Here is a copy and paste.....................From elsewhere
This next comment is not directed toward your rifle or tests in any way, just a general comment for other members. Sometimes, especially in factory rifles chambered in large magnums, if you seat the bullets to contact the lands, you will get some group stringing. In my testing, this has been a result of bolt preload.
Bolt preload, especially in an out of box factory rifle can cause noticeable and measureable stringing in groups. This is generally a result of inconsistent bolt lug contact to the receiver bolt lugs support or simply an unsquare situation as a result of machining in the total receiver. GENERALLY, uneven bolt lug contact will result in vertical stringing. Unsquare condition in the receiver can result instringing in pretty much any direction.
Anyway, back to what can cause bolt preload. There are three major things that cause this.
1. Bullet contacting the lands of the rifling which puts pressure against the bolt head when the bolt is closed causing bolt preload.
2. Cases that are tight in the chamber, when the bolt is closed on a case that contacts the should or the chamber solidly, again, there is pressure on the bolt head when the bolt is closed.
3. Foreign matter between the bolt nose and the barrel breach can also cause bolt preload and serious accuracy issues.
Again, these three issues are seen mostly with factory rifles. WITH custom receivers and rifles, generally things are much more finely machined so bolt load does not cause as much of an issue.
Now why does bolt preload cause stringing, well, simply put, it will create inconsistent vibration patterns in the bolt, receiver and often barrel, the larger the diameter of the case head, the more dramatic it can be as seen on target.
This is why I have always said to build a rifle in a 223 that shoots well under 1/2 moa is EASY, to do the same thing in a 338 Lapua is MUCH more challenging. Not hard, anyone can do it, just more attention to detail is needed whereas with the small diameter 223, you can get away with ALOT more because the bolt thrust generated by the very small case head is extremely small and often times, even a bolt with only one bolt lug supporting the case will still shoot very well because the 223 does not have enough bolt thrust to compress the bolt to the point where it flexes enough that the floating lug I forced to contact its support surface.
In a 300 RUM however, or example, again, not referring to your rifle, just in general, if there is a floating bolt lug, even moderate pressure loads will produce enough bolt thrust to flex the bolt head until it contacts the bolt lug supports. This will result in inconsistent vibration patterns in the rifle system and as a result, generally stringing on target.
