Secant vs Tangent
- Thread starter Paddyd
- Start date
You should do Berger's recommended full seating testing so that you'll actually know.
https://www.longrangehunting.com/th...-from-berger-vld-bullets-in-your-rifle.40204/
https://www.longrangehunting.com/th...-from-berger-vld-bullets-in-your-rifle.40204/
garandman
Bolt Gun Bodacious
My practice ( repeatedly successful) Has been to start 20 off, settle on a powder charge level, then come back and test seating depth in 5 / 1000 increments.
ETA: by "successful" I mean 0.36 -38" @ 100 yd in 2 different calibers.
But even Berger says that's wrong. Lol
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Secant ogive have for me been more sensitive to seating depth. As far as getting them going, once I find a powder charge that makes me happy with velocity consistency & a nice wide velocity node and shows promise grouping wise, then I'll play with seating depth. Start at .010, .040, .080, .120 off. Look for the trend and follow it only as far as improvement is enough to waste the powder chasing it.
I decided to start these .025" off the lands. 5 ea 22.0 22.5 23.0 23.5 of 8208XBR. See if i get one to show promise. Then I'll try .2gn up and down. last take best charge and play with going shorter and longer. Probably end up shooting damn near the entire box of bullets before I get the best possible load, but it sure is fun when it all finally comes together.
I have some 70 RDF bullets I plan on testing next and I will try this method. Thanks.
I have an old M70HV in 223R thay either was chambered with a long throat or it has suffered lots of erosion. The Hornady 68's won't get under 1/2" until they jump about .090" Between .090" and .100" they shrink into the low 2's . Right now I'm trying to work something up in a new Criterion barrel with a throat so damned short it's almost non existent by comparison. Just trying to get .040" jump puts the bullet WAY deep into the case.
Ned Ludd
Silver $$ Contributor
Although VLD (secant ogive) bullets have the reputation for needing to be seated into the lands, I find that to be somewhat anecdotal. Certainly the transition of a secant ogive bullet from ogive to bearing surface is far more "abrupt" than a tangent ogive bullet, and it could easily affect the way they enter the lands. In my hands, VLD bullets do tend to be slightly more finicky with regard to seating depth than tangent ogive bullets. Nonetheless, I have developed plenty of loads with VLD bullets seated off the lands, and they shoot just fine if you're willing to rigorously test seating depth in fine (.003") increments. Any more, I start pretty much every load development, regardless of the bullet used, with the bullet seated at .015" off the lands. Once a satisfactory charge weight has been identified, I will start by testing seating depth in .003" increments from .006" off to .024" off. Notice that the initial charge weight seating depth of .015" off the lands in exactly in the center of the total seating depth test range. That is so you're not changing the seating depth more than one half the total range in either direction, which in my hands is not sufficient to dramatically alter pressure or velocity with jumped bullets. I have rarely found a bullet that wouldn't tune in somewhere within that range; however, it is certainly possible.
In the event I don't find an appropriate seating depth between .006" and .024" off, I might try seating bullets next at "touching", .003", .006", and .009" into the lands. I have also found that some VLD bullets will tune in very nicely somewhere in that range, but the optimal seating depth window generally seems to more narrow than for the seating depth optima off the lands.
If you use the Berger VLD bullet seating depth approach, just remember that it is meant as a very coarse test to find one seating depth region where the bullets shoots better than the others. In other words, you're testing a very wide range in coarse increments to narrow down the testing zone. You still want to go back cover that single region using a fine increment test to optimally tune tune the load. Also, changing the seating depth of a bullet that far, both into and off of the lands, can create pressure issues as you may be changing the effective case volume by a significant amount. So it's not a bad idea to use a slightly reduced load for safety reasons. Many people have found Berger's approach to work very well, especially for bullets that did not seem to tune in well using a more standard approach.
Paddyd, FWIW - there have been a few reports here that some of the RDF bullets seemed to prefer a fairly good jump, maybe in the .030" to .050" (or more) range.
In the event I don't find an appropriate seating depth between .006" and .024" off, I might try seating bullets next at "touching", .003", .006", and .009" into the lands. I have also found that some VLD bullets will tune in very nicely somewhere in that range, but the optimal seating depth window generally seems to more narrow than for the seating depth optima off the lands.
If you use the Berger VLD bullet seating depth approach, just remember that it is meant as a very coarse test to find one seating depth region where the bullets shoots better than the others. In other words, you're testing a very wide range in coarse increments to narrow down the testing zone. You still want to go back cover that single region using a fine increment test to optimally tune tune the load. Also, changing the seating depth of a bullet that far, both into and off of the lands, can create pressure issues as you may be changing the effective case volume by a significant amount. So it's not a bad idea to use a slightly reduced load for safety reasons. Many people have found Berger's approach to work very well, especially for bullets that did not seem to tune in well using a more standard approach.
Paddyd, FWIW - there have been a few reports here that some of the RDF bullets seemed to prefer a fairly good jump, maybe in the .030" to .050" (or more) range.
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There never was basis for past notions about VLDs having to be into lands, or even close to lands, to shoot well. And still today, and likely tomorrow, full testing will be needed to actually know.
That's not pulling a seating depth out of your butt and going back for fine tweaking after powder.
It's up front full coarse testing, -and then fine tweaking after powder -for tightest group shaping.
That tweaking is within a relatively narrow window for secant ogives -vs- tangent.
That's not pulling a seating depth out of your butt and going back for fine tweaking after powder.
It's up front full coarse testing, -and then fine tweaking after powder -for tightest group shaping.
That tweaking is within a relatively narrow window for secant ogives -vs- tangent.
Well, last night I loaded some of those Hornady 68's over IMR 8208XBR. Seated them exactly .025" off the lands. 23.0 gns seems to be the one to tweak as the first 4 bullets went virtually into the same damned hole. I got excited and pulled #5 high to open the group to .303" center to center. I'm going back out tomorrow to try the same load again and to try the same charge weight, but .005" longer and shorter to see the difference.
I started with a mid length of 2.84 with a hybrid and went .04 each side of that 2.88 (.005 off the lands) down to 2.80 and found the 2.84 to 2.80 showed no difference in accuracy. 2.88 was definitely worse. Jumping the VLD's and/or hybrids might work. I've definitely seen a difference in group sizes by changing bullet seating depths by .04.
Real simple, secant ogive bullets shoot best at jam or about .005 off. Find the touch for your rifle and then start at .010" off the touch. This is more to find the sweet spot for your bullet and powder combo without exceeding pressure limitations. Then you work off the jam in .003" increments and into the jam in .003" increments. Find the powder that works first! So many start with "I have a ton of Re-16 on hand" and never try anything else, me guilty of that, LOL!! If it's the wrong powder for your barrel and load, it's the wrong powder whether you have 50 pounds or 1 pound! Have fun and enjoy finding that right load !
Well so far in my CBI 223 they ( Hornady 68 match) shoot best with a .025" jump. Tried it both directions and it goes south both ways away from .025" off.
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