I was surprised at the chronograph results I got using this accurate military ammunition. GP11 (DA 4-78).

Standard Deviation 46 fps (1.8%) of the mean instrumental velocity of 2,543 fps (muzzle velocity 2,552 fps)!

I chronographed several other military surplus loads and fully expected them to have a greater SD than the GP-11 ammo. Not the case.

Just proves SD isn't always the best predictor of accurate ammo.

Nope. S/D makes a difference, but so does the B/C, and many, many other things. All in all, I think the Swiss really did their homework when they designed the K31 and the ammunition, etc. Definitely one of the sweetest shooting guns I've ever been privledged to fire.

mike

Don't get me wrong. I'm a BIG fan of the K-31 and the GP11 ammo.

I was just a bit surprised with my chrono results. The SD was greater than I expected.

Conventional "wisdom" says that ammo with a SD greater than 1% will probably not be that accurate. WRONG in this case!

Would someone mind explaining (S/D) and (B/C)? I'm havin a hard time figuring out what yall are talking about.

milrif

I believe what they are talking about is:
S/D= Standard Deviation, Plain and simple it's the spread of velocity in feet per second from the lowest to the highest round in a string of shots.

B/C= Ballistic Coefficent, A number that says basically how slippery a bullet is flying through the air, usually the more pointed and slender a bullet is the higher B/C it has. Not sure what formula is used to come up with these firures.

Anyone who thinks the above explaination is wrong or can clarify it please feel free to do so. Not sure how close I was to being right but I think I was headed in the right direction.

Steve E...........

ahhh that makes sense, thanks for clearing that up

Generally S/D stands for Sectional Density, wich is a ration of a bullets size to it's weight. B/C is Balistic Coeficient, and is generally speaking as to how aerodynamic a bullet is.

mike

in this thread s/d is standard deviation, a way to measure how consistant the velocity is in a given lot of ammo.

I recently chrono'd my 1911 with the same ammo...devaiation of only 35fps and with an average of 2620.

I was impressed.

quote:

Originally posted by Steve E.:
milrif

I believe what they are talking about is:
S/D= Standard Deviation, Plain and simple it's the spread of velocity in feet per second from the lowest to the highest round in a string of shots.

Anyone who thinks the above explaination is wrong or can clarify it please feel free to do so. Not sure how close I was to being right but I think I was headed in the right direction.

Steve E...........

Headed in the right direction but not quite correct. S/D, Standard Deviation, is a mathamatical probability that the majority of rounds fired will fall within + or - of the computed mean velocity of those 10 shots. In other words if you have a S/D of 20 fps with a 10 shot sampling you can be pretty sure that 80% or so of the next 100 shots of the same load will fall within + or - 20 fps of the mean velocity. If the E/S was 50 fps then you have a good load. However if the E/S was 75 fps it is telling you that 2 shots were way outside the S/D and probably is not be a good load. The only way the S/D can be 100% correct is to shoot up all that lot of ammo.

But what about the 20 or so rounds that did not fall within the S/D range? The Exteme Spread, E/S, is the measurement of the lowest to highest velocity. This figure is the more important one if you shoot long range. Also there is a relationship between the S/D and the E/S of good loads. Again the only way to know for sure is to shoot up the whole lot, but then what then do you have left?

I have been using Oehler chronographs since '75 and find the most consistantly accurate loads have a S/D that is 30+% of the E/S with a 5 shot sample. With a 10 shot sample I prefer the S/D to Be 40+% of the ES. Of course the smaller the E/S the better. With big game loads a 10 shot sample is sufficient but with competition or varmint loads I go with a 20 shot sample.

For those who want to tell me of the low S/Ds they've got with 3 shots please don't. I've got lots of those also but find most often they are not a true sample of the loads capability. I use 3 shots when working up loads and then refine them with 10 or 20 shot samples when I'm in the performance range I'm looking for. I also have some loads that have very low S/Ds and E/Ss in the single and low double diget figures. However, I have found these to be rare and mostly occur with finely prepped reloads for match or bench rest rifles. Particularly with 20 shot samples.

Larry Gibson

Excellent post, Larry!