Barrel slows down after 300 rds?

I have shot the same load in my 28 nosler from May tell September. Today I went and shot and up until last week I had an es of 6 at 3043. I dropped down to 3000. Accuracy was still good. Es was still 6 fps. But everything stayed at 3000. Any ideas why a barrel would slow down? Have about 3-400 rounds. Haven't cleaned it in a while so did that tonight. So far I have checked.
Powder lot same
Primer lot same
Neck tension 1.5 though
Powder charge was same.

Bullets were loaded a few months ago and have sat in my shed with hot weather?

Out of curiosity I would say pull a bullet and weigh the charge. There has been a few threads on humidity changing the burn rate of powder.

I did pull one when I got home. It was spot on. Would humidity change the burn rate based on when you seat them or when you fire?

From what I've read it can change while being stored in loaded form.

Maybe you have some throat erosion, check how far you are from the lands and compare it to where you started at. You might have to increase your powder charge to make up for throat erosion.

I wouldn't think throat erosion would be it. Last week I went a shot a very similar load. I did load them fresh and shot them however. Loaded then headed down and shot them. I was getting what I always had. It could be the fact that the bullets weee stored for a couple months in the summer heat in my shed with no ventilation. I hadn't cleaned in about 150 rounds. Accuracy was still .3 or less. I cleaned it real good with bore tech. I'll have to load 5 shots and shoot them in the same day. I did move all my primers inside next to the powder.

What was Temps when you shot 3043 compared to 3000,, Depending on powder used and Temps that could be you issue. I ran into this with RL17 in a 6.5 Creedmoor that had almost 75 FPS swing between Under 90s to low 70s.

I have checked temp stability with N570 from 48 degrees in may to 80 in the sun in June. Always shot the same. Today was 70 degrees.

Environmental issues such as temperature, humidity and barrel temps could cause this...even how hot you cartridges were when you loaded them in the gun...try comparing them again to letting them sit in the sun and sit in a hot chamber before you fire them, bet it's more than what your showing also...air conditions in your reloading room while loading the shells...lots of reasons

I went and shot this morning. Velocity was 3010 on a powder charge from 78.3-79. 79 use to give 3040. So about 20-30 fps difference. I did shoot 79.5 but the low was 3026 and high was 3057. I am going to re check the lands. I used to be .020 off. If I am still accurate and say 30 or 49 off, could it just be the loss of pressure from being further from the lands?

So 78.3 -79.0 are the same velocity?

I bet you will find that you have throat erosion. I have seen 15 fps with seating depth changes.

They are the same velocity. 78.3-79 had 9 shots between the range and an es of 6.
Low was 3008
High was 3014

What is your powder charge your using