Wanting lower pressure but maintian velocity

A few minor tweaks you might try:

Norma brass is usually on the soft side. If you're willing to try different brass, try Winchester; you'll also gain a little bit more powder space too, new manufacture Winchester are 6-7 grains lighter than most Norma. Lapua is harder, but it's about 10 grains heavier than Norma, so you'll be losing powder capacity, which is probably not what you want.

Are you shooting Sierra 140s or 142s? (the 140 Sierra is not a premium long-range bullet). Lapua 139s are larger in diameter than Sierra 142s, plus I think they might have a thicker jacket too. I seem to recall that Lapua 139s needed to be backed off by half a grain or a full grain, relative to Sierra 142s. Both Lapua 139s and Sierra 142s seemed to shoot equally well for me.

With my 6.5-08 (disclaimer: I never was able to get it to work as a competitive F/Open rifle!), I found H-1000 just slightly too slow. I mostly shot a way-too-hot load of Varget, or a load using RP-5 powder (still have a bunch!), which was about the right speed. IIRC I was in the low 2900s (30" MacLennan, 139 Lapua and 142 Sierra).
 
With Win .243 brass in my chamber all the prep work done, full length sized and neck turned, the brass will fit into the chamber, as soon as bullet is seated into the case the bolt will only close on about 50 percent of the loads. I suspect that the neck has a donut formed at the shoulder junction, switched to Norma 308 cases, also tried Win 308 cases they function fine. This rifle actually is a 6.5/08 chamber as well as my dies. I just started the post as a 260 Rem just to keep it simple.
 
I using 139 Lapua's they shoot great, less than 1/2 moa at 600. Picked up H4831sc to try, I might just rechamber to 6.5/08 AI this will ensure that I have enough room for powder and resolve the pressure issue, Still will have a bit better barrel life than the 6.5x284, but not much.
 
See if you can get the throat long enough so that the bullets of your choice (presumably one or more of Lapua 139, Sierra 142, Berger 140 VLD) are seated far enough forward that the base of the bullet is ahead of the neck/shoulder junction. That way you get the most capacity you can get out of your case, and you also avoid any "doughnut" problems.

Agreed that getting slightly more powder capacity would be worthwhile (in that it would open up the option to use H1000 Rx22 H4831SC).
 
Hmmmmm... if you are getting pressure signs because of a "bullet release" problem, that's most troublesome.
The donut can be reamed. If you are close and have a Forster trimmer, I'll loan you my neck reamer. Use it on a fired case and it should not touch the neck, proper, it'll just cut out the constriction.
 
Have you tried using different primers with the powder you're already using? Switching primers can make a difference of several KSI.
 
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