It's soft steel and it's drawing against polymer, don't overtighten it... finger tight is plenty. If the screw is etching or deforming the forearm, it's too tight.
But that's unlikely to be the problem.
I'm assuming you have a scope on it..
Most likely culprit (especially if said scope was factory mounted) is the base(s) aren't tight.
Next, I'd suspect the rings..
If everything is tight, I'd question the age of your powder. Triple7 is way more hygroscopic than black powder, and I've seen plenty of groups widen (and fizzles) from aged pyrodex and 777.
It's also possible something worked loose in the scope (especially if it's the Konus usually included in their package deals... usually they're good to go, but they do have a higher failure rate than most of the major brands)