Phorwath,
My answers are conditioned on the disclaimer - IMO
How many shots does it take to get a reliable and "real" SD?? is it 100??
Approximately 20-25 shots. Having stated that, I want to make it clear that I don't proceed to shoot 20-25 shot groups for purposes of collecting Avg MV, ES, and SD in one setting (one firing session). Because that's not how I develop, identify, and field proof my pet handloads. I find that by shooting over my chronographs during load development, after I've settled on a load, and then while field-proofing the accuracy and field drops of that load, that I often end up with 20 or more shots fired over the chronograph. I log atmospheric temperatures every time I shoot, because temperature is one known factor affecting MV. Doesn't do any good to shoot 5 groups of 5 at 0F, 20F, 40F, 60F, and 80F, unless you've already demonstrated that the powder and primer don't yield differing MV at the differing temperatures. I seldom shoot more than three-shot groups for accuracy. But I don't shoot one single 3-shot group to establish accuracy. I shoot a number of 3-shot groups over time at different ranges. I will, on rare occasions, shoot more than 3-shot groups solely in order to collect additional MV data, but generally only AFTER I've selected/finalized my load and I want to field proof the Avg MV, ES, and SD.
Is a shooting chrony a good enough instrument to get a true SD? Or does it take alot more expensive (and accurate) chronograph, like an Ohler 35P or similar?..........Garbage in, Garbage out type of thing.
I've never owned a Shooting Chrony, but based on numerous posts I've read regarding user experiences with the Shooting Chrony, I think not. One LRH member posted that the only thing his Shooting Chrony was good for was to confirm a bullet left the muzzle and passed over the skyscreens. Another posted that the best shot he fired over his Shooting Chrony was the one he killed it with. Without pumping any specific brand of chronograph, I will say that the chronograph must produce accurate data to be useful for the collection of any beneficial data - whether that be Avg MV, ES, or SD. If the chronograph data aren't accurate, the chronograph not only provides misleading SDs - it also provides inaccurate Avg MV and ES. Here's a link to some photos, and a description of, my current chronograph setup:
http://www.longrangehunting.com/forums/f19/new-oehler-35p-new-skyscreen-rail-first-test-data-60778/
Some will argue to the death saying that a 3 or 5 shot group isn't enough to really determine average group size. The same argument (I am assuming) would apply to the SD's given by the chronograph if only 10 or 20 shots are fired through one??
I would agree that a single 3 or 5 shot group is insufficient to determine the average accuracy (group size) of any rifle. But five 3-shot groups fired over a period of days while field proofing the rifles capability will begin to more closely represent the true average accuracy of the rifle. Ten 3- shot groups will add an even higher level of confidence of the rifle's true accuracy capability. Same with the determination of Avg MV, ES, and SD. 20 shots with identically loaded cartridges fired at similar temperatures will provide a high quality determination of SD. I shoot thinner tubed barrels than most of the members on this Forum because my hunting is backpacking in 4-10 miles and then back out - hopefully with some wild game meat. I see my MVs change some on my thinner tubed barrels with increasing barrel temperatures. They tend to display a decreasing trend in MV as they warm up. I don't ever read anyone else posting the same thing with the heavier tubed LR rifles in common use. But I observe this often enough to believe that my warming barrel increases my ES and SD. This is another reason I fire 3-shot groups rather than 5 (or more) shot groups. Again, I don't shoot 20 shots in one sitting solely for the purpose of collecting 20 MVs. I collect these MVs over time as I'm developing and then field-proofing my pet handloads and confirming drop data at variable distances. By the time I've fleshed out my chosen load, I find that I usually have collected enough MV data to establish a very representative Avg MV, a realistic ES, and a realistic SD.
If I measure the length of 20 fence posts out of a pile of 200 and figure the SD based only on those 20, wouldn't it be entirely possible to be an incorrect value?
Yes, it would be possible. Even probable - without knowing something about the methods of fabrication of the fence posts. Like were they all intended to be fabricated to the same length? Some due diligence is required in the selection of those 20 posts from the 200 posts, and the measurements need to be carefully collected and recorded. Just like the loads being fired over the chronograph need to be hand crafted carefully and fired at similar ambient temperatures which most closely resemble the first shot or two from the fouled barrel. MVs can be vastly different from a squeaky clean bore, when compared to a fouled bore. I've had clean bore velocities reduced as much as 120 fps compared to the second fouled bore shots from two of my rifles. Generally speaking, collection of MVs over the chronograph requires some thought and attention to factors that could bias the MV data high or low, compared to the conditions under which you'll be taking the shot at your game animal. The collection of MV data should attempt to duplicate the conditions under which the rifle will be fired during the hunt.
Back to the fence posts; if the same person cut all 200 fence posts using the same equipment and methods, and you selected your 20 fence posts for measurement by collecting one from every 10th fence post cut, and then carefully measured and recorded the length of these 20 fence posts - your Avg fence post length, Extreme variation in fence post length, and SD from the Avg fence post length should all be pretty close to the reality of what's likely to be present in the remaining 180 fence posts.
The fence post example is not a very good analogy to the collection of MV data. But I tried to identify some pertinent factors that should be considered with the collection of the fence post length measurements. There are also factors to consider with the collection of MV data, if a shooter hopes to establish Avg MV, ES, and SD that are representative of the true Avg MV, true ES, and true SD likely to be in-play when pulling the trigger on the round that's intended to slay the prey.