Excessive Standard Deviation in Bolt Gun???
What are some issues that could cause excessive standard deviation when working with handloads in a bold gun, assuming the bolt gun is in perfect working order, the bullets are consistant, and powder charge is consistant?
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Do you mean velocity or accuracy deviation ? Are the cases all the same length, size and weight? Is the powder and primer correct for the load? Are the bullets all seated to the same depth? Any of these can cause accuracy and velocity deviations. 0 -
quote:Originally posted by zimmden
Do you mean velocity or accuracy deviation ? Are the cases all the same length, size and weight? Is the powder and primer correct for the load? Are the bullets all seated to the same depth? Any of these can cause accuracy and velocity deviations.
All of this plus crimp (if any). Is the case close to or full of powder, or is there a lot of air space?0 -
High SD implies inconsistency. Potential sources of variation are numerous and include the following:
-Primer depth of seating.
-Primer type/lot.
-Overall case length
-Overall case thickness/weight (affects case volume).
-Powder quality (eg, humidity-exposed powder may be inconsistent).
-Powder charge weight
-Bullet quality
-Bullet depth of seating/ overall cartridge length.
-Bullet crimp consistency
Then there are variables specific to shooting:
-Temperature
-Humidity
-Wind
-Consistency of primer strike
-Position of powder within cartridge (generally only an issue in cartridges with lots of air space).
-Quality of chronograph (this is more theoretical, it shouldn't be an issue with a good modern timer).
Note also that standard deviation just tells you a sort of geometric average distance from the mean. While its a pretty good rough estimate of data-set variability, it won't tell you the whole story.
For example, suppose you had a set of chronograph data where ten shots were all between 1000 and 1010 fps and ten more shots were all between 1200 and 1210 fps.
If you just looked at the SD, you'd see a fairly high number, implying inconsistency and suggesting that your shots should be grouping all over the place. In reality, with this example you'd probably have two separate, but individually match-quality groups, and probably have a pretty simple explanation of what was going on (ie two different bullet weights, etc).
The point is, SD is useful, but its probably still worth graphing or taking a quick look at the data set to see what's going on. IE, are the data points evenly dispersed or not?0 -
check to see how much neck tension you have. If it is at or under .001" you need to increase it. 0 -
quote:Originally posted by JustC
check to see how much neck tension you have. If it is at or under .001" you need to increase it.
Even that depends a lot on the powder. While most rifle powders require a fair amount of neck tension to ignite and burn reliably and consistently, some fast powders or black powder, it has very little impact on. In fact many black powder cartridge shooters use NO neck tension whatsoever.
We need more info on caliber, powder, etc.0
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