Why Does My PCP Air Rifle's Zero Move Between Sessions?

Step 1: pin the problem down before you touch anything. Pick one distance, 25 or 30 yards is fine, and use the same rest, the same tin of pellets and the same aim point. Shoot five shots, mark the group, and write down the date, the air temperature and the gauge reading. Do the same at the start of the next session before adjusting the turrets. After two or three sessions you will know how far the zero moves and in which direction. A shift that is always vertical points towards air pressure or velocity. A shift that goes sideways or in random directions points towards something loose or the scope.

Step 2: look for anything loose. This finds more wandering zeros than any other check. Go over the ring cap screws and the clamp screws that hold the rings to the rail, using a torque driver set to the mount maker's figure if you have one. Check the rail itself if it is a separate part screwed to the action. Then check the stock screws, because an action that shifts in the stock moves the point of impact too. Finally check the front end: the moderator, the shroud, and any barrel band or clamp. Hold the action and press gently on the muzzle. If you can feel the barrel or the barrel assembly move, you have found a likely cause, and on a rifle under warranty that is one for the retailer or importer, not a home fix.

Step 3: rule out the scope. Start with parallax. If the parallax setting is wrong for your distance, the reticle appears to move on the target as your eye moves behind the scope, so a slightly different head position each session gives a different point of impact. Set the side wheel or adjustable objective by looking, not by the printed numbers, which are only approximate. Then move your eye a little up, down and sideways. The cross-hair should stay still on the target. If it floats, adjust until it does. Next, check the turrets. After a zero change, some scopes take a shot or two to settle, so always confirm a new zero with a fresh group. If you have a second scope you trust, swapping it over for one session is the quickest test of all. A scope that has spent time on a springer deserves extra suspicion.

Step 4: check fill pressure and the regulator. An unregulated PCP does not give the same velocity across the whole fill. Velocity typically rises to a peak and then falls away as the pressure drops, so a rifle zeroed on a full fill can shoot to a different point when it is half empty. Find the pressure range where your rifle is consistent and fill to the same figure each time. A regulated rifle is steadier, but only while the cylinder pressure stays above the regulator's set pressure. Once the gauge falls below that, the rifle behaves like an unregulated one and the group drops. Regulators can also creep: pressure slowly rises behind the regulator while the rifle rests, so the first shot or two after a break leaves at a different speed. If your first shot of the day is always the odd one out, suspect this.

Step 5: think about temperature. The pressure of the air in the cylinder changes with temperature, so a rifle filled indoors and shot outside on a cold morning has less pressure than the gauge showed at home, and a rifle left in the sun or a warm car has more. Seals and lubricants also behave differently when cold. How much this moves the zero varies a great deal from rifle to rifle, so do not rely on a rule of thumb. Let the rifle sit outside for a while before you check zero, and note the temperature in your log. If your zero is always low on cold days and back to normal on mild ones, you have your answer, and the practical fix is to check zero at the start of each outing.

Step 6: use a chronograph if you can. Everything in steps 4 and 5 shows up as a change in muzzle velocity, and a chronograph turns guesswork into numbers. Shoot a ten-shot string at the start of a session and compare it with the last one. If the average has moved, the cause is on the air side. If velocity is the same and the group has still moved, go back to the mounts, the scope and your hold. A chronograph also tells you that the rifle is still under the 12 ft-lb limit. In Great Britain an air rifle above that figure needs a Firearms Certificate, so never adjust a regulator or hammer spring without measuring the result.

Step 7: check pellets and the barrel. Keep to one tin while you diagnose. Different batches of the same pellet can shoot to slightly different places, and mixing two part-used tins is an easy way to create a puzzle. If you have just cleaned the barrel, expect the first shots to land differently until it settles. Clean rarely and then re-check zero, not the other way round.

Step 8: look at your hold and rest last. A PCP is forgiving compared with a springer, but it is not immune. A rifle rested on a hard surface, on a bipod, on a bag or on sticks can shoot to slightly different places, and so can a rifle rested at the front of the stock compared with the middle. Tilting the rifle, known as cant, moves the pellet sideways and low, and the effect grows with distance. Zero the rifle the way you will shoot it in the field, keep the cross-hair level, and use the same support point each time.

If you have worked through all eight steps and the velocity is wandering with no pattern, or the barrel moves by hand, stop there. Pressure parts on a PCP are not for experimenting on at the kitchen table. Take your log and your chronograph figures to a gunsmith or back to the shop. A clear record of what moves and when is the fastest route to a fix, and it is hard to argue with.