I own a Kimber 8400 Select Grade in .338 Win Mag in French Walnut. It shoots Nosler 180 grain AccuBonds at 1 MOA but when I shoot 200, 225 and 250 my spread increases to 2.5 inches at 100 yards. At 1 inch my long range is limited to say 600 yard 6 inch kill zones. Kimber says the 8400 is best shot in the 200 grain and 210 grain heads and suggests Barnes Triple X which I have not shot. I shoot Nosler's in most all of my rifles because they use gilding metal and not soft copper.
I hate cleaning copper out of Rifle bores so I stick with gilding copper. A break in of the rifle was performed with regular barrel cleaning but I never followed a shoot 5 and clean process as some suggest for 50 rounds. I don't know many folks that do that and hey perhaps they should.
Today it is known by serious shooters and rifle gunsmiths that thinner sporter barrels that are free floated are more prone to resonance, torquing and the like. Free floating is best on a heavier barrel, again experts say. I am not an expert, let me say that. I do experiment however after significant study and am good with tools and the like.
I have placed a Limbsaver sporter barrel deresonator (donut) on the 8400 and it shoots the 180 grain Noslers at sub MOA at almost 3/4 inch groups. But as you well know it is an eye sore on such a fine rifle. The ultimate fix many experts suggest is to use upward stock pressure, say 8 to 10 pounds on the front tip of the stock. I found an article by Glass Bedding a Sporter Rifle
that instructs on Glass Bedding and making a separate 2.5 inch contact surface at the tip. He uses Devcon 10110 Putty to form the contact surface and utilizes Kiwi natural shoe polish as a mold release agent for the barrel and surrounding wood surfaces as it is cheap and does the release trick nicely. I called Larry recently and he suggested using business cards at the front tip until there was enough pressure to tighten groups. At that point you can use the putty forward of the business cards. Alternatively his article suggests just adding a 8 to 10 lb weight to the sling swivel while the barrel holds the weight of the rifle during the curing process.
I have ordered the putty and kiwi shoe polish from Amazon.com and it seems that Amazon knows that trick as they suggest the Kiwi shoe polish to add to the Devcon sale. I bought in at $58 dollars delivered and am waiting for the package of putty and polish to arrive. Larry's experience with this process of forward pressure on sporter barrels has taken a so so shooter rifle and now consistently shoots 1/2 inch to 3/4 inch groups and it looks just like the factory made it that way. This process and knowledge is not new, it has been around for many many years, just seek and ye shall find. Craig Boddington wrote about adding business card shims in his book. Make it accurate.
I have not gone to the range with the business cards yet but will add threads as I go through the process. Note: Kimber gave me a torque value of 60 inch-pounds for the Allen head lug screws for my model but a Kimber tech offered that since it is aluminum pillar bedded, the screw will bottom out on the pillar and does not necessarily require torque just a tight Allen wrench will do. I have two torque wrenches. I use a Wheeler fat wrench that goes to 60 inch pounds (you can buy it from many hunting suppliers, a great tool as I torque my scope screws to a value as well).
Inspection of the Kimber bedding I found to look very solid and very professional. The tolerances for inserting the barrel onto the stock are very tight. Talk again soon...