I simply go by past experience and interpolate from similar cartridges. For instance, with the .270x.225 DVH, I used both the .257 and the 6.5 JDJ as gauges. So if the 6.5 gets "x" velocity with a 120, it stands to reason the .270 will be able to match or potentially exceed that due to a slightly larger bore diameter, although many variables can come into play. I consider other factors, too, and all of this gets me into the ballpark as far as a target MV.Originally Posted by BT
My goal for this 20.5" barrel was to reach 2600 fps with the 120 grain SST and 2850-2900 fps with the 95 grain Barnes. The round is actually capable of a bit more in the Contender, but I like to stay on the conservative side. Plus, with only 100 pieces of brass, I want to make them last as long as possible.
As to the chronograph, I currently use a Caldwell Ballistic Precision unit and always try and use it under the same type of lighting conditions. So far, it's done very well, though the "trust but verify" aspect does come into play with me. If something seems amiss, I dig out my well-worn, 8" Dan Wesson .22 LR that I've had since the late 80s. It's been tried on a number of chronographs, including the Oehler I had for about 10 years. With one particular load, you can count on it registering between 1048 and 1060 fps as long as the ambient temp is above 65. It's been remarkably consistent over the years. I actually took out the DW yesterday, and the two rounds fired across the chronograph went 1050 and 1052 fps. It may not be a scientific approach, but it's good enough for what I do and actually outed a Pro Chrony unit I had a few years ago when it began reading in the high 1100s with the DW.
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