Michael,
Check to see the condition of the inside chamber surface. If it's severely pitted or frosted from the blood splatter, it might be adding enough friction to the extraction process to put off the timing.
Use some masking tape on the rear of the receiver and look for impressions of the rear toggle hitting it. If there are no marks whatsoever there is not enough recoil force driving things back. If there is too much force (the paper is being hit hard, heavily marked or cut into) the toggle train may be bouncing back very quickly.
Double check how you are supporting the Luger during cycling. You need a firm, solid stable hand hold to oppose the frame recoil force.
Try a 21 or 22 coil recoil spring. This is what was used during later Mauser production and may apply to your HK.
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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