Checked your bolt fit? Fat o-rings can help get the gas to the front, but the front needs to have a decent fit in the breech as well. How far the bolt goes into the breech with the closeness of fit to the bore will affect the blow by.
Put two balls in the open feed neck, shoot one and see how high the second one goes up. Then cock the marker and just put one in the feed neck and dry fire. If the dry fire is consistently lower in blow back, you're getting significant leakage from the front. Tighter barrel fit on the paint makes that worse. Otherwise you're probably getting leakage from the bolt o-rings.
If you are using air, I wouldn't worry too much about sweet-spotting. For that same matter, you might make sure your reg doesn't have a built up ring of wear or old lube in the one area inside that you are trying to run it in. That's some annoying stuff there.
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