comments:
Quick update: Having been impatient while waiting for shipping from a popular hardware vendor, I ordered this from amazon: https://www.amazon.com/dp/B0B6J1JPF8 for use/testing as a recoil spring. It was cut down to 44 coils and the bolt weighs in 65 grams, give or take half a gram. (Taken down from 125g original weight) Build deets: mp5K, ftn.5 rimfire flow (with venturi tube), and Remington subsonic 40gr .22lr ammo. The action cycles well now, but feeding from the mag is still inconsistent at best. Sometimes it's the follower having shifted in the mag itself, and there's also a bit of forward/back play in the mag when it's locked in place -pulling the bottom of the mag rearward seems to help put the rounds at a better angle to load without issue.