thanks, MVW

 

[MarkS]

The only thing I do to bullets is to run them thru a Bearing surface compairtor http://www.6mmbr.com/medlerTUBBBSC.html. Bullets of the same weight, but different Bearing surface will be slower or faster. This will be the best thing for ES.
Also play with your neck tension, most of the time lighter is best.


Mark Schronce

 

[jcampbellsmith]

MVW

Extreme Spread is not a useful measurement. All ES tells you is the difference between your fastest and slowest shot, it tells you nothing about the distribution of the intermediate velocities. The Standard Deviation is much more useful. Here's an interesting article on this.

http://riflemansjournal.blogspot.com/2010/02/reloading-velocity-decisions.html

I've got my Standard Deviation below 10 on some loads by focussing on neck tension. Firstly I have used bushing dies to full length resize (without the expander ball) and secondly I have neck turned to try and get consistent necks. Thirdly, I sometimes sort the necks as per the following article.

http://www.6mmbr.com/casenecktools.html

Good luck. JCS

 

[dreever]

Don't know what primers you are using but I'd suggest CCI 250. My .308 E.S. is 10 fps with a S.D. of 4 for a ten shot string which I credit to the primers.

Danny

 

[jbpmidas]

Neck turning gave us the "most bang for the buck" in consistent velocities. Consistent neck tension is crucial!

 

[MVW]

Thanks Guys,

Danny: I am using CCI BR-2 Primers. Do you think those are as better/worse than the CCI-250?

Mark: I have been using a Redding comparartor gauge to measure the distance between the base of the bullet and the ogive. That is not exactly the same as bearing surface, so I am wondering if you think the TUBBSSC will give a better representation of bearing surface?

JCS: Interesting article on SD. My only thought about ES is that if you shoot a 20 round string at 1,000 yards as you do in an F-Class match, you could have a shot spread greater than the 5" X-Ring no matter how good the SD turns out to be.

I did turn the necks and that does seem to lead to a better result than the non-neck-turned Lapua brass.

Mike

 

[Leaddog]

The 175 SMK's are not a consistent bullet for 1000 yards. Try the Berger 185 LRBT's.

 

[dreever]

I would give the magnum CCI 250's a try. They work really well in my .308.
I'll go along with Charlie in that the Sierra's may not be the best bullet out there.
I'll agree that the Berger 185 Long Range is a much better bullet for 1000 yards. In the 175 family the Berger 175 Tactical OTM bullet has it all over the Sierra's in my GAP Hospitaller. It is a fantastic bullet in my opinion.

Danny

 

[MarkS]

MVW,

The TUBBSSC will give a better THE of bearing surface of the bullet. You are only getting half of it. If you get the ES down you will have good SD. The load I shot my IBS records ran 9 ES and SD of 3.

Mark Schronce

 

[jcampbellsmith]

I did turn the necks and that does seem to lead to a better result than the non-neck-turned Lapua brass. Mike

Mike. What I found with the neck turning was that even after neck turning, some necks were better than others. So using the Sinclair tool I split the necks into good necks and not so good necks. Regards JCS

 

[ordnancemarine]

I shoot 175 @ 1K and I found that Federal large rifle match primers work good in my rifle. I also achieved a lot more consistency by getting a Gempro 250 scale that is capable of measuring to 2 thousandths of a grain accuracy. One other trick is placing a #7 size o-ring (from ace hardware) between the locking ring and the reloading press to help with concentricity. Neck tension is also very important so you may want to look into neck turning. I've found that it makes everything group really tight. BTW anyone who says that SMK aren't good rounds for 1K shooting obviously doesn't know how to shoot or to reload. I've noticed an equipment bias with some circles of shooters and it distracts from learning about what works for our individual weapons. Some projectiles work better in certain weapons... so you may want to try to experiment with a variety of different powder/ projectile/ primer/ load variations if you are not getting the accuracy you seek. Learn from your own experiences with your gun, not some one else's.

