Determining optimum shot string size
by B.B. Pelletier
Lots of talk about the new Benjamin Marauder last week. In fact, it is the most talked-about PCP this blog has ever seen. Most of the talk has centered on accuracy and wondering whether the Marauder lives up to all the claims people, including me, are making for it. Well, on Friday, I had a very interesting phone call from Ray Apelles, the guy I told you was working on the Marauder with Crosman.
Ray's conversation boiled down to one point, "What is the criteria for determining the size of a shot string?" He clarified that by adding that he is interested in the accuracy at 50 meters (55 yards), which is the maximum distance in national and international field target competition. His reasoning is simple. The max shot string for him is always the most shots he can get from a charge and expect them to stay on the impact point at the max range. He defines "stay on the impact point" as giving a one-inch group or less. For Ray, any shot that won't stay inside the specified group is one he doesn't want in his effective shot string, because he competes in field target. He wants to know that every shot has a chance of hitting the target.
He then asked me what kind of velocity variation I would accept in an optimum shot string. When I answered 30 f.p.s., he explained that he didn't care about velocity, so long as all the pellets met his accuracy criteria.
A voice from the past
That took me all the way back to 1995, when I was first introduced to PCPs by Rodney Boyce. He taught me how to determine a PCP's performance curve with a chronograph, and how to relate those numbers to the pressure gauge during the fill. This is stuff we have been going over in the past several weeks, so you regular readers should know what I'm talking about.
Rodney then told me to select an aim point at 50 yards (this was in the days before the field target maximum distance was lengthened to 50 meters) and to shoot pellets within the performance curve (only those pellets going fast enough to make it into your optimum set of velocities) until they started to wander away from the group. The maximum number of pellets that stayed in the optimum group was the limit of usable shots for that particular rifle--no matter what the chronograph says.
Here's how it works
Let me give you an example. Say your chronograph suggests you have 31 shots that stay inside 27 f.p.s. maximum variation. You might be tempted, as I was, to call that your usable string. But if you also shoot a group at 50/55 yards, you may discover that after 21 shots the group starts to open up. Which number should you use?
Well, use 27 shots if numbers are all that matter to you, but use 21 shots if you actually want to hit what you aim at. See the difference?
...or, think of it this way
Here is how Ray said it. "I see your five-shot groups are all very tight. What would happen if you overlaid five of those groups on each other? Would the resulting 20-shot group be larger?"
Before you look at those four Marauder targets I posted last Friday, let me tell you that I adjusted the scope between them. Those were taken from 20 different targets I shot with the rifle, but there was some scope adjusting going on. However, Ray's call has raised an important question. Should I be trying to show you the optimum pellet spread based on group size instead of velocity variation?
This will add a lot of time to testing
It's far easier for me to do velocity variation, because all I have to do is pick a set of numbers from a larger set. To show optimum group size will mean many more hours at the range, because I will be shooting 20 to 35-shot groups, depending on the gun. And, because I won't know how large those groups should be before I begin, I will have to shoot several of them to find out. And all of that comes AFTER I have discovered a good pellet for the rifle!
Guns to be hand-held
You can tell me to just attach the gun to the bench with a vice, but that is impossible with the current equipment I have. The benches I shoot from are all movable, plus many of them are portable. Maybe I can have a heavy custom vice rig built for this, but at present I don't have what I need to clamp the guns in a vice. I will have to hand-hold the guns for now. I'm not complaining, but my groups will be a little larger because of it.
What about springers?
Now, if you are one of those people who wants everything in life to be "fair," you are no doubt thinking that spring guns aren't the same as pneumatics. What will I be doing about them? Well, I spent a good portion of the weekend thinking about this and my solution is to shoot 20-shot groups from every spring gun at 35 yards. I picked 35 yards because, even though there are RWS Diana 54s and TX200s in the world that can hold their own with PCPs, there are also Benjamin Super Streaks and Mendoza RM2000s that can't. And occasionally I'll pick up a rifle that can't even keep up with the average springer. With a 20-shot group at 35 yards, you will get to see the accuracy potential clearly. Obviously the velocity doesn't fall off at any point with a springer, but just as obviously to those who have experience with them, springers are ten times harder to shoot well than pneumatics and CO2 guns.
Speaking of CO2 guns, what shall I do about them? Well, there is the problem of temperature to consider. All my long-range shooting is done outdoors, and that will limit me to warm days without wind, only. I have decided to treat CO2 guns like springers and shoot 20-shot groups at 35 yards, but I will adjust this as we go. I will also pause between shots to allow the gun to warm up, and the amount of time in each pause will vary with the temperature.
Target guns will not be tested this way. Them I'll continue to shoot at 10 meters, because nothing else matters. And obviously BB guns will also not be included.
There will be no more bullseye targets. Once I have sighted-in I will shoot at small aim points and the scope will be adjusted to group far enough from the aim point so I don't destroy it with my shots. I will also make reference lines on the targets to align the scope reticles, to eliminate the possibility of error from canting.
And don't expect to see me shoot ten groups like this, so you can see a relative size. That would take days of time that I simply do not have. You can certainly do it with your one or two rifles, but I test hundreds of airguns and there's no time for unlimited testing. While determining the optimum group size I will also get a feel for the rifle, so the group I do shoot will be based on that experience. If anything happens to spoil the group I will stop and shoot another and I'll tell you about it in my report.
I'm going to give this methodology a try. The last time I tried anything this ambitious I was shooting 10-shot groups while testing different versions of a Ruger 10/22 with many types of ammo, and it took days to get the test completed. This isn't that ambitious because I'll only be shooting a single type of pellet in one gun instead of eight different types of ammo in two different guns, but it's still plenty of work. The results should be more believable, though, because you will see clearly if the shots are migrating during the test. I think I will call this the Mega Test.
In a related note, Crosman called last week and offered to send me the very latest version of their valve for the Marauder. Since they sent my test gun 45 days ago they have continued modifying the radius and shape of the air transfer port and they have made a breakthrough that they say bumps the usable shot count upwards by several shots. It may also make the velocity variation tighter. When it arrives I'll install it and report the results to you. That will also be a good time for me to explain how to adjust the fill pressure level and the velocity adjustment, because I'll have to do both after completely stripping the gun.