January 04, 2011
By David Tubb
Neither rifle nor shooter will shoot to its potential without suitable ammunition.
By David Tubb
The author does the majority of his on-target testing using a machine rest. It's an easy and accurate way to test variables such as seating depths and primer brands. He will, however, always confirm load selection shooting from position. Group size (especially elevation) determines his load.
The low-end loads in good reloading manuals are always a good place to start when testing a new load for competition. I live in the country and have no neighbors for miles, so I set up outside my shop (earthen backstop) and chronograph reduced-charge loads, working my way up to the velocity I'm trying to get, and I don't try to shoot anything as fast as it will go. There's no target involved yet.
New brass never shows the pressure once-fired brass does, so I shoot the initial tests with new brass. I start with five new cases and use those same five cases as things progress. When I seat the primers again after that first firing, I can easily tell the kind of pressure I have. A lot of people are measuring the case head and looking at the fired primer to see how flat it is and so on, but my gauge is the primer pocket. How well the new primer fits the pocket of a fired case is the acid test for me because it directly reflects on how much case-head expansion is occurring. This is why I fire the same cases over and over. Doing so lets me duplicate the fatigue I can expect in subsequent loadings.
After I've fired these five on the new brass, I go up a grain or half-grain, load them again, shoot them again, then feel the primers seat again. I also now have a velocity readout. Five shots isn't going to give me a standard deviation worth looking at, but I can see if there's a big spread and also, of course, what the approximate velocity is.
To make it a good test, I try to reduce variables and establish controls. One way I do that is by soft seating all the bullets in these initial test rounds. That is an important control factor. This method is done by running a light-enough case-neck tension to allow the rifling in the barrel to seat the bullets to their final depth when I close the bolt on the rifle.
TRIAL BY TARGET
Let's say I am trying to find a 600-yard load. After I find a load in the area I want velocity-wise and pressure-wise, I load 15 of that one and then 15 more with a charge a half-grain higher and 15 more with a half-grain less, so I have a 45-round test: A, B, C. I clean the barrel thoroughly and go to the range. Once there, I shoot 12 to 15 of my short-range loads, so the gun will be fouled to close to the same condition as if I were coming off the 300-yard line.
Transporting successful loads to other regions of the country is still one of the great obstacles--or goals--in ammunition performance for me. Hotter, drier and higher conditions increase the performance of ammunition in a few key areas. Bullets drift and drop less at higher altitudes. Ohio is lower, more humid and cooler. It's not possible to anticipate all the changes, but it is possible to be more aware of the potentials and to err on the safe side, when there are options. Otherwise, these regional differences can represent minutes-in-zero changes, not just a click or two. One key for me is to find a propellant that demonstrates stability over a wide range of ambient temperatures--especially when the temperatures are on the low side--and produce very low variances shot to shot. Temperature insensitivity has become very important to me.
I always start with the mildest charge I brought because I shoot on the edge, to a degree. I then shoot two to three of those over a chronograph and to get my elevation on the 600-yard target. I then fire 10 rounds for a group. The reason I load 15 of each is that I shoot two to three and then my 10 for the test, and I always save a couple of rounds of each test load to bring back. I come back and measure the overall length, pull the bullet and reweigh the charge. That way I double-check myself. At the range, I mark the cartridge box and line up the rounds and fired cases so I know the loading order and can review the progression of my test. I mark the case heads so I can't mix them.
I like to test three loads at a time. That's about 50 shots--figuring barrel fouling, sighting and group--and that's a good test. I don't want to shoot more than 50 shots of full-power ammo through the gun (it leaves too much residue), so I clean the gun at that point.
I make my notes about which I think shot the best. Loads B and C shot better and closer to the same points of impact than load A, and there was a half-grain difference in each. Then I come back and load the B and C load again and probably add a third factor, like a different primer in the B load (when I get a load tuned where I like it, I change primers to test that). That becomes B1. If it's shooting well, I probably won't change bullet-seating depth. If it's not, I test varying seating depths (moving the bullet back, away from the lands). I might also bump up the load a little more just to see, and that change will now be the D load. I do this because I can tell when I seated the primers to make up the rounds for this next test that there were no pressure problems in the C load.
If the B and C load gave good SDs, and the B1 load (which is the primer change) also gave a good SD, but when I seat new primers in the D load they feel a little looser (this is the third load on them), then the extra half-grain was too much. That tells me that the C load is going to be edgy when I go to Oklahoma City, where I know it will be 100 degrees. That makes me lean toward the B load, which was a half-grain less than the C load. With the B loads (and let's say that both primers shot well and velocities were within 20 fps of one another--they're usually not the same with different primers) I now shoot a 40-round test with 20 of each, B and B1. I have zero and elevation established and will shoot 10 and 10, 10 and 10 (not 20 and 20). I shoot two 10-shot groups twice because in the middle of the string, I might have a wind change, and I try to level the playing field and give each two chances.
I am adamant about shooting the same powder and bullet-jacket material across the course, so that means I don't shoot a 4895 at 300 and switch to Vihtavouri 160 at 600. I've seen some strange things from combining barrel residues. It's another variable; why introduce it? I used to shoot a lot of different combinations at 300 yards and sometimes threw in another bullet for 200-yard events. With the 6mmXC, my 300- and 200-yard load now is a reduced charge of my 600. The same bullet, definitely the same jacket material, definitely the same powder.
Let's say I choose the B load because, of course, it produced the best group in my test. I then usually run one experiment with seating depths just to make sure I don't overlook any variable that might have a positive influence (hopefully, I have eliminated those with a negative influence by now). Since I have been soft seating and already have the bullet snugly into the lands, I move the bullet back. I normally will go to .010 and then 0.020 inch. These rounds will have different case-neck tension since they are jumping.
Be sure and record all your test data. By testing ammunition thoroughly and in a manner in which it will be used in competition, you will eliminate one more unwanted variable from the firing point.
To learn more about David Tubb and read more of his information materials, go to www.zediker.com or www.davidtubb.com. Tubb's new book, The Rifle Shooter, is available now from www.zediker.com or Superior Shooting Systems, Dept. RS, 800 N. Second St., Canadian, TX 79014; (806) 323-9488.