Is this accurate...?

[1stmar]

I took this off the Lee precision site. This is different then what I understood the benefit of seating the bullet close to or touch the lands. My understanding was it reduces the "jump", helping to ensure concentric entry into the bore. This is using lead length to control pressure. Is this a good practice, in that is it predicatable and consistent, accurate? Do bench rest shooters typically use light bullet pull? Recognizing they will neck turn etc...


"Maximum accuracy is usually achieved by seating the bullet out far enough to touch or almost touch the rifling. This provides the shot start pressure normally supplied by the crimp. "

 

[Strafer Gott]

Definitely a benchrester technique, and definitely not possible all of the time due to magazine length versus chamber throat. Loaded in a bolt, this can help accuracy, but it didn't work out in my AR, due to bullet slamming on loading.

 

[1stmar]

It's surprising to me that this would yield more consistent pressure spikes.

 

[Unclenick]

Consistent start pressure reduces muzzle velocity variation and barrel time variation. The idea is to swamp out neck grasp differences with the higher pressure needed to swage a bullet into the throat. However, the burning characteristics of different combinations of powders and bullets and case capacities and primers is complex. Berger has found, for example, that their VLD designs sometimes want rather a great deal of jump to be their most consistent on target. So, in the end, while it is true more often than not that close to the lands gets good precision, it's not true in every case and you just have to try different seating depths with each load component combination to see what you get.

Here's a link to Berger's method. Dan Hackett, writing in the Precision Shooting Reloading Guide tells how he accidentally turned a seating micrometer the wrong way one day and wound up with 20 rounds of .22 Swift seated 0.050" off the lands instead of 0.020" as he'd been using. At the range, the gun, which had never previously shot better than 3/8" at 100 yards, turned in two 1/4" groups and two bugholes in the 1's with those "too short" rounds. So, you don't know until you try.

 

[mrawesome22]

And some jam seat into the rifling.

It's all about getting es and sd as low as possible.

sudo apt-get dist-upgrade

 

[1stmar]

What are good numbers for es and sd?

 

[wncchester]

What are good numbers for es and sd?


1 or 2.

 

[Unclenick]

Small numbers. If you can get a medium power rifle SD down to 10 fps, you are doing very well. ES will depend more on the group size, as, even with an accurate SD, the more rounds you put on the target the greater the chance that one will be randomly farther out than usual. But for 10 shots with 95% confidence the expected value of ES will be about 3.1 times bigger than SD; for 5 shots, 2.3 times bigger, for 15 shots, 3.5 times bigger, for 3 shots 1.7 times bigger.

 

[Bart B.]

I agree with Unclenick on SD and ES, but sometimes the lowest numbers are made by powders that slam the bullets hard enough into the rifling to distort and unbalance them. Even when a bunch of 'em leave at exactly the same speed, they jump off the bore axis when exiting the muzzle and that sends 'em further away from the aiming point.

Seen that happen with several ball powders. Maybe this is why benchresters prefer extruded powders for their metered charges used up through 300 yards.

 

[Bart B.]

Unclenick, you say for 10 shots with 95% confidence the expected value of ES will be about 3.1 times bigger than SD and for 15 shots, 3.5 times bigger.

Why's the ES for 10 shots a smaller multiple of SD than what 15 shots have? I'd think more shots per group reduces the SD multiple for ES. Unless you've pulled one of my goofs and got two multiple's numbers mixed up.

 

[1stmar]

Sd is 25 and es is 59. I'm not neck turning and I have no desire to so 25 might be about as good as I can get.

 

[Bart B.]

1stmar, how old's your firing pin spring?

I've noticed that when they weaken down to about 80% or their original rating, muzzle velocity spreads increase.

In my Win. 70's, their factory springs' are rated at 23 pounds; I replace 'em with 26 pound ones. Even that much of an increase over a new spring gives much better SD and ES numbers.

In my many years of competitive shooting, a couple friends have asked me if I had a spare 'cause they were getting big elevation spreads at long range. One of them used the new one I loaned him and won the next match. The other did shoot better scores.

 

[1stmar]

Thanks for the tip Bart, it's cant be more then 15 years... . I never would have considered it. Is that some thing that can easily be done with limited knowledge ? Any effect on trigger pull?

 

[noylj]

has anyone EVER seen low es and SD actually producing increased accuracy?
I haven't--not in my rifles and not in my pistols.
Maybe, and I mean maybe, in very long range shooting, but again, does anyone have proof in several different firearms?
The only I have learned in 40 years of reloading and shooting is that my assumptions going in are often proven wrong when put to the test.
About all I can say is that a poor bullet will not shoot well, but it is amazing how often a fairly inexpensive bullet turns in amazingly small groups compared to some "premium" bullets.

