Re: Keeping E.S. under 20fps.
All the items mentioned by S1 are necessary to obtain extremely low velocity spread amongst a group of loaded cartridges, but I have found that a couple of specific things must happen even after paying attention to the things previously mentioned.
The first has to do with brass. The following method is one which seems to work to get a few excellent groups of brass which will work to get consistent velocity from. AFTER doing all the requisite preparation steps to get a good group of brass, load up a pile of ammunition with a good proven load using proven components. I normally do between 50 and 100 rounds at a time but you can do more or less, depending on how much brass you wish to end up with. Next shoot all the loaded ammunition across a chronograph using the method mentioned in the second important item and make sure to use a black marker to write the chronographed velocity on each piece of brass. After all the shots have been fired sort the brass by velocity and group it in groups of 10 pieces based upon velocity. When doing this, often I can get 4 or 5 pieces of brass which will give identical velocity for the initial firing. In general, I get between 30 and 50 fps spread across 100 pieces of brass (depending on brass quality) but after they are sorted from lowest to highest velocity I can often get one or two groups of ten which have less than 10 fps spread. If you wish to be really picky, repeat the procedure and fire the brass across the chronograph a second time to verify the consistency of velocity. Again, the quality of the brass will determine how well the first velocity relates to the second velocity. The two times I tried doing a group of 50 cases twice across the chronograph ALMOST all of the brass showed consistent performance by staying in the same position in line. I say almost because both times there were a few cases which simply did not give consistent results and both times I tossed those pieces in the trash. What I did end up was two different boxes of 40-45 pieces which can be counted upon to give less than 10 fps spread for each group of ten pieces of brass as well as less than 30 fps spread amongst the entire box. Once I sort the brass in this manner each piece of brass stays in the same spot in my ammunition container from that point forward until I toss the entire lot as being worn out.
Note that the velocity from the first firing will probably NOT match the firing from the second firing due to external changes such as temperature, powder lot, primer lot, etc. What will remain consistent (except for the bad brass) is the relative position of each piece of brass in line and a low velocity spread when testing loads. It gives you the confidence to test other components and know that you have minimized the effect of brass in the loading process.
The second item has to do with the specifics of chronographing a load. There are probably many ways to get consistent results but this is how I do it.
Try to shoot all loads being compared at a single setting to minimize changes in external conditions. A load shot at a lower temperature will usually chronograph slower than when the temperature is higher.
I shoot a couple of initial shots thru the rifle in order to get the barrel and action up to the temperature which the chronographed loads will see. I normally touch the barrel with my finger and verify the temperature as being slightly warm to the touch. If you take a long enough break to allow the barrel to cool completely make sure and fire a warming shot before re-chronographing.
I fire one shot each several minutes as consistently as possible and feeling the barrel between shots to make sure it has the correct temperature. I only do this when the weather is cool because if the temperature is 100 degrees in the shade the barrel simply wil not cool down sufficiently between shots, which will give bad chronograph readings.
As I mentioned, there are probably many ways to get low velocity spreads but this is one method which works extremely well for me.
Oh yes, one thing to make sure and do to give yourself a better chance to get a load which has a low velocity spread is to use proven combinations even if they are not what you intend to end up with. For instance, one time I bought 5 boxes (100 loaded shells) of Federal Gold Medal 308 loaded ammunition and shot it all across the chronograph in order to give me the best possible chance to come up with some good brass. I know that buying brand new ammunition and shooting it at paper simply to come up with good brass is very expensive but it does give a consistent starting point.
[ 06-09-2003: Message edited by: Bruce Gordon ]