Twist rate for 88 and 90 grain bullets -elevation

[6br788]

I am new to the forum and soon to be to the 6br cartridge though I have wanted a rifle chambered in this round for years. I live at an elevation of around 5500 feet, and the longest range we have here,not even in quite yet) is 600 yards. I am planning on shooting the 88 grain FB LD berger, 87 grain v-max,for varmints), and 90 grain scenar out of the standard 6 br. This will be a field and bench rifle that should weight in at 13 lbs or so when I'm done. I do not plan on shooting 1000 yards in the forseeable future, and wanting to shoot to at most 600 yards at varmints as well as paper I am thinking the bullets to 90 grains will be good. Considering my elevation, would you think I should go with a 1 in 9" twist? I would go with a Hart barrel for 1 in 9" or possibley Lilja though my smith prefers Hart. If someone could give me solid reason to use a 1 in 8" I would go with my favorite barrels, the Broughton 5C. And, of course I am also asking if a 1 in 10" would even be a possibility. Thanks.

 

[mikecr]

I would go 9tw for those bullets. If that is the heaviest you ever plan to use. Have fun

 

[6br788]

You know, one question I have now is at 600 yards is there any advantage of the 88 and 90 grain bullets over the 107 SMK? Is it foolish to think that I am going to gain something by using the 88 and 90 grain bullets to 600 yards rather than just going with an 8 twist and 107 grain bullets if I use the rifle mostly at the range but occasionally on varmints?

Also, is it worth my time at this point in the game to neck turn, or should I have my smith use his reamer not requiring neck turning?

 

[Forum Boss]

You know, one question I have now is at 600 yards is there any advantage of the 88 and 90 grain bullets over the 107 SMK? Is it foolish to think that I am going to gain something by using the 88 and 90 grain bullets to 600 yards rather than just going with an 8 twist and 107 grain bullets if I use the rifle mostly at the range but occasionally on varmints?

Also, is it worth my time at this point in the game to neck turn, or should I have my smith use his reamer not requiring neck turning?

All valid questions that many of us are mulling over. First as to the neck-turning--a lot of people have reported zero,or negligible) reduction in accuracy going to a no-turn neck, provided you do mic the necks before loading your match rounds. We've seen an occasional case that's .002 thicker on one side than another. Cull those and use them for varminting. We do think neck-turning may reduce doughnut problems, provided you have good tools and technique. That said, if your chamber is optimally throated, the bullet won't be near the doughnut so it's not a big issue. It really comes down to this: do you want total control over neck wall thickness or will you sacrifice the ability to correct maybe 1 case in 30, in order to significantly boost your load output. For most varminters, who might shoot 500 rounds in a hunting weekend, the answer is clear: no-turn. For match shooters, there is no consensus at this time.

Regarding the bullets--both the Berger 88LD and the 90gr Scenar have shown exceptional accuracy, as has the 87gr V-max for that matter. You'll be giving up about .12-.13 in BC compared to the 105s and 107s. Run the ballistics. With a 10mph cross-wind at 600 yards, drift with a .53BC 105gr bullet is about 22.3", assuming 2850 fps muzzle velocity. With the same conditions, drift with an a .395BC 88LD is about 31.9", assuming 3000 fps muzzle velocity.

Will that make a difference? It might, if you are shooting in a box canyon with shifty winds. If you are shooting mostly inside 400 yards, we'd probably go 1:10" twist and shoot the 88s and 87gr V-Max. But if you do plan to spend much time at 600, you may wish you had the ability to shoot the 105s. And that means going all the way to an 8-twist barrel.

There's really no "right" or "wrong" answer. Chances are the lighter bullets may group a little tighter than the 105-107gr bullets, so that's another factor to consider. And you may be able to drive the 88s FASTER than 3000 fps, so that changes the ballistics. With a long barrel, you can probably push 88s to 3100 fps, and then the wind deflection is only about 30.2", 8" more than a 105.

Addendum: Further research by Henry Childs has shown that the Berger 95 VLD really needs at least a 1:9" twist and Henry feels a 1:8 is "safer" for all temp/altitude conditions.

 

[6br788]

Thank you for the input. I appreciate your answer. Over here it would be mostly coyotes we are after when it comes to the varmints. I have been told the match style bullets do a decent job at saving the pelts, and the 1-8" may be the way to go. Of course then again it sounds like the lighter pills will make it to the 600 target and I will need to correct for wind drift a tad more. I will need to get with the gunsmith I usually use and see what throat length his reamers are set up for. Thanks.

 

[Changeling]

6BR788, You were given some very good advise from all of the above.
You seem however to be more interested in the varmint aspect, in particular "Yotes". In my opinion, if you were going to use some of the target bullets not specifically designed for varmints you should give a lot of thought to the 8 twist. It will give you more rotation speed and better performance on the Yotes.
I also have been trying to find out what kind of accuracy guys with 8 twist are getting with the really light bullets,55,70,80) but it seems no one or not many have tried. If anyone has, I am really interested.

 

[Shelley]

Changeling

I do have a pet gun with a 6BR chamber in an 8 twist barrel. The reason that I've never tried really light,short) bullets is because the short bullets won't reach the lands and still have much bullet base in the neck. I have no idea if this will be a deal breaker as far as accuracy is concerned.... just my thoughts.

I also suspect that some small bullets will blow up if you push them too fast from a 8 twist barrel. The rpms are really up there. I'll bet that Alinwa over on BR.Com has done this. If no one here has a good answer you can try Al.

Shelley