Conflicting information

[46490]

I have used the Berger twist calculator and in my 6BR with a 1-7.5 twist all of the Berger bullets are stable from 2800 to 3000 fps. Do I need to plug in the exact FPS or is a guess accurate enough (using load data)? As of right now I plan on using 104 and 115 on top of H4350. Any comments or real world experience is appreciated. Also Lapua cases and Fed 205M primers (27.5" barrel).

 

[sandwarrior]

I have used the Berger twist calculator and in my 6BR with a 1-7.5 twist all of the Berger bullets are stable from 2800 to 3000 fps. Do I need to plug in the exact FPS or is a guess accurate enough (using load data)? As of right now I plan on using 104 and 115 on top of H4350. Any comments or real world experience is appreciated. Also Lapua cases and Fed 205M primers (27.5" barrel).

Guesstimates from the book ususally aren't even close. Your chamber is probably different enough it will make a differendee. You should chrono your loads, then you'll have accurate info to use.

 

[INTJ]

I have used the Berger twist calculator and in my 6BR with a 1-7.5 twist all of the Berger bullets are stable from 2800 to 3000 fps. Do I need to plug in the exact FPS or is a guess accurate enough (using load data)? As of right now I plan on using 104 and 115 on top of H4350. Any comments or real world experience is appreciated. Also Lapua cases and Fed 205M primers (27.5" barrel).

115 grainers, H4350, and the 6 BRA don't normally go together. The Dasher is about the smallest capacity case that might work with 115s.

H4350 is usually too slow a powder for the 6BR. Not sure you can get enough in the case to get a reasonable velocity even with a 103-105 grain bullet.

The usual powders for the 6BR are H4895, IMR 8208 XBR, Varget, RL-15. RL-15.5 ought to work too.

 

[RegionRat]

Do I need to plug in the exact FPS or is a guess accurate enough (using load data)? As of right now I plan on using 104 and 115 on top of H4350.

You can guess for the purposes of planning builds and selecting bullets. Once you buy the barrel and select the chambering, things are real. Like @INTJ is pointing out, trying to push heavy for caliber bullets is much like trying to push very light ones. It can be done, but you must accommodate the selection and expect to give up a little on the other opposite extreme.

The SG of a 115 at 2800 FPS is 1.49 at Sea Level. That puts it just at the transition between what they call Comfortable Stability to Marginal Stability, so you are okay and odds are you will rarely be worse unless they hold a match in DVNP. The SG only gets better with altitude.

Remember, the stability calculator is more of a rule of thumb based on lab observations being turned into what we call a parametric model, not some sort of direct calculation of forces that predict bullet motion. It has been around a long time and has been tested pretty well.

If you push the SG value down from 1.5 towards 1.0, there is a lot of ground covered in terms of twist and speed for a given bullet length (weight). You can observe examples of lower values remaining stable, and higher ones tumbling as you go down in SG. There isn't a guarantee of success or failure in that grey region, just a guideline to judge it against. The closer you get to SG 1.0, the sooner you can expect trouble.

It is not always convenient to swap barrels to use a longer bullet on one hand, and there are limits to how fast you can twist a bullet at high velocities before is wants to spin apart on the other hand. There is enough ground in between to err on the fast twist side to be able to shoot the longest (heaviest) bullets, while still being able to run the lighter ones in most calibers, but selecting a twist is always agony. YMMV

 

[46490]

You can guess for the purposes of planning builds and selecting bullets. Once you buy the barrel and select the chambering, things are real. Like @INTJ is pointing out, trying to push heavy for caliber bullets is much like trying to push very light ones. It can be done, but you must accommodate the selection and expect to give up a little on the other opposite extreme.

The SG of a 115 at 2800 FPS is 1.49 at Sea Level. That puts it just at the transition between what they call Comfortable Stability to Marginal Stability, so you are okay and odds are you will rarely be worse unless they hold a match in DVNP. The SG only gets better with altitude.

Remember, the stability calculator is more of a rule of thumb based on lab observations being turned into what we call a parametric model, not some sort of direct calculation of forces that predict bullet motion. It has been around a long time and has been tested pretty well.

If you push the SG value down from 1.5 towards 1.0, there is a lot of ground covered in terms of twist and speed for a given bullet length (weight). You can observe examples of lower values remaining stable, and higher ones tumbling as you go down in SG. There isn't a guarantee of success or failure in that grey region, just a guideline to judge it against. The closer you get to SG 1.0, the sooner you can expect trouble.

