I received a query this morning from a gentleman who was upset and looking for a quick answer to a problem. He had chronographed a .308 load and put the resulting velocity, his bullet BC, and other relevant atmospheric data into a ballistics program, and saw output that indicated that his bullet SHOULD be super-sonic to 1,000 yards. During his range session however, his groups were falling apart before he reached 1,000. He was naturally perplexed, as he expected a super-sonic bullet to group well, right?... Well, there is an answer, but it isnt quick. I didnt know exactly where to start on the laundry list of things that could cause this outcome, so Ill arbitrarily start at the most obvious and work down from there... 1. Trans-sonic buffeting of a bullet begins to happen long before its velocity decreases to subsonic. A bullet which is not spinning fast enough to be gyroscopically stable can begin to yaw well before the falling into subsonic. So, twist rate and bullet stability factors matter. 2. His chronograph could be lying. They do it with a frequency approaching that of the average politician, but still only half that of the current POTUS. It is still way too often. Without using a second (or third) control it is nearly impossible to tell when it is happening for the newbie, although (as in politics), most of the old-timers know how to spot a lie. 3. The BC he is using may be incorrect. Without getting into all of the intricacies of G1 v. G7 drag models or BC measurement methods, I will say that BC is a shifting relative number that can even be rifle-specific. A rifle that has a slight mechanical tolerance differential from the norm may launch bullets that are engraved out of concentricity, which causes them to exhibit a different BC until they go to sleep. I once had a rifle which consistently shot a particular bullet with 30 BC points difference from what the same bullet exhibited in my other rifles. No, I am not making this up...LOL 4. External ballistics factors like a difference in air density between his ballistic models conditions and actual conditions, Shit in is shit out. The user interface on many ballistics programs make it easy to incorrectly input data or overlook important data. 5. Ammunition issues or inconsistencies trump ballistic program output. If extreme spreads are too radical, it doesnt matter what the chronographed average is. Load all of your bullets with .015 runout, and you have effectively changed your BC. 6. Strong headwind can even affect velocity lapse, and cause a marginal, ragged-edge load to fall short. 7. Shooter error. Heavy mirage can often create enough visual distortion to open groups, even if the round is staying stable. A lot of times I have seen long-range groups go to crap because of the shooters inexperience and failure to shoot under the same condition. Later, these poor groups were blamed on the load not having enough ass to reach 1,000. Bottom line here, is that the ballistics program is a godsend but you you have to know what the ballistic factors are to put into it to get good output, and how to collect that data properly, apply it correctly, and where to start looking if it doesnt work out. That means a fair amount of ballistics knowledge is still required. Here is my quick-guide to accurate output expectations: Basic ballistics app / very basic knowledge ( manufacturers BC/ cheap chrony data) = 500-600 yards. High-end ballistics software / very basic knowledge = 500-600 yards. Basic ballistics app / moderate level of knowledge / basic data collection skills = 800-1,000 yards. High-end ballistics software / moderate knowledge / good data collection skills = 1,000-1,200. Basic ballistics app / strong knowledge / excellent data collection skills = 1,200- 1,500. High-end software / strong knowledge / excellent data collection skills = 1,500-1,800. High-end software / strong knowledge / excellent data collection skills, a horseshoe up your arse and an angel on your shoulder = 1,800-2,000. High-end software / strong knowledge / excellent data collection skills and a post-retirement book deal = Unlimited. Objects in orbit not out of the question...
Posted on: Tue, 09 Dec 2014 18:00:12 +0000
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