Sd And Accuracy?

[Pablo1234]

What is the coralation between Standerd Deviation and Groupings. I'v been told that it means verry little in 50 yard range shooting when it comes to accuracy. I have a hard time beliving this; but I'm new to reloading. I'm shooting a CZ .40 cal in my bullseye leage and I have a group size of 2.75" at 50 yards.

I shoot

180g FMJ

titegroup 3.5g

sorted brass Winchester, RP, and Federal

@ 747fps S.D. 8

When I was using mixed brass consisting of every thing under the sun I had an S.D. of 36

I never checked my groups with the mixed brass; but I belive it helped a lot. On my bullseye forum ther telling me S.D. dosn't mean anything and mixed brass is as good as sorted brass. What do you think?

[GuildSF4]

What is the coralation between Standerd Deviation and Groupings. I'v been told that it means verry little in 50 yard range shooting when it comes to accuracy. I have a hard time beliving this; but I'm new to reloading. I'm shooting a CZ .40 cal in my bullseye leage and I have a group size of 2.75" at 50 yards.

I shoot

180g FMJ

titegroup 3.5g

sorted brass Winchester, RP, and Federal

@ 747fps S.D. 8

When I was using mixed brass consisting of every thing under the sun I had an S.D. of 36

I never checked my groups with the mixed brass; but I belive it helped a lot. On my bullseye forum ther telling me S.D. dosn't mean anything and mixed brass is as good as sorted brass. What do you think?

<{POST_SNAPBACK}>

I have had SD of 50 and groups of less than 2" at 50 yds (from a rest) (Gun was 9x25 115 WJHP, 1700 FPS. I have also had groups at ~1" at 25 yards (from a rest) mixed brass .45 Auto 180 LSWC powder WSF SD ~25. (Have had smaller SD in some loads that grouped worse.)

IMHO low SD is better, but the real test is how to gun/shooter groups the shots. May matter more in a rifle or pistol at longer ranges, especially if you are looking for sub 1MOA.

[kevin c]

Agreed (for what it's worth - it isn't like I'm a professional, here ).

A small SD basically means that the shot to shot variation in velocity is less than a load with a higher SD. It may seem logical and sensible that the less variation in velocity you have, the more consistent the load and therefore the greater the POTENTIAL for accuracy, but the SD is only one factor in many.

If your bullets tumble in flight, if the HP cavity isn't concentric with the the rotational axis and flight path of the bullet, if the jacket and core separate in loading or firing, if you nick or score the base of the bullet loading the rounds, a small SD won't matter. The bullets could all leave the barrel at exactly the same muzzle velocity (zero SD, I think?), and still be all over the paper.

I'm not a ballistician, but I'm pretty sure that the amount of bullet drop caused by a change in muzzle velocity is very small at 25 to 50 yards. You could take two loads with zero SD but with velocities 50 fps apart, shoot them at the same target through the same gun, and might not see much more than a tiny bit of vertical dispersion, if even that. At rifle ranges it might be significant, but at pistol ranges and with a 6"X11" "bullseye" in IPSC, I don't think it's critical in our game.

As far as mixed components go, the same applies, I think. Consistent components, along with consistent loading technique results in a more uniform product, with greater POTENTIAL for accuracy, but no guarantee of it. And, if there is more accuracy to be gained, there is the question of if it makes any practical difference for the use you will be putting the load to.

Really, what matters is how your load groups out of your own gun.

[gm iprod]

Good SD is better than poor SD.

But as kevin c points out it is only one factor. The most important (assuming you are bench resting the firearm????) is lockup. If the sights and the barrel are pointing the same place each and every time, then the bulets will not group well. Having a good lock up and then having a good SD will improve the chances of a good shot. The way a handgun cycles is important, a good SD will make the gun cycle more consistantly.

But if your SD is poor, especially if it is one or two rounds out of a group of twenty tested, then it is going to cause the group to be poor. The larger the sample the better.

The lower the velocity the more important it is for the SD to be small. The bullet is moving more slowly and will spend more time in the barrel affected by how the shooter is holding the firearm. This is a real problem for bullseye. Less of a problem for IPSC as we tend to run a slightly higher velosities shoot at shorter ranges and run around.

For NRA AP and Bullseye accuracy is paramount, I get small Sd and small groups for my NRA AP gun, a freind who's gun shoots equally as well as mine usuing completely different components has a slightly larger Sd and still groups less than 2" at 50, prone.