6.5x55 Swedish - what's going on with same loads 2 brass types?

[Bergara HMR]

Guys I think I'm posting this in the correct thread, move it if not. I'm new here. OK my problem:
My rifle - Steyr Pro-hunter 6.5x55 Swedish. 22" barrel, moderated. This has always been a very accurate rifle. 1/2 MOA when I'm focused.
My issue - The last few days at the range I've noticed something weird that I fail to understand.
I home load. I loaded up 40 round, everything was identical except 20 had Federal brass & 20 had Lapua brass. At the range (200mtrs) I noticed the Federal was grouping well over 1 MOA. The Lapua 1/2 MOA. So I repeated the test. Same powder, load, primers, bullets, only the brass was different. Went to the 100mtr range. Same result.
Brass history - Federal brass shot about 10 times neck sizing only. The Lapua brass shot about 3 times neck sizing only. Suggestions were made that the Lapua my have less internal space raising the pressure. That said, this rifle dislikes hot loads. So time to test:

Capacity test - I took my 20 rounds and weighted each round, 10 Feds (average) then 10 Lapua (average)
Then filled each with water, did the average for each brass set, subtracted the brass weight to get the internal capacity. (sorry I don't has the actual findings at hand), however, the Lapua had a bit more capacity than the Federal.
So I'm no further forwards.

Note - I've shot Federal in this gun for years with great results.
Now I'm thinking about full case sizing to start again with the brass back to factory. I did check OAL & that's fine. I have downloaded the SAMMI stats but have not yet checked the shoulder with my comparitor, but they all chamber fine.
Anyone have an idea of what is wrong??
I'm so sorry for the length of this post, but I couldn't make it shorter.
If you read this far, thank you.
Kind regards

 

[Bugholes]

Measure the case head diameter. There are variations in the 6.5x55 Swede. Check and see if your brass goes into, and falls out of your bolt face , with ejector removed
some mfgs make it a 308 sized head, but it’s actually a little larger. Some bolt faces/heads have the additional clearance for this discrepancy and some do not

 

[Evlshnngns]

It not often a 1/2moa load will work between different brands of brass.

I ran lapua 6.5x55 vs ppu over the chrono with the same load, It was something like an 80fps difference. Full length sizing and annealing could make a difference.

 

[hlee]

You need a chronograph. Load 10 (minimum) of each and shoot across the chrono…

 

[Rider47]

My guess - Neck tension.
Different and inconsistent neck tension due to 1) differing neck brass thickness between the to brands (Lapua is thicker), and 2) poorer quality (consistency) of FC cases.

 

[Bergara HMR]

Thank you guys for your replies. I don't have a way to systematically anneal my brass, but I will full length size & chronograph them. Never thought about chronograph them for comparison.