I’m just terrified that I’m going to half to use a massive charge to get any kind of a group from it. All I’ll be left with is squirrel pate. ...
No matter what speed you hit a squirrel with a 390/395 ball you're going to do some serious damage. Maybe don't look so hard for problems that likely don't exist or won't occur?
No one knows for sure what that rifle will do but he/she who shoots it. Put 200 shots down range and you'll know a ton better what to expect from it-and if you don't beat the snot out of it while doing such, it should retain most of it's resale value. Send it back now and you shall never know how well it could shoot.
Check the twist! --or not-- it's
irrelevant once you start shooting and SEE what it does.
40-grains is no good, only if 35 or 45 groups better. A missed squirrel cares not how fast the ball was traveling.
Two things you can do: Shoot it or send it back, and I think sending it back without determining the actual twist in that bbl is foolhardy. I know I'd be burning powder because barrel makers tend to avoid using twists that make their barrels perform poorly
--think about that (despite what all the experts might say on the internets).