 

[ordnancemarine]

The 175 SMK's are not a consistent bullet for 1000 yards. Try the Berger 185 LRBT's.

Thanks Charlie Watson, I guess I should tell that to my 175 SMK's next time I'm hitting X's at 1K. They apparently didn't get the message.

 

[DennisH]

I've got my Standard Deviation below 10 on some loads by focussing on neck tension. Firstly I have used bushing dies to full length resize (without the expander ball) and secondly I have neck turned to try and get consistent necks. Thirdly, I sometimes sort the necks as per the following article.

Could someone explain "Standard Deviation" ???

 

[GermanS1]

Dennis, the simplest way to think of stadard deviation is that it is the expected variance from perfect for any given (random) value within a group of values. For instance, SD of 5 in muzzle velocity with an average MV of 3000 fps means that you can expect that any given shot will be no more than 5 fps from the average. How true that is in practice depends on how many shots went into the data, for instance, if you fired 20 shots to generate your SD number, it is much more likely to be correct than inf you fired 5 shots for it. The article on my website quoted above (Velocity Decisions by Ken Oehler) should explain that with more precision.

Comparing ES to SD is a bit like comparing a group target to a score target. In a group target (benchrest) you measure the distance between the two widest (worst) shots and ignore the others. On a score target you measure (with scoring rings) the distance of each shot from center - every shot counts and is measured. Same with ES and SD: ES is a measure of the difference in velocity of the highest and lowest shot, ignoring the cluster in the middle where the vast majority of your shots are; whereas with SD every shot is compared to the average and you get a fuller view of your load's performance.

As for the concern expressed in an earlier post about having a high or low shot, yes, that will always happen, there's no way to avoid it. However, by working on the SD you have a much more complete data set (you counted all the shots) and can make a better decision about loads.

For what it's worth, in my experience, once you have a good, accurate, consistent load, the key to smaller SD is perfectly consistent neck tension (turned necks) and the mildest primer that will reliably ignite your charge. Look through the primer articles on the site for more info, beginning here: http://riflemansjournal.blogspot.com/2009/06/primers-small-rifle-primer-study.html there are more that can be found in the index. There are also plenty of articles on neck turning, and other features of careful, precision reloading. Enjoy!

 

[Terry]

It is the deviation from the standard.

Example: 3,000 to 3030 velocity range, 3015 average, if you can add or subtract 5 fps and cover most of the shots then your SD is 5.
If it takes 10 fps then your SD is 10. It is not necessary to cover 100% of the shots. In a group of 5, I would be happy to cover 3
or 4. In a group of 10 7-8 would be fine.

 

[DennisH]

Thanks for the top two responses.

Just wondering:

How does my Chrony figure SD. ES is not a problem to fully understand on my Chrony printouts.

 

[Nodak7mm]

Dennis,

Your Chrony should be doing that for you. I dont have experience with the printout, but with my chrony; I shoot my desired number of rounds and then review the data (low/hi, avg, es, sd) by pushing the "FU" button. This is assuming your using a Shooting Chrony brand chrony.

Rod

 

[DennisH]

Your Chrony should be doing that for you. I dont have experience with the printout, but with my chrony; I shoot my desired number of rounds and then review the data (low/hi, avg, es, sd) by pushing the "FU" button. This is assuming your using a Shooting Chrony brand chrony.

My Chrony does figure all the above! I was just wondering how SD is calculated by the Chrony. I will review some of my tapes and if necessary, I will post a few that didn't add up to me!

Dennis

 

[jcampbellsmith]

Dennis

Excel will work out the Standard Deviation for you too, as per attached.

Regards JCS

 

[DennisH]

I understand it will work it out, I would love to figure out how the total is calculated.

2625
2627
2618
2609
2620 AVG
8.1 SD
66.3 VAR

If you have a avg speed of 2620, a var of 66.3, How is the 8.1 SD calculated? These figures come off the smk 142's