That's a nice looking spring compressor you built. Can you post photos of the whole thing? Looks like 3 what we call 2x4s, a couple of threaded rods, nuts and washers, and a couple of ~26-27mm "holes." Is that about right?
Could be just the piston seal affected by temperature. Oring style seals like the Vortek offerings are least affected.
Have you done a tissue test for the breech seal?
I just realized something; should I be using a dampa mount at these kind of power levels or will my Hawke Vantage stand up to this violence?
As I remember, the front screws bracket does float.If your POI changed all of a sudden I would suspect the scope or mount.
I would consider the scope still. Those Hawkes were built for 12 ft/lb and under. Mine still shifted on a rifle doing 12 ft/lbs, it now resides on a .22 lr and the replacement for that Airgun is doing just fine.
I'll have to look it up. There was a thread awhile back about that. People were having issues, Hawke was offering to replace with an upgrade. Usually a 30 mm tube. One airgunner shared an email response that flat out said they recommend their 30mm tube models for anything over 12 ft/lb. Some posting in the thread said they had scope issues, others didn't. I believe both. In your case, up to you what you do, but it wouldn't be a bad idea to put a proven scope on that rifle to rule out the scope.
https://www.gatewaytoairguns.org/GTA/index.php?topic=120000.msg1161236#msg1161236Not the thread I was referring to but there's a reference from "buldawg" partway down. I'll keep looking. Don't think Hawke would mind you contacting them.