I bet it's kinda similar to the forend stiffening job I did on my .338 4x4 (also synthetic stock). I bought 3 packages of JB Weld at Wally World along with a bag of hard plastic craft beads. I drilled 1/4 holes through the forend sideways all the way through the ribbing and stock. Then I put masking tape over the holes to keep the JB Weld from draining out. And I roughened up the inside plastic with a die grinder and burr. Then I poured in the hard plastic beads so they filled in all the voids between the ribbing. Then I mixed up a bunch of the JB and slowly poured it over all the beads (I used a bamboo skewer to poke and prod them around to even it out and make sure the epoxy was flowing through the beads, not making air pockets). The beads are like gravel in concrete... filler and adds some structure to the adhesive. Anyway, it has a nice stiff forend now.
But, on to your suspected movement. What took the groups down nice and tight on mine was using JB to 'bed' the magazine holder into the stock. With the barrel/action removed, try wiggling that magazine holder piece side to side in the stock. Check both the rear and front part (near the two big screws that hold the action to the stock). I bet you'll find play there. I bedded the front part first and it didn't take much JB. After that cured (action bolted into stock with Pam cooking spray to ensure it would release from the JB) I could tip the forend down and JB the rear of the magazine holder. In the back, the JB goes in the openings to the sides of the rear action screw hole. You'll see, it's obvious.
The other thing I found with my .338 4x4 was the barrel would badly copper foul within 15 shots. It's slowly smoothing out, but it doesn't smooth out the bore unless there's no copper fouling. That means a lot of tedious copper cleaning after every few shots if your barrel is doing the same.