My reloads in once-fired hulls vary about 15 feet per second, give or take a couple. Factory loads from the same lot vary about the same but the variation between lot numbers can be much larger. Also, ammo factories load their shells on numerous machines and like all things mechanical, some will produce shells with more variation than others. Accordingly, you can even encounter large variations within the same lot.
I've often said that the handloader using his one machine can turn out more consistent ammo than the factories, especially if his loader is powered by a controlled system like hydraulics or an Automate. A uniform dwell period at the bottom of the operating cycle is key.
Frankly, I don't think I can blame my shell for any target I've missed, so it's sort of a non-issue - these aren't MOA rifles we're shooting.
I chrono my favorite loads often just to make sure they are consistent. Also, I like to see what effect temperature and humidity have on performance.
When I load shells I select somewhere between 11 and 20 at random for chrono testing. That way I'm testing what I'm actually going to shoot, not some carefully weighed and assembled shells.
I find my reloads match the performance of STS Light Target and Nitro 27 shells. I don't pay a lot of attention to the extreme spread, although that is usually around 25fps. I pay more attention to SD. If the string of between 11 and 20+ shots returns an SD in the single digits, I'm happy. That means the load is consistent, even if it did include one shot at 15fps above average.
I'm no longer in the game but when I was chronographing I would say my results would agree with the findings above from ZZT. Some other suggestions are to use the same gun, same choke (best wide open), and consider different powders for comparisons. Of course temperature can make a difference and even the length of your test barrel. Be consistent with your testing and then expect consistency in your product. Re powders; I found 700X to be the most consistent over the years with Standard Deviation readings ranging from 7 to under 15 with the average being single digits. It's been, however, several years since I've played with the gear.....breakemall....:Bob Dodd
OK guys I just got a cono for Xmass, Help me understand,SD I shot mine today, 3 shots,1246/1223/1206 thats a 1225 ave. is that normal? what's the SD? Is it good or bad? I shoot, Nitro's-17.5-700x-CB wad, out of my MX3. thanks, Butch H
Butch, SD or Standard Deviation is a math. formula that resembles a simple average of several samples. I'm not a math. person and do not know just how much the difference is between that and "average velocity" of a several shot sample. The lower the SD the more consistent individual shots are in your test string. A large number in the SD suggests your velocities are spread out over a larger span. If you don't have a chrono that calculates the SD for you, I suggest you simply write down the velocities read and calculate an average velocity then compare the shots to see how much deviation from the average each shot is. If that deviation from average is large - that's not good. In your case, your 3 shots averaged 1231 (rounded off) and the 1246 deviated 15 fps; the 1223 deviated 8 fps; and the 1225 deviated 6 fps. Those 3 deviations = 29 or a deviation average of 9.6 fps. Had you taken say 10 shots per string I'd trust the numbers more and with a little math work you can see your 3 shot test resulted in a pretty good "deviation from the average shot." I also would consider tossing out a radically high or low test shot if you encounter that. Necessary? Heck no, just understand it would be VERY rare to have velocities that were precisely the same at the velocities we deal with. Maybe calculate the average and realize reloading is not rocket science. Hope that's understandable.....Bob Dodd
Butch, Standard Deviation is a statistical concept that essentially tells you how close to your average velocity your shots are likely to be. Consider the two 10 shot strings below. They were for 7/8oz loads and the only thing different was the wad.
1221,1203,1206,1244,1213,1217,1216,1242,1230,1260 Avg Vel 1225.2fps SD 18.4
1223,1215,1239,1204,1217,1222,1216,1228,1227,1226 Avg Vel 1221.7fps SD 9.4
I consider the first string to be bad. This load with this wad always seems to give ten shot SDs between 18 and 29. IMO it's crappy load.
The second string is from my favorite 7/8oz load. It is good. There is one flyer that raises the SD a little, but this load always returns SDs between 7 and 9 something. It is consistent, plus it patterns well. That's why I use it.
Just to give you an idea how much that one flyer skews the results, look at the string below. I changed the 1204 to 1213.
1223,1215,1239,1213,1217,1222,1216,1228,1227,1226 Avg Vel 1222.6fps SD 7.8
Place your muzzle 48" from the center of your screens.
Record velocities for 8 shots.
enter them here:
Having chronographed shotshells for 20 years, including countless factory rounds, I've concluded anything less than 30 fps extremme spread and a sd=15 is good.
Load carefully enough and check often enough and you will find handloads that routinely give less than 25 fp and sd <10.
Provided you chrono at least 8 rounds, the sd is predicting what range of velocities you can expect for the batch.
95% will be within +/- 2 sd's of the average, 99% within 3 sd's.
So, for an average velocity of 1200 fps with sd=10, 95% of the shells should fall within 1180-1220 and 99% within 1170-1230 fps. You can't count on factory premiums to be better than that. SAAMI allows them to state "1200 fps" as long as they are 1110-1290 fps.
The best thing about chronographing shotshells is not knowing the exact velocity, but having confidence in the consistency of the load. Three rounds from a box might be 1100, 1200 & 1300 fps. Another might be 1190, 1200 & 1210 fps. Both average 1200 fps....which do you like?
Also, remember that "hobby" chronographs, which are what most of us common folk own, clock the leading pellet instead of the mass of the shot charge and therefore give readings that are slightly faster than actual. My chronograph manufacturer told me to deduct 35fps from the readings to be real close to actual.