Why does my rifle shift its aimpoint? - Part 2
by B.B. Pelletier
Before I begin, I'm going to break my rule about commenting on forums. I read a thread about me on the Yellow Forum yesterday and was surprised to see some very complimentary remarks. I usually don't get that. Several who commented also mentioned that they don't agree with me all the time, and I'd like to say here and now - NEITHER DO I! I have changed my mind many times in the pursuit of this hobby and I bet I'm not done, yet. Just a few weeks ago I discovered a new wrinkle on the artillery hold that makes it work a lot better than the way I described it in the Beeman R1 book.
I'm not fishing for compliments, but I'd like to thank everyone who added to that thread.
Okay, to the task at hand. Our bore is clean, so let's mount the scope and get to testing. With an AirForce Condor, however, there are some other things that should be checked. Barrel tightness is an important one. Four screws hold the barrel in the Condor, and if it's a recent one the screws go through the bushings to contact the barrel. Older Condors have two long screws that contact the barrel and two short screws that contact the barrel bushings.
The other thing I checked was the clearance of the top hat. It's set at the factory at 0.090" clearance, and should be left there for best results. This one was okay. Then, I mounted the scope and tightened it down. I used an AirForce 4-16x50 scope I use on both my AirForce rifles. It's in a prototype B-Square adjustable one-piece mount with ultra-high riser blocks. It provides too much elevation, but it works on every AirForce rifle I try it on, so scope mounting takes about one minutes. Time is money. Let's shoot!
Shooting from the bench
I promised someone I would report about shooting techniques from a bench rest, so you probably think I have a high-tech bench to use in testing. Nothing could be further from the truth. My "bench" is a rickety folding table I drag from range to range. It's a TV table on steroids. When I shoot PCPs, I use a long shooting bag filled with crushed walnut shells. The top is arranged in a "V" and the rifle lays between the upright legs. A slender Condor fits this bag perfectly.
Sight-in at 10 feet
If you haven't read my Sight-in article, I recommend you do so now. You don't need boresighters, lasers or any other gimmicks to sight in an airgun. Just start shooting at 10 feet. Your goal is to get the pellet printing as far below the aimpoint as the center of the scope is above the center of the bore. Of course, you want to be aligned with the aimpoint vertically. The picture shows this much better than I can explain. Remember, this is shot at 10 feet. Because of the terrain on this range I had to shoot at 12 feet. Everything worked as it should.
Move to 20 yards
After you're on at 10 feet, move to a target 20 yards away. Refine the sight adjustments until you're on at that distance. This takes another 5 minutes (took three shots). On to 35 yards, because that was the distance at which Hegshen complained of point-of-aim shifts of 1-1.5 inches.
Move to 35 yards
At 35 yards, I started shooting JSB Exact 15.8-grain domed pellets with the power wheel on No. 4. On my Condor, that's where I get great accuracy. But not on Hegshen's rifle. The pellets were all over the place. His rifle has the latest valve (we checked it before I started this test), and we filled the air tank to 3,100 psi. After all the shooting I'd done to this point, the gun was probably down to 2,800-3,000 psi or so.
When pellets are all over the place and seem to be moving slowly (as these did), I turn up the power. In this case, I went to power setting No. 8. The point of impact climbed several inches, which was perfect, because I didn't want to destroy the aimpoint.
This time, the pellets went to the same place, so I shot 20 rounds to see if there would be a point-of-impact shift. There was none. From this test, I know the rifle isn't shifting its point of impact, so now we can move on to more likely culprits.
Before we move on, a word about this 20-shot group. It's not a five-shot group. Had it been, it would have been about 0.30" center-to-center. But, shoot 10 shots and that'll increase to 0.70". Shoot 20 shots and it increases again. The group that you see measures 1.124" c-t-c. Could this rifle have done better had I shot a second 20-shot group? Of course. Could it have done worse? Absolutely. Don't get hung up on the group size, because it doesn't matter. Look at the shape. There is no POI shifting going on, and that's all we care about today.
Okay, the rifle is fine, so AirForce packaged it up and sent it back to Hegshen, along with the 35-yard target that I wrote some notes on. What if he still has the point-of-impact shift when he gets the rifle back?
I'm pretty sure he will, because I know what's happening. When I was the technical director at AirForce, I took all the phone calls and emails about POI shifts and accuracy problems, and I have seen this happen many times before. Before I tell you what it is, let's review this case. Hegshen told me he would get his rifle sighted-in, then come back to it several days later and the point of impact would have shifted. He also said that sometimes he would shoot five or six shots and then the point of impact would shift 1-1.5 inches to the left at 35 yards. He also told me his rifle shot to one side close up and to the other side far away. What's wrong?
Look at the picture of me shooting the rifle. See how high my head is above the bore of the rifle? It has to be high because that's where the scope is. If I do not put my face at the same point on the buttstock every time I shoot, my point of impact will shift, too. I've had to learn how to position my head in the same place shot after shot, or my POI would shift, too. But that's not all.
Look at where the center of the 35-yard, 20-shot group is. Now look at where the first 35-yard group of shots with the lower power setting is. It's several inches lower and slightly to the right. In other words, changing the power changes the point of impact. And, changing where you position your head changes the point of impact.
It's difficult to see, but just in front of the scope on this rifle you can see a scope level. If you go back to Part 1 and look at the first picture, you can see it there, too. Hegshen has glued it to his rifle, which isn't the best way to mount a level, but it works. I used that level on every shot to get the 20-shot group. Had I not, there would either have been point-of-impact changes or a larger group.
Here is what I'm telling Hegshen - and anyone else who experiences point-of-impact shift. First of all, the first shot from a cold rifle will probably not go to the same place as the shots that follow. That's true for springers, for CO2 guns and for PCPs. It's even true for firearms. Airgun barrels do not warm up as the guns are shot, but the valve of a PCP needs to be exercised occasionally to deliver consistent performance.
Second, you have to work on your hold, so your eye always ends up in the same place relative to the scope. This takes a lot of practice, but it returns more consistent groups.
Third, find a power setting and a single pellet and stick with both. Every time you adjust the power, you'll have to sight-in the rifle all over again. Hegshen has been shooting Kodiaks, which are good pellets, but I recommend that he try JSB Exacts and Crosman Premiers. Both will outshoot Kodiaks in a Condor, as long as the power isn't turned up all the way.
Fourth, if you want your groups to move straight up and down instead of from one side of the vertical reticle to the other, center your scope optically. Don't take shortcuts. Do the labor and you'll get the reward.
I'm not finished with this report, yet. There'll be another part that covers all the other reasons for POI shift. But that will be another day.