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Same as my experience. 400s showed pressure signs sometimes well below book loads.
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It's very common for light loads to show false pressure signs such as flattened primers.
It's because the casing is being held forward first by the firing pin then by the expanded casing holding onto the chamber wall.
This allows the primer to back out of the primer pocket and expand. Then often the pressure gets high enough to break the hold and the casing slams back against the bolt face with the backed out and expanded primer trapped in between.
So what you end up with is a primer that "looks" over flattened when in fact it was caused by a under pressure load.
I did a side by side load work up using CCI-400s and CCI-450s using identical loads and bullets and all LC-11 brass.
I saw no significant difference between the primers during the testing and after the testing was done and the casings mixed together there was absolutely no way of telling which were which.
I believe most of the difference people claim to see is specific firearm related and not component related.
I used 1500 Remington 6 1/2s in my VTR-15 without a single issue (before I found the warning) which I was alerted to by someone on this forum after posting about a different rifle having piercing them.
Same load different rifle different results.
Motor