VLD flight stability

[mac86951]

Ok let me start with a little back-story here. I have a 20" WOA 1:7 (yes I measured it) 223 wylde barrel. Single loading I have plenty of accurate loads using 77gr Conventional, but when I tried 75 A-Max or 75 JLK or even 70gr Berger VLDs a strange phenomenon would occur, the VLDs would barely group inside a 8.5" x 11" target at 100y. My good groups from other bullets are sub 1".

I'll spare you the details of my load testing, but thinking from a bigger picture I don't want to just toss out the "this barrel doesn't like VLDs" until I've exhausted all possibilities.

to narrow down the question, lets look at the JLK VLD 75gr, but the findings have applied to all VLDs as of yet.

Possibly, the jump to the lands was too great? (Chamber COL to lands just touching was 2.450" but for other reasons I'm trying to load to 2.260" (A-maxes I loaded to 2.390"). When I tried a jump of 0.010" I received a similarly large group.

Possibly, the run-out is too great? I have since found lots of concentricity issues and resolved them with a Forster seating die. I've yet to try this solution on the VLDs, but I did load lots of bullets and sort by run-out. Alas, I didn't see a significant improvement, but I need to pursue this one more time.

Last is my question: We specify barrel twist rates, but is that velocity dependent too? I wouldn't think this to be the situation, but I'm sure you all know better than me. Example: A 20" barrel, 2650 is a good medium load for my 77 conventional bullets. I would expect a 75VLD to perform fine in that situation too, but is it possible that a change of ~100fps could return stability? Hypothetically, what happens to twist as velocity changes?

IE would a 1:7 be the right choice for a cartridge shooting 1700fps, or another shooting 3500fps? (again hypothetically)

Finally, has anyone had a barrel that would discriminate from VLDs all together, or is it more likely that I'm the culprit?

Sorry for the long post and thank you for your time.
-Mac

 

[fivering]

My experience with VLD bullets is that they perform best when jammed into the lands a few thousandths.

 

[watercam]

I agree, perhaps jamming would be better in this case. I have a variety of barrels and every one likes something different from VLDs. From jammed .010" to jumped .020". Go figure.

 

[Keith Glasscock]

Just my opinion -

Going from ~ 1MOA to ~8 MOA is not going to be jammed vs jumped. There is far more in play here.

Tell me, what do the bullet holes look like? Are they round and really small or a little oval or larger than the 77's?

What happens when you shoot round-robin with both types of bullets?

To answer your question about increased velocity, the twist rate remains the same, but bullet RPM (and thus stability) raises slightly. It might be enough to improve grouping, but that would be a guess.

 

[Bryan Litz Ballistics]

If the VLD's are seated much further down in the case (base of bullet protruding below the shoulder) then the big groups might be caused by ill effects of the pressure around the unsupported base of the bullet.

To test this, you would try a reduced charge (3-5 grains lower) or a slower burning powder.

I had a Palma rifle that was throated too short for the 185 Hybrids and it shot them into a 3" group at 100 yards. Same rifle shot 155.5's into 1/2". My theory has been that in some cases, having too much bullet below the case neck can be a problem. There are exceptions to this so apparently it's not a general rule.

-Bryan

 

[mac86951]

Thank you all for your replies, and here are my responses:

Bullet holes are nice and round. No key-holing here which is exactly what I would expect given groups this size.

Working out to touching lands (2.450"), I also saw no improvement, but with keeping run-out minimal I'll repeat this with various powders in case velocity has a stability effect, as well as cases in the possibility that the neck is releasing unevenly.

Honestly, I'm impressed I can get such a large group without key-holing but I don't expect this to start a new competition.

Thanks everyone!
-Mac

 

[ronemus]

I shoot the 75 gr A-Max in a .223 from a 1:7 at ~2930 fps to 1000 yards with no issues, and they routinely group under 2/3 MOA. I get the best results jumping 0.030", although they're not very sensitive to jump in the chambers I've used.

I originally used a 1:7.7 twist, but that was marginal at 1000 yards on a cold day; however, it worked well to 600.

 

[Laurie]

If the VLD's are seated much further down in the case (base of bullet protruding below the shoulder) then the big groups might be caused by ill effects of the pressure around the unsupported base of the bullet. [Bryan Litz]

Mic McPherson in his recent tome Metallic Cartridge Handloading refers to this in a section on ideal / non-ideal bullet base positions vis a vis the neck. He says the unsupported base is struck by a series of primer initiated pressure waves before the charge ignites and the bullet moves, these being powerful enough to distort / expand the bullet base producing excessive pressures once the expanded section hits the rifling.

It's an interesting hypothesis, but certainly doesn't apply in every instance, as seen by the number of .260 Rem shooters who happily load 140gn VLDs to short-action magazine length. For a rifle to produce 8.5X11-inch groups at 100 yards, methinks there is something not so happy about that barrel and/or the way the chamber has been cut.

 

[SBS]

I would shoot this load at 300 for curiosity's sake. Also, run them over the chronograph if you've not already done so.

 

[jerrysharrett]

If the VLD's are seated much further down in the case (base of bullet protruding below the shoulder) then the big groups might be caused by ill effects of the pressure around the unsupported base of the bullet.



-Bryan

Bryan, when I was working with Ferris Pindell in the 2002-2003 period on the 40 degree shouldered PPC he kept referring to something called "turbulence point" that the longer necked cartridge offered. I never did get an explanation from him about that term that I could understand. Any ideas??

 

[Laurie]

The 'turbulence point' (TP) concept that been pretty widely discussed on this forum as well as elsewhere. It's a cartridge design issue rather than a ballistics matter.

Take a case drawing and extend the shoulder lines forward with a pencil to see where they cross as it's believed that's where the TP Occurs. If it's ahead of the case-mouth, it is 'bad', that is not confined within the case but produced in the barrel. The TP itself, so the hypothesis runs, is a superheated mass of unburned, partially burned powder kernels and gasses which are hot and very abrasive. It erodes barrel throats fast.

A 'well designed' cartridge has a sharp enough shoulder angle and/or long enough neck to contain the TP well within the case-neck so preserving the barrel steel. 6mm BR, the PPCs, 6XC, 6.5X55 and even more the AI version fall within this 'good' category; anything based on the 308 Win case other than AI versions don't.

A commonly used comparison is the 243 Win and 6mm Remington, the latter a 'good design' because of its relatively long neck. Despite them having very similar case capacities and the Rem version rated at a higher SAAMI allowed MAP (62,366 psi v 60,190) it is claimed that 6mm Rem barrels last longer.

All this is depending on your point of view: gospel ... an interesting but unproven and almost impossible to prove hypothesis .... total moonshine! It is indisputable that many well designed and effective cartridges use sharp (30-deg or greater) shoulder angles and longish necks, ........ but whether TP is an issue in that mix?