It is not always convenient to swap barrels to use a longer bullet on one hand, and there are limits to how fast you can twist a bullet at high velocities before is wants to spin apart on the other hand. There is enough ground in between to err on the fast twist side to be able to shoot the longest (heaviest) bullets, while still being able to run the lighter ones in most calibers, but selecting a twist is always agony. YMMV

Thanks for and much appreciated information. I am @ 4300 feet - so I am expecting better results across the board since I am not at sea level.

 

[Lance21]

115 grainers, H4350, and the 6 BRA don't normally go together. The Dasher is about the smallest capacity case that might work with 115s.

H4350 is usually too slow a powder for the 6BR. Not sure you can get enough in the case to get a reasonable velocity even with a 103-105 grain bullet.

The usual powders for the 6BR are H4895, IMR 8208 XBR, Varget, RL-15. RL-15.5 ought to work too.

While not 115 grains, my bra runs 107s, h4350 and Alpha brass at an easy 2855. Prs rifle, highest velocities not needed there.

 

[RegionRat]

Thanks for and much appreciated information. I am @ 4300 feet - so I am expecting better results across the board since I am not at sea level.

It is free to play with that web tool, so I encourage folks who have not had to formally study ballistics to play with it until they get a feel for what is going on.

Go ahead and play with the altitude and see what it does to the SG value. Then, play with the twist rates and velocity too. You will get the hang of how strong each parameter is on the SG change.

A different view, is to learn how much spin a bullet can take before it breaks apart. Once you gather that research, you can start to play with the SG values due to different twists and see which bullets you can use.

It isn't easy to gather that research for a given bullet, but a few manufacturers will specify their maximum.

https://www.accurateshooter.com/tec...ing the formula, this is,x 60, or 270,000 RPM.

https://www.accurateshooter.com/tec...ing the formula, this is,x 60, or 270,000 RPM.

https://castboolits.gunloads.com/showthread.php?226044-Maximum-velocity-(rpm)-for-lead-bullets

 

[INTJ]

While not 115 grains, my bra runs 107s, h4350 and Alpha brass at an easy 2855. Prs rifle, highest velocities not needed there.

OP said 6 BR, not BRA, though you do make a good point--OP didn't say what discipline he was shooting.

In 1000 yd BR we routinely run 103-105 grain custom bullets at 2900-3000 fps in 6 BRAs. A straight 6 BR would usually go about 100 fps slower. I don't know if anyone running a 115 in a straight 6 BR case that is competitive.

Now PRS does have different requirements and guys do run them slower. Still, a Dasher would be a better case than a straight 6 BR for a 115 in any discipline.

 

[mgunderson]

Question and not criticism. What is the benefit of running different bullet weights? Practice vs match or short line vs long distance.

 

[LRPV]

Question and not criticism. What is the benefit of running different bullet weights? Practice vs match or short line vs long distance.

None.

Except of course you have a shelf full of it already and can’t find more of your favorite.

Another thing I’d recommend on the stability twist calculator is to measure the OAL of YOUR bullets and insert it in the box. Back when I used to use this tool some of the auto fill data was very optimistic on the length of the bullets. Must be a guy thing…

 

[David Milisock]

I have used the Berger twist calculator and in my 6BR with a 1-7.5 twist all of the Berger bullets are stable from 2800 to 3000 fps. Do I need to plug in the exact FPS or is a guess accurate enough (using load data)? As of right now I plan on using 104 and 115 on top of H4350. Any comments or real world experience is appreciated. Also Lapua cases and Fed 205M primers (27.5" barrel).

How far you shooting? A 7.5 twist is surely going to work for the 104 at stated velocities, the 115 depends on how fast and how far.

One twist calculator has my 1 in 9 twist 6M Remington barely stable with 105 projectiles, another calculator says it's unstable.

I get very reasonable groups, 1 3/4" at 300 yards with hunting bulletts with no key holes at 3,200 FPS. With that said I would not shoot deer sized game at 300 yards with my 6MM Remington unless I had an accurate load and optimal conditions. I suspect that adding a few hundred yards would possibly separate the wheat fron the chafe.

So the question remains, how far?