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Hello everyone. Yesterday I checked the muzzle velocity of ammunition ( seleir & bellot .308win 168 grain) and to my amazement I saw that the muzzle velocity that was about four years ago 2650 fps for the same ammunition decreased significantly to 2520 fps in the same rifle and under the same conditions. Anyone know how to explain this?
Storage in a dry place and at a normal temperature for a living space
Thank you

Same chronograph placed in exactly the same place??

Alan

And following on from Alan’s post, what was the weather like when you first tested it and were you on the same range?

Altitude, temperature, humidity etc all have a part to play…

You should always record air temperature, atmospheric pressure and range altitude when chronographing your ammo. Nearly all ballistic calculators will require you to give this info to calculate your drops.

Short answer, it was much warmer day 4 years ago when you originally chrono'd the ammo.

Presumably your rifle has been used a fair bit in those four years resulting in some wear to the barrel?

Thanks to all the respondents! Regarding the shooting conditions. Same range same height more or less the same temperature (not measured) same month , different chronograph. about four years have passed and I fired a rifle at about 2000 rounds and the barrel wears off a bit. In the future I will record temperature accurately. There are about 3 potential reasons left: barrel erosion, using a different chronograph, and the thing that scares me the most is that the fire fire powder inside the cartridge has deteriorated. Can this happen?

All nitrocellulose propellants will deteriorate over time, they deplete their stabilisers and eventually can even self-initiate under extreme conditions. Four years seems a bit short for that to be the case though, given the stated storage conditions. You generally only start to see that sort of deterioration after 15+ years under normal storage conditions.

Thank you Dave

The modified Arrhenius equation has a key term Temperature in kelvin K in 2 places. So if you reduce T you significantly slow down K the rate of a deterioration reaction. So i store my powder (not BP) in a 'beer' fridge in a locked garage becuse its a constant temperature & its cool 4C or 277Kelvin vs hot in summer etc. They are in sealed poly tubs. Anything else in there is sealed like bottles or cans so introduce no moisture to it.

There was ammo form sunken ships some years ago that was old but worked fine - i saw some RAF 303 (larger primer to blast a round out of the barrel so prevent jam ups if misfire) it was 1939 & shot fine. At the bottom of the Irish sea its probs a constant 2dec C.

So now we al need a Beer fridge, then a 'beer' fridge or a refrigerated ammo safe!

Wow...................I sometime do think we overthink things