Bruce Potts advises what rifle shooters need to know when the temperature drops and it's darker

Q: Will cold weather affect the velocity of my .243 stalking rifle and do I have to change zero during the winter months?

Rifle velocity in cold weather

A: Here in Britain it is unlikely that winter temperatures will make any real difference to zero.

In terms of changes to velocity, in hot weather, the cartridges can get very hot and the pressure on firing can rise and therefore impact velocity. In winter the same cartridge will be slower but dependent on other factors – such as altitude and air pressure density – which will all make a small degree of velocity change and therefore trajectory shift. For example, I tested some .223, .243 and .308 bullets packed in snow to see the change.

At normal ranges up to 200 yards, I only had variations of 50fps to 100fps, resulting in no more than 0.5in to 1in variation maximum dependent on bullet weight. At greater distances yes, a bigger correction would be needed.

Try it yourself with a cooler bag or snow and shoot your rifle and ammunition combination at your normal ranges so that you can see the difference. The real trouble comes if you reload in winter and the cartridge pressure is OK – if you then shoot the same ammunition on a hot summer day you will find that the pressure will increase excessively.

Shooting the .243 calibre

Rifle shooting the .243 calibre

The .243 calibre is a user-friendly cartridge for shooting deer, mostly roe, and perhaps the occasional fox at ranges out…

Bad light

Q: Does the point of bullet impact change when a scope is turned down for poor light? If I use a variable magnification scope zeroed on the highest setting and in good light, would the point of impact change when the scope is turned down in dim light?

A: You are right to be concerned regarding the possibility of your rifle’s zero shifting from one magnification setting to another. This can be a common problem and is one that many shooters forget to check when they purchase a scope. It is usually the cheaper scopes that suffer from this displacement problem.

I test a scope’s repeatability by setting a target at 30 yards for a rimfire rifle and at 100 yards for a full-bore, and then shoot a three-shot group at each magnification setting to ascertain if all the shots fall at the initial point of impact.

A small degree of shift is acceptable, and for you to decide, but at longer range it could be the difference between a hit and a miss or, worse, a wounded beast. It is worth spending more money on a quality scope, or stick to a fixed-power scope that will obviously not suffer from this problem.