Same charge loaded a month apart yielding different Velocities.

[Syncrowave]

My first reaction is that the velocity difference doesn't look all that large to me, at least not over a 20-round sample size for each group. I'd be interested in seeing the numbers with larger sample sizes.

If the velocities are actually different, the first thing I would look at is temperature difference at the two shooting sessions.

Next guess would be that the RH was different at the two loading sessions, which changed the actual charges of propellant. Powder takes up, and gives off, moisture, making charges "appear" to be hotter/colder when in fact the amount of propellant is the same. I think it was in Glen Zediker's book on reloading for competition that he said he basically abandoned weighing each charge in favor of charging the cases by volume (with a benchrest-type Culver measure) for that reason. I think he said he got better or at least equal accuracy/repeatability by charging volumetrically rather than by weight.

 

[JCardenas]

I believe this has happened to me. I shot at a match after my loads had sat for more than 45days because the previous match was cancelled. I decide to just shot what I had loaded and it shoot great. From what I am reading, I now believe that the added pressure could have produced those groups. Thanks for the heads up on neck tension, I will also have to see if jamming my bullets will reduce the cold bore.
So my question is...How long does it take for a bullet and brass to cause this cols bore effect.