Originally Posted by Boss Hoss
If your shooting is 300 yds or less then boattails are a waste of time.
Agreed, though not so much a waste, but not a necessity either, especially at these velocities.
Look at the BR guys NOBODY uses boattails in the 100 or 200yd game...
Good point, but then NOBODY shoots a 6.5-300 Wby *that* close . . . ;-)
BT have to fly approximately 300 yards before they become completely stable in flight
I'd respectfully disagree with this generalized statement and would say: "NOT if everything is correct."
I have a fast twist .243 shooting 105 Scenars, a .260 and a 6.5-284 shooting 140 Bergers and a 6.5-300 Wby shooting 123 Scenars and ALL of them punch bugholes at 100. If they didn't, they'd go back for correction.
If your bullet is not stabilized at closer distances, and needs to "go to sleep" (sic), the throat geometry is clearly allowing in-bore yaw to occur. In-bore yaw produces a bullet that is not rotating around it's center of gravity at the muzzle and at closer ranges. No more mystical than that.
The combination of an elongated ogive in conjunction with a boat tail produces an extremely short bearing surface. This is exaggerated in VLD-style bullets. This short bearing surface requires a close fitting leade or ball seat area of the chamber to produce a straight run to the rifling. If the diameter of the ball seat area is too much greater than the bullet diameter, the bullet's short bearing surface is not long enough to keep itself straight. This problem allows the bullet to slump slightly off center as it accelerates forward and it is then in a slight yaw attitude when it engages the rifling. This yaw continues to the muzzle.
When the yawed bullet emerges, it rotates around it's center of form, while it's center of gravity is eccentric to this rotation - yuk. After a certain time of flight, the bullet is forced by it's rotational forces to begin to rotate more so around it's center of gravity and it stabilizes somewhat, but, it will NEVER be as accurate as a bullet that emerged correctly to begin with.
Groups at distance will typically not be bigger at the same rate as it would as the result of linear angular dispersion. Due to this mechanism, your 300 yard group may be about the same size as a 200 yard group, but it will never be smaller.
As BuffaloBob points out, this problem can definitely be a product of freebore, which accentuates any in-bore yaw by allowing it to continue longer and to a greater degree. The 6.5-300Wby is originally a 1,000 yard BR wildcat cartridge based on the necked .300 Wby and does not typically use the same "Weatherby Freebore" illusion as the legitimate members of the family. If you did not specify exaggerated free bore with your build, it should not be present in this long range precision BR cartridge chamber and on the reamers commonly found for it.
The very first thing I would do is suggest that expectations be aligned with the **hunting** bullets you are trying to get to perform. What level of accuracy is expected? What are you getting? 1/4 moa may be unreasonable, while 1/2 to 3/4 moa may be more in line with the game bullet's limitations.
Ever try "good bullets"? I never had amazing accuracy with GKs in my own 6.5-300Wby, and it may due to forces vs bullet construction. Nosler 120 BTs shot great in my rifle, perhaps due to the hard base resisting setback deformation better.
If best accuracy with a 140/142 SMK or 139 Scenar exceeds 3/4 moa in a custom rifle, this in itself may be a problem and something is likely broke.
I would suggest checking and/or touching up the muzzle crown, which is considerably more critical to accuracy with boattails than with flat base bullets. In fact, we're really not sure at this point that it will shoot FB bullets any better anyway, right?
How many rounds are through it? Throat erosion will enlarge the ball seat area and it might benefit by a slight setback. My own 6.5-300Wby had to go back at 750 rounds, and we had to take almost 2" off to get it to indicate with enough runout to provide proper reamer piloting for a decent chamber again.
I would next try seating the boattails longer, as close to the rifling as possible. This will negate as much slump as possible while not changing bullets.
The problem will be more apparent with a bullet like a Scenar, which will be a little "skinny" at .2635" or so, and a "fatter" bullet can sometimes help, but 140 SGKs are typically a little fat already and it should shoot them.
You might also consider trying moly-coated bullets, which generally show better results when there is a geometry problem. This is because the coated bullet enters the rifling with less resistance and less potential deformation. It straightens itself out better, stabilizes sooner and leaves the muzzle with less difference between it's center of form and center of gravity. While the resulting effect is sometimes attributed to the moly being "slipperier" in the air, while it is actually because the coated bullet stabilizes faster, thus achieving it's published BC sooner, thus giving less drop/drift.
Another obscure problem inherent to these rascals is the hard carbon ring, which will both increase pressure prematurely and will also reduce throat diameter enough in some cases to deform the bullet. The slow powders necessary for this guy WILL leave some serious crap behind, a diamond-hard combination of carbon and calcium-based deterrent coating residue.
Considering the reasoning process behind getting a 6.5-300Wby, it **should** shoot boattails and I personally would be very unhappy to be forced to shoot flat-base bullets in it. What's the point? If you try these things and see no improvement, it may be time to take it back to the builder for correction.