@ 08:42 am (GMT)
Nathan FosterHi David, just a couple of important points to go over first.
Never shoot 10 shot groups with this rifle, that is to say, do not let this barrel get hot. If you want to put 10 shots on a target, try to keep the rifle at ambient temps all the way through (up to an hour or more). If you want to test to see how the rifle holds its zero under pressure (for follow up shots on game), perform this session once, after developing your optimum load.
Do not use scotchbrite on this particular chamber. This form of lapping should only be used to clean up very rough chambers (generally other rifle brands) during the break in. Following this, you should avoid any harsh lapping until the throat is showing wear (although with 10 shot groups, there will be significant fire cracking at around 200 rounds). But generally, it is best to avoid doing much at all with the Tikka unless you have to (e.g fitting bullshit attachments to the rifle - bipod, muffler, bubble level, Fitbit, phone holder, cup holder, a framed photo of you and your mum with identical tatoos and tactical beards at Disney Land).
There are some rough Sako and Tikka rifle chambers, but these are very rare for modern rifles. You will know if you have one of these because the rifle will produce both heavy copper fouling and groups of around 3 to 6" at 100 yards. But as a reminder, any form of lapping should only be utilized after all copper has been removed. never use lapping materials or compounds to remove copper. The bore needs to be stripped to bare metal before one considers lapping.
One of the main problems with this rifle is heat. As the barrel heats up, it swells. As the barrel swells, dimensions are reduced. As this happens, we will see more copper fouling. Along with this, if the barrel is either fluted or threaded, the barrel will eventually swell in these areas, eventually losing accuracy.
The SST bullet is not a wide bullet. It comes in at .284", around .0005" less than some of the newer bullets. The jacket alloy is thick but soft, helping to ensure reliable expansion. Fouling will be most evident with the SF loads as these are much faster (friction) than other factory loads.
The LRX alloy is harder, therefore it produces less fouling. But this should also be an indicator of its downrange performance. If I collected some stones from our creek, packed them into a box and labelled them LRS, you would not be fooled into thinking that these are suitable for long range. The same applies here.
The Tikka chamber design often works well in conjunction with the Hornady ammo, both the ogive shape and the COAL. Its a fluke. In some instances, the 154gr will shoot a bit better than the 162gr. But quite often, both will shoot well. Hand loads can take accuracy a step further. But it does take some discipline on the part of the shooter to make the Tikka Magnums perform.
All I would suggest is that you obtain a harsher solvent (e.g Sweets or Wipe-out etc), then take more time with your cleaning. Let the solvent soak in, come back every half hour or perhaps once an hour and scrub it with a bronze brush, then leave it alone again. If it takes a half day to clean, so be it.
If however, the rifle shoots terribly with a clean bore, then you may wish to experiment with only partial copper removal (swabs, no bronze brushes). Common sense will guide you in this matter, you already know what heavy copper fouling looks like compared to mild fouling.
So basically, all I am suggesting is that you leave the rifle alone at various stages. Leave solvents to soak. leave the rifle to cool more between groups.
I think you will be OK. I doubt any harm has occured - especially if the rifle is still producing some copper fouling. In any case, your question will most likely help others in the same boat.