Grouping question

[Llama]

I got into the precision marksmanship game a little over a year ago. For a short background, I'm shooting a factory Remington 700 in a magpul PRO Chassis with a Leupold Mk4. It's a .308. I've been shooting Federal Gold Medal Match (168) this whole time, and today to Hornady ELD-M (168). The first 5 group (pictured below) with ELDM was less than impressive. This was fired at 100 yards.

After letting the barrel cool down, I went back to my stash of Federal, and fired another 5 round group (Pictured below). This group was also fired at 100 yards. This is the type of grouping I am accustomed to.

I failed to take anymore pictures, but I let my partner shoot 2 groups and he had similar results to mine. However, as the day went on, we continued this and the groupings on Hornady tightened up significantly.

I understand that certain guns prefer certain ammo, but for the life of me I can't figure out how the groupings of the Hornady tightened up so much. Typically I would blame myself, but with the Federal grouping right, and my partner getting the same results, I'm hesitant to blame myself. Is there any science to explain why the group would tighten up throughout the day?

 

[5RWill]

How many rounds are on it?

 

[Llama]

How many rounds are on it?

Probably 7000 in total. The gun was used previously. Close to 500 or so since I did a deep copper strip on the barrel.

 

[5RWill]

I was thinking maybe it was newish and the factory chamber might’ve had a rough spot that would smooth out. I’m not sure though. Some rifles are just picky with loads. I have a friend with a custom 6.5 saum and it won’t group his prime ammo below an inch but shoots dimes with his loads. Much like bullet jump and powder charge can drastically change a group.

 

[Llama]

Is the hornady ammo all the same lot?

It was.

The only theory I can come up with is, maybe with the slightly different projectile shape, the barrel is getting relined with copper a different way, and just naturally tightened up. But I’m not sure if that’s voodoo science or there is real validity to that theory.

I went from approximately a 3 MOA group down to around 1 over a hundred rounds.

 

[whatsupdoc]

Not sure if it was a typo but if that barrel really has 7000 rounds through it I would replace the barrel.
Ammo is too expensive to waste on a questionable barrel.

 

[5RWill]

Definitely getting up there for 308

 

[TimK]

That's a stumper! I'm surprised the Hornady ever shot that poorly. Have you noticed that the barrel opens up when it gets hot shooting the FGGM?

 

[Llama]

That's a stumper! I'm surprised the Hornady ever shot that poorly. Have you noticed that the barrel opens up when it gets hot shooting the FGGM?

I haven't noticed any significant grouping changes with the federal when heating up no.

I'm going to take it out this weekend again and see what kind of results I get. Was just hoping someone could explain the conundrum to me, but sounds like everyone is just as perplexed. I'll keep yall updated.