SD vs Accuracy

[Walt]

In the Wolfe Publishing online article that came out yesterday there was an interesting story on test-firing a Smith .45 Colt from a Ransom Rest with various loads. What I thought was most intriguing was the relationship between standard deviation and accuracy. Lowest SD is not an absolute indicator of best accuracy but it suggests that a relatively low SD, such as between about 5 and 15fps seems to do the best.

[Scott Tschirhart]

It makes sense that lower SDs would lead to better accuracy. But I am not convinced that there is a direct correlation, particularly at typical handgun ranges.

[AJMD429]

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Consistency in as many factors as possible would usually be key to hitting the same place repeatedly.

I've seen many expert reloaders try 'refuting' things like weighing powder charges, neck reaming, precision seating, and so on, and it is always interesting.

One of my most accurate loads/rifles was 6mm Remington, loaded with the 'Target' version of the little Lee Loader kit, with inside neck reamer and micrometer seating. However with IMR 4831, I could just 'dip' the cases in a cereal-bowl of powder, shake the case enough to get powder out of the neck area of the case, and finish loading the round the usual way, and still get 9 or 10 shots in a dime-sized group at 100 yards. So for that load and gun (Ruger M77V), no precision at all was needed in terms of powder charges. My thinking is that perhaps the very slow powder simply didn't completely burn while the bullet was in the barrel (there was always a big muzzle fire-ball), and 'metered' by barrel-length, the excess simply making a variation in the fire-ball.

[Scott Tschirhart]

I’ve had loads that varied quite a bit that shot very well. Then again, I’ve had loads that should shoot well that didn’t.

There may be more at work than we think.

Many years ago I had a Remington 600 that shot lights out with cheap red box Federal 150 gr .308 soft point. No other factory ammo would do as well. No handload I tried would do as well.

But it liked that load.

These days I shoot .308 Remington factory 150 gr Coreloct and I don’t worry too much about groups and I don’t bother loading.

[AJMD429]

These days I shoot .308 Remington factory 150 gr Coreloct and I don’t worry too much about groups and I don’t bother loading.

No matter how much I practice, unless I'm at a bench with a rested long gun, MY inaccuracy is going to be way more than the gun's inaccuracy.

So although I do try to find a 'good' load (reload or factory load) for a given firearm, what I elect to use for ammo is more based on the bullet construction and 'reasonable' accuracy than it is on anything else. Having a hunting or protection gun that I'm trying to get to shoot 1/2" vs 1" groups with at 100 yards may be 'fun', but it just isn't realistically practical - unless I'm shooting from a padded rest while hunting deer or protecting the home, my inherent abilities are going to limit my practical accuracy of fire to at best maybe 4" at 100 yards, and that will do. If it's with a handgun, I don't think "minutes of angle" wouldeven be the right term - four inches at four yards is probably 'enough' accuracy for the rapid-response world of self-defense, and the best I'll get in that kind of shooting - that would be 100 inches at 100 yards for 'accuracy'...100 minutes-of-angle...

[Keith D]

Some bullet shapes just shoot better than others. Makes sense that wadcutters are no good at long range...no matter the SD.

[Walt]

It just doesn't make sense that in these tests where the hold on the gun, the aiming technique and the trigger pull were mechanically made consistent, would have extreme spreads in the most accurate loads of sometimes 60 or 70fps. Is it differences in bullet composition, is it harmonics, is it barrel temperature variations? It's a puzzling business and that's what keeps us coming back.

It's kinda like Taffin's Perfect Packin Pistol. There's no such thing but the search for it is the attraction.

[marlinman93]

I've always felt that the lowest SD I could get from my hand loads for rifle or pistol will always result in the tightest groups. Velocity in general means less to accuracy, unless the velocity also gives tighter SD readings.