Getting Your Inputs Squared Away Part: 6
Time to get your muzzle velocities taken care of. This is why you needed the 50-100 rounds of your chosen ammunition. We’re going to assume that you want to get your butt to the range and start getting your data. So we will also assume that you have a chronograph and batteries for it and you’ve familiarized yourself with its operation. You’ve got your tripod and test mounted your chronograph to it and learned how to adjust it.
Grab some dry ice and stick it in the bottom of your cooler wrapped in a layer of cloth. Drop your wet ice on top of that. Your wet ice will now stay solid for much longer than it would have. Take your ammo supply and put it in a zip-lock bag and put that in the cooler. You’re going to need to let the ammo sit in there an hour or so to cold-sink. Grab your infrared thermometer and head off to the range after a quick stop at the drug store. Pick yourself up a box of Therma-Care brand heat wraps. There are other brands but TC heats quickly and evenly up to just short of 110 degrees and they’re very consistent which helps with safety.
The object here is to test 5 rounds at each of several temperature regimes. You want to gather each shots’ velocity. Start with a cold bore (this could take a while, you should have all day, maybe 2 days). Grab 1 round from the cooler and close it back up. Measure the temp of that cartridge with your infrared thermometer. You may have to measure several places on the case to get a consistent reading. When it comes up to 0 degrees F stick it in the chamber and touch it off as quickly as possible. You’re not target shooting so don’t bother trying for precision. I don’t even put a target up when I’m doing chronograph work. Leave the action open and let the bore cool for a few minutes. Grab a 2nd round from the cooler and when the temp comes up to 0F fire it. Rinse and repeat till all 5 0deg rounds are fired, noting each velocity in order.
After the gun is fully cooled off grab a 6th round and let it come up to 10 degrees F or 20 degrees F. With ball powders I’d usually recommend doing 10 degree increments. This is because I’ve found ball powders to be more routinely temperature affected. With stick powders 20 degrees has treated me well.
Once you’ve repeated this test at each temperature regime and worked up to ambient air temperature and you’re not needing the cooler anymore, pull any remaining ice and dry ice out of the cooler. You’ll now be working the other direction. Use the Therma-Care wraps to heat up ammo as necessary for each temperature regime. You’ll want to measure the amount of time it takes to get a single round up to temp for each temperature and then wait that amount of time for each following round in the shot string. This will help assure even heating.
As each set comes up to temp, fire it and log the data. Don’t get impatient here and let your barrel get warm. You don’t want it imparting additional heat to the system. Do this test for each temperature regime above your “station” temperature (whatever outside air is in the shade as an average where you like to shoot).
After you’ve burned through 40-100 rounds you’ll have a fantastically useful set of data for non-standard temperature regimes. Now you should take an additional 10 rounds at 60-70F and send those downrange and collect that data. You want more data points for your “station” temperature so we can sort out the standard deviation and extreme spread and average and get a useful number to put in as “the” spec muzzle velocity. There’s no harm in getting more data points for any particular temperature range. It’ll help the statistics be more useful to you.
Put the collected muzzle velocity data for all the loads in order into the Shot Log & Statistics tab of Ballistic_XLR and you’ll get some useful statistics. Standard Deviations over 10fps are getting near the too big zone. Extreme spreads over 20fps are also getting too big. Ammo should be velocity consistent or we’ll never get your elevation right at extreme range. Long range maybe extreme no.
You should have string data for 40,60,70,80,90,100 as a practical minimum. If you shoot where it’s really hot a lot (Oz) or really cold (Norway) then you’ll want to do some extra velocity testing in those conditions just to get the most accurate data you can.
Now take your noted 70 degree average velocity and add it in the appropriate box in column A on the Pocket PC inputs page. Take the average number for each temperature specific shot string and add that to the appropriate cell in the Muzzle Velocity Variation table on the Pocket PC input page for the appropriate temperature and velocity regime.
Ok, your standard and non-standard-temperature velocities are in MVV. Muzzle velocities are done man!
NOTE: If you only put in your 70 degree muzzle velocity in Column A and you leave the MVV chart alone then the MVV chart will make a lot of assumptions and do some BS math. The assumptions it makes are not far off what the average person should expect from the average load with an average case capacity and average bullet weight with average powder. That said, the numbers produced via guesswork have nothing to do with reality. By adding your numbers to each temperature zone you’re ditching my BS numbers and putting in real ones that will work.
Stay tuned for Part 7.