I’ve been loading for a 300 wm for several years. I just started shooting it again from last year’s ammo. At the end of last year the last shot I took was at 385 with all the previous years dope. That last shot also blew up the casing in half. I had to have Shilen get the casing out with a hand reamer. Now, with same ammo, my dope is way different. FiremanJG helped derive my corrections when I took his class. I just shot at 400 yards. All the previous years my dope was 1.8, now I’m shooting 1.5. At 200 in .1 different, at 300 I’m .1 different. Thoughts

What changed? Temps? Ammo velocity, pressure? Load details of ammo? Is your zero the same? Everything stems from a 100% dead on zero. If zero is off, everything else will be off.

Agreed.

And, how long has that ammo been loaded?

I don't remember what month you came out, but my bet is, it was not 100° outside. Did the ammo sit in the sun? What powder are you using?
Same base, rings, and scope?

I’ve shot all times of the year and the data is the same. I shoot 230 grain Berger match hybrid targets with 73.3 grains of retumbo. Ammo was in the shade as usual and was loaded roughly a year ago. Zero changed too. Nothing on the gun or with the ammo changed. The only thing is Shilen had to take my gun apart to hand ream that case out. They had to take it apart once before but didn’t have to team it. That was a broken extractor issue. I’m concerned because this is my Colorado gun. I could shoot 600 yards where I’ll be hunting

How many loadings on the cases? Case head to case body starts out strong but through continual firing and resizing it gets weaker as brass flows from that area forward and you wind up trimming the cases.

POI shift an happen when things are taken apart and put back together, torqued differently and such.

But I’ve taken it apart multiple times and it has always stayed the same. Ammo is the same. Loading number isn’t a factor. It was doing 1.8 at 400 last year, now 1.5. Same batch of ammo

Same weather, different correction?

What's the total round count on the barrel?

So all times of year wether it was cold or it was 100F outside it shot 1.8 drop at 400?
That doesn’t sound correct.
Did you Chrono the loads?

I can believe that it used to hold 1.8 at 400 yards in all weather. I have several that do the same, and they are much less powder charge, less bullet weight, and less BC.

I'm leaning toward two things.

1. Barrel speed up
2. Cold metal welding, bullet to brass. It's been loaded for a year. Bond between bullet and brass = stick = more chamber pressure = faster load. And he got a stuck case, to show for it.

I thought he OP said the case came in half... May not be pressure related that is usually caused by oversizing and creating excessive headspace.... it’s a belted case but setting the shoulder back too much will eventually get a case separation.

That is why I ask how many loadings on the case that split, case head separation can and does happen, pressure, sizing techniques, chamber several things contribute to case head separation, why I ask how many loadings on the brass. I have had some factory ammo that I would not use the brass from it to load because there was the bring ring and could easily feel the area brass was being pulled from. Those were 264 Win Mag loads.

Everything yall are saying is correct, of course.

But you're not addressing the change in DOPE. DOPE that is showing a phenomenon of a faster load.

Cold metal welding, and a barrel that sped up can also cause the problems yall addressed, as well as as the change in DOPE.

I’ll load some fresh ones to try them out.

^^That is exactly what I was hoping for!!!

Please, please tell us what happens.

If the dope change it can only be a two things I can think of (assuming same zero)

Ballistics
Either bullet changed velocity or bc changed somehow. I think velocity shift is more likely assuming same bullet

For velocity to change some ideas
Exterior conditions like temp
Chambering dimension change when shilen took a reamer to it
Bad powder throw (mis zero scale, too much, etc)

If it were me...
First step for me is always to ensure all pieces are tight and installed correctly
Re zero to establish that my zero has not shifted
Remake ammo watching for above factors. I would measure everything
Test over a lab radar
Record data and compare to previous notes for clues

Good luck

Did you have Kyle measure your powder?

Can you measure a case that was fired before repair and compare it with a case that was fired after the repair?

Well, after taking a rifle apart, I always re-zero and then re confirm my data. A shift like you are seeing at only 400 is a lot.

Another thing to try, is on your year old ammo, seat your bullet about .001" deeper to break the bullet and brass bond that's formed.

Fresh ammo that I loaded last night shot at my original dope. I went to my 400 yard plate and shot with my correction at 1.5 and missed then I adjusted to 1.8 and hit 7 of 9 times in a crosswind. It’s an 8 inch target.

Great news!

Cold metal welding.

Did you re-seat some of your old ammo, like Chad suggested?

Going to that tonight and shoot tomorrow

Awesome!

Let us know what you learn, please.

So many THF members don't realize how important this data gathering is. I will be one to remember it, and reference it when the time comes. This is so good for everyone's benefit. Thank you for bringing it public!

tips and tricks can always be added to the sticky thread of tips and tricks to reference later

Sounds like you got it figured out. One other thing to remember in these situations is the change in scope height if you changed the scope and rings - that can impact DOPE. Same thing if your zero isn't perfect.

I unseated my old rounds with my impact puller just enough to break them free then reseated them. Shot them just now at 400 and I’m back to 1.5 correction.