Too much jump?

[Batsin]

Size only 1/2 of neck with bushing. Unsized part fully expands to chanber, soon or later.

Believe it or not, many modern cartridges, especially the Creedmoors, prefer the opposite. They tend to be happier with a slightly looser case fit since the chamber use the shank of bullet itself for centering*. The standard SAAMI 6.5 and 6mm Creedmoors have almost .2" of very tight (half a thou clearance) freebore** and typically, the shank of the bullet is inside this section when the cartridge is chambered. The OP mentioned that this chamber was throated deeper than normal and the photo in post 21 shows over .25" of the shank extending from the case. The tight section of freebore should start somewhere around 1.94" from the base. In other words, the shank of even the shortest bullet is positioned inside the freebore.

* For comparison, many cartridges, including the SAAMI 30-06, 270, 280AI, 300 WM and 300 WSM, only have a forcing cone into the rifling and no section of snug freebore for bullet alignment. These cartridges greatly benefit from the "1/2 neck sizing" technique.

** The SAAMI 308, for example, has less than half this length of freebore with 4 times the slop.

 

[243winxb]

the photo in post 21 shows over .25" of the shank extending from the case

Your "shank" seems to be different then mine?




https://bulletin.accurateshooter.co...-boat-tails-and-other-bullet-design-elements/

 

[Batsin]

Possibly so. I use the word "shank" the way that Brian LItz does in the linked article. In the article, Litz defines the shank as "the full caliber straight section". With the bullets measured in this thread, the ogives terminate at the full or "caliber diameter" of 0.257", which also happens to be the shank diameter. I agree that terms can be different with bore-riding bullets where only the driving bands are the full "caliber diameter" and the shank would typically be the major, bore or land diameter (terms that are generally used interchangeably).

Nevertheless, with all of the loadings shown in post 21, there will be a section of "the full caliber straight section" inside the freebore of the chamber unless there is something very odd about this particular "25 Creedmoor" freebore/throat. It appears that the Original Poster's chamber has an unusually long freebore. I wonder if the gunsmith misunderstood the request to make the chamber for "really long, high-BC bullets". Some high-BC bullets are long because they utilize unusually long ogives and relatively shorter shanks. With these bullets, you actually need a shorter freebore. The freebore on the reamer drawings for the 131 Blackjack bullet, which is said to work well with the 133 and 135 Bergers, for example range from .115" to .131". As can be seen from the JGS reamer drawing, any section of bullet extending beyond 1.9413" will either be in the .2575" dia freebore, throat, or rifled bore

Article:
Tangent, Secant, Hybrid Ogive Bullets:
Bryan Litz Explains Characteristics of Different Bullet Ogive Designs

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[1mechanic]

If the throat is minimal over the bullet diameter it will help with accuracy and it will not be critical of the jump but the chamber is the key if the throat is loose it won't like a long jump.

 

[Black Hat]

I shoot Hornady XTP 32 Caliber 100 Grain Jacketed Hollow in my .303 just for fun, didn't bother measuring the jump, though they shoot relatively well considering.

Bear in mind, my Lithgow is throated for the 180 grain round nose.

 

[Stammster2]

My concern would be around bullet alignment and blow by around the bullet. It can't be good for the base of the bullet to exit the case mouth before the bullet starts engraving.

Not sure how that might affect accuracy and velocity, but I suspect it would.

 

[Lou270]

I have seen some terrible accuracy with shooting vld type bullets in rifles with long throats where could not seat near lands. For, ex I tried 165 berger vld and 200 eldx in a 300 with long throat. 7-8 in groups with berger
And 3-4 with eldx. I thought my scope crapped the bed. However shoots sub moa with 180 ABs, Hammer and other bullets. I ran into similar issues with bergers in other rifles with magazine restrictions when I first started shooting them. So, think it was the bullet type which is more sensitive. The very thin jacketed bullets with sleek ogives seem to be the most sensitive to a lot (>> .1) jump. Accubonds and ballistic tips and standard soft points like hornady interlokt seem very jump insesitive in my experience.

Lou

 

[snox801]

Yep!
View attachment 544116

With 145 Black Hole, I can touch the lands on my .257 WBY (SAAMI). The current load is .015" off the lands.

That is one long bullet if you can touch the lands in a .257 bee.

Jealous you have enough twist.

 

[Batsin]

I have seen some terrible accuracy with shooting vld type bullets in rifles with long throats where could not seat near lands.

That is not uncommon. Some rifles do not like the sharp transition from the ogive to the shank of secant-ogived VLD bullets. With a tangent ogive, there is a gentle curve away from the shank instead of a sharp break. It is also difficult to make the sharp break perfect and consistent from bullet to bullet. Most rifles seem to prefer the gentle transition, which is why Berger developed their "Hybrid Ogive" design. The reason that secant and hybrid ogives are popular is that it is easier to get a higher BC with them.
Some companies still make both secant and tangent ogive bullets. With Sierra bullets, it is often advised to use the tangent ogives to determine the accuracy potential of a rifle if there is some doubt if it will ever shoot well. Sometimes people get terrible accuracy with the secant bullets and think they have a defective gun.

 

[Black Hat]

That is not uncommon. Some rifles do not like the sharp transition from the ogive to the shank of secant-ogived VLD bullets. With a tangent ogive, there is a gentle curve away from the shank instead of a sharp break. It is also difficult to make the sharp break perfect and consistent from bullet to bullet. Most rifles seem to prefer the gentle transition, which is why Berger developed their "Hybrid Ogive" design. The reason that secant and hybrid ogives are popular is that it is easier to get a higher BC with them.
Some companies still make both secant and tangent ogive bullets. With Sierra bullets, it is often advised to use the tangent ogives to determine the accuracy potential of a rifle if there is some doubt if it will ever shoot well. Sometimes people get terrible accuracy with the secant bullets and think they have a defective gun.

Your reasoning makes sense, my .243WIN likes a .060" jump with 95VLD's, I tried jam out to 0.080", one hole groups at 0.060".
This rifle wouldn't shoot match kings either, so probably something with the chamber.