One reader’s PCPs give lower power during the winter than they do in the summer – is this normal?

Q: I like to check the muzzle energy of my rifles regularly, and over the last couple of years it seems to me that it varies with the seasons, not by a huge amount. Whilst I understand this is normal with springers, my PCPs tend to give lower power during the winter than they do in the summer. Is this normal? Should I just re-zero and carry on, or do I have a problem?

A: It is perfectly normal for airgun muzzle energy to vary according to the season or, to be more precise, according to the temperature. Co2 guns show the greatest variance, because the pressure at which Co2 changes from a gas to a liquid drops as temperature falls, so there is significantly less pressure in cold weather, so much so that I would strongly advise always checking zero before shooting a Co2 if the temperature has changed more than perhaps 10 degrees since it was last zeroed.

Spring airgun muzzle energy varies with temperature for a variety of reasons that can see energy climb or fall at lower temperatures, and to make matters worse, pellet point of impact (POI) changes can be out of all proportion to accompanying changes in muzzle energy, or even go in the ‘opposite’ direction, so that lower muzzle energy can give a higher pellet POI. Things seem to happen if the temperature approaches zero, or rises by around 20°C since zeroed, so check zero if there is a significant temperature shift.

The PCP should be least affected by temperature, although it is by no means immune to changes. As in spring airguns, low temperatures can thicken lubricants, which would take energy from the hammer, so the valve opening could be less, or of a shorter duration. As in spring airguns, different materials expand and contract at different rates with changes in temperature, again altering friction and impacting on muzzle energy. In my experience, changes in muzzle energy due to thickening lubricant or friction are far less in the PCP than in the spring airgun.

Even though the PCP is least affected by temperature fluctuations, it is always worth checking zero before a shooting session, if for no other reason than pellet POI can change for other reasons, such as the barrel or scope accidentally getting a knock in transit.

Finally, don’t worry so much about changes in muzzle energy as changes in muzzle velocity. What seems a substantial change in energy can be just a few fps, and far too little to change pellet POI.