Thread: "HELP" T3 with shocking grouping ???????

As mentioned above checking that recoil lug is well engaged. The bottom of the gas port on the left side of the action should be level with the top side of the stock.

A mistake often done by many ( included myself in the past)when taking the tikka out of the original plastic stock and putting it back is the over torquing of the action screws. Despite the manual stating quite a high torque ( around 50 or 60 inch pounds on top of my head) the screws are definitely not torqued as high from the factory.
Probably more around 25 inch pounds. When the action screws are over torqued , the shape of the bedding into the stock is modified and the support is not the same anymore and that can affect how well the rifle shoots.
One Remedy is to cut some coke can aluminium foil and place them between the stock bed and action in a way that can be located evenly under the "round" part of the action either side of the action screw location.one layer is usually enough. This helps to rise the action from the previously crushed area and can help restore accuracy .
That solution is only a quick fix for the synthetic stock issues and to test the rifle and determine if a proper bedding is required or not.
I fixed my second hand 270 wsm like that.
A bedded wooden stock or any good aftermarket stock should not have those issues and action screws can be torqued at a higher poundage.

The barrel could be stuffed.

We have a tikka in 223 at work that is not a great shooter with most ammos except the outback 69 grains sierras.

I also once worked on a tikka 223 barrel that had its chamber with 0.15 mm of run out to the axis of the bore. I cut the chamber out and rethreaded and rechambered the barrel in fresh rifling with less than 0.01 of runout of the chamber. That did help it to shoot better.