 

[old roper]

here is good article on ES read this part "Making Superb Ammunition -- The Key to Accuracy"

http://www.6mmbr.com/1000ydpg02.html

Here another good article on ES/SD

http://www.gunwerks.com/Long-Range-...les/Powders,-Primers,-and-Long-Range-Shooting

 

[Unclenick]

Bart,

The numbers I gave result from the confidence being fixed while the number of samples (shots per measurement string) are changed. Suppose you have a constant standard deviation and a 95% confidence interval and fire 20 shots. Since 95% of the population of a normal distribution is inside two standard deviations from the average, out of the 20 shots the expectation is that one (5%) will exceed 2 standard deviations from the average. If you fired 100 shots, the expectation would be that 5 shots would exceed 2 standard deviations. But if you fired only 10 shots, there's just a 50:50 chance that one will exceed 2 standard deviations, and if you fire just 5 shots, there's only a 25% chance that one will exceed 2 standard deviations. What is happening as you increase the number of shots in each group, the chances of getting an outlier increases. So, the chance of the ES being a larger multiple of SD increases with the chance of getting outliers, and that, in turn, increases with sample size.

Good tip on the springs. That's the kind of practical information you often can't look up conveniently. I found a site one time that showed how badly small aircraft engine valve springs took sets and weakened and eventually even deformed (lots of help from the heat), but after cryogenic treatment they didn't take a set beyond 15% loss in force. Since then I have had a number of springs cryo'd and so far haven't lost any of those. My Marvel 1911 conversion came with several extra recoil springs, as those light springs are not expected to last well, but I had mine all cryo-treated and after several thousand rounds the first one is still running fine. I expect the rubbing surfaces of the springs will eventually wear enough so they'll need replacement, but so far so good. The cryo-treating seems to be an effective spring life extender.


Noylj,

Range is everything. Because the bullets slow in flight, each successive 100 yards takes longer to traverse, giving gravity and wind and any other influence more time to act on them than during the previous 100 yards. The result is the error resulting from differences in muzzle velocity grow exponentially with range. For example, the 175 grain .308 MatchKing at 2650 fps and 2600 fps will produce a 0.1" difference in drop at 100 yards, 1.1" difference at 300 yards, and a whopping 20.5 inches difference at 1000 yards. So, 0.095 moa, 0.35 moa, and almost 2.0 moa for those three ranges, respectively. So you might never notice the difference in the noise, even at 300 yards, but at 1000 yards it will affect your scores significantly.

 

[1stmar]

If I read the last paragraph correctly in the gunwerks article, neck tension should be no more then .002. Very light, I would think you would have to be conscious of recoil etc from changing the seating depth.

 

[Mike / Tx]

If I read the last paragraph correctly in the gunwerks article, neck tension should be no more then .002. Very light, I would think you would have to be conscious of recoil etc from changing the seating depth.

I cannot remember where it was that I read about it, but the jest of it was a test using differeing amounts of pressure on the butt pad of the rifle to vary the numbers.

I have proven that to myself, having great numbers one time out and not so great numbers the next time, but the accuracy stayed pretty consistent.

This said, unless your shooting competition you would probably never notice it. About the only time I shoot across my chrony is when testing loads. I use it while working up to the area I am happy with, then agin several more times during the course of a year with different conditions to check how much the temp changes have worked for or against me. Once there I simply load and shoot until I am out of those particular components. If I run out or change anything I will always work back through at least to see how the newer component compare to the last used.

Also like UN mentioned with the Berger bullets, and Dan Hacket's findings, I more times than not seat to accommodate the length of my magazine. I usually start off .020" form the max length and shorten from there. In my friends 300 RUM he is seating the 185gr VLD's .125" off the lands and getting stellar groups.

 

[1stmar]

Yesterday's loads looked better statistically, 13.9 std, 30.3 es but the weather was rough, heavy rain, then clear and sunny, the overcast. Groups opened up a littke but I was struggling with mirage and my eyes.

 

[Bart B.]

1stmar on firing pin spring weakening:

Thanks for the tip Bart, it's cant be more then 15 years... . I never would have considered it. Is that some thing that can easily be done with limited knowledge ? Any effect on trigger pull?

Easiest way is to take your firing pin spring out and compare it to another new one for the rifle. If the old one's shorter unloaded, it's probably weaker. With a 15-year old spring, I'd just buy a new one made by Wolff that's 2 pounds stronger than what your old one's specs are. Wolff's website has some good data.

http://www.gunsprings.com/Rifles & Shotguns/cID2

Spring force can be measured by slipping one over a 1/4" steel rod, then pushing a board with a bit larger hole than the rod over the rod onto the spring. Put a board on a bathroon scale and put the rod & spring on it then push the holed board down until the spring's compressed to the same length it is cocked in the bolt. Read the scale. Yes, a bit complicated but it works good enough.

If you know someone who works in a machine shop, they may have a spring gauge that'll measure it very accurately.

I doubt anyone could tell the difference between a new and weakened firing pin spring in trigger pull that's more than 2.5 to 3 pounds. Lighter pulls might let some folks tell the difference. It depends on the trigger's mechanical makeup, too.