Sounds like the barrel will be reinstalled so a tension relief cut is out of the question.
The 16 is a small ring so start with the bolt installed and don't over-tighten your action wrench. Work the bolt as you tighten wrench. If you feel the lugs start to bind, back off just a hair. Use a cheater on wrench but leave about 8" of handle showing close to head. Preload tension on the cheater and smack the exposed handle with a 2 or 3# hammer about 6" from the wrench head. Once will usually pop em loose but sometimes 2-4 whacks are necessary.
If that doesn't pop it loose be patient. Stand it on muzzle and saturate circumference of barrel stub inside reciever with a penetrant such as Liquid Wrench, Rust Buster, or 50/50 mix of ATF and laquer thinner overnight. And give it another go.
Out of curiousity is your 16 still a 7x57 or is it a 7.62 CETME?
JM2c