Can you post a pic?
Barra updated the hammer adjustment lug and that seems to be the way all the new guns are shipping. My first one was delivered on 5/7/25 which has the original hammer design you see in all the vids on YT. The second one delivered 8/6 and I posted a couple shots of the new version here.
I'm not going to touch the valve spring, at least not any time soon. I'm hopeful the plenum will be the answer. I'm not an expert on fluid dynamics and have just a basic grasp of physics. The extra volume of air at pressure might be what closes the valve faster due to the lowered momentary drop in pressure when the valve opens.
I'm not shooting out in the middle of nowhere, there are houses all around me. So I need to get the report down a few decibels. Removing the slug kit has helped some and 1.5 turns in has the fps back in the 960's.
Also, Barra changed the design of the hammer...
I'm at 2500psi running in the 930~940 range with a 30 grain slug, but accuracy isn't there yet. The adjustments aren't the same as the original version and I think it's because the lug is further in on the new design but they used the same spring and the lenght is the same. This is putting more tension on the spring when the lug is flush. I've backed it out so far that the lug is protruding almost the it's full length and still can get it to stop wasting air. Another problem with it is, when the lug is that far out the back, the action can't be removed from the stock without tightening it back up close to flush. It's gotten so frustrating I'm hardly shooting it right now. I started trying to tune it at 2000psi but the valve will not shut fast enough. So I went up on the pressure to see if that would to it. No love. I've even thought porting this one too but I'd like to get it running right first so I have a good comparison between the two.
I'm planning on trying some different springs to see if it makes a difference. If a shorter spring does work then this is on Barra in my mind. I do plan on reaching out to them soon, maybe even today.
If I can't get it working I'll probably step up the slug weight, something like 34 grain, and see how they run. It's either that or return it.