Bullet Jump: is less always better?

[Lemonaid]

https://precisionrifleblog.com/2020/03/29/bullet-jump-load-development/

This article is very interesting! Check it out, it may change how you reload!

[grayfox]

Definitely an interesting article. Those PRS blog guys find some neat stuff.
Could possibly be that Arnie's compound throat is a "more forgiving" design than conventional throats for the Grendel, which would be along a similar vein to the final points in the article. Maybe his design works well for our cartridge for similar reasoning.

[Fess]

Bullet construction also has an effect on optimal bullet jump. Monolithic bullet often do best with larger jumps than lead-cored ones. Barnes Bullets states:

https://www.barnesbullets.com/load-data/

A NDIA report from a number of years ago suggested that part of the issue is the force required for initial engraving of the rifling, when the bullet can still tip slightly off-axis. It is possible that the compound throat may have different engraving properties than a standard 1.5 degree one, though.

[grayfox]

I'm thinking something like that, plus it may have a more gradual erosion profile... certainly offers a more gradual alignment process.

Couple of guys on here also say that the 120 SMK, for example, with its tangent style ogive, is a pretty forgiving bullet as far as accuracy goes. That shape more than a secant style or compound style... maybe all these things are related.

[centerfire]

I think bullet shape has more to do with jump tolerance than material composition. I don't think jump distance is much of a surprise these days. Most of the guys I shoot with now use a chronograph to find their optimal charge and a seating test for group size reduction.

[jmkhenka]

I have always fascinated by the focus on bullet jump, and loading close to the lands.

As a swedish reloader, and with experience mainly from 6.5x55 swede, i would tell you that unless you make a custom rifle you will not be able to load close to lands with most, if not all, 130-140 grain bullets.

And that has never been a issue with this particular caliber, as we have been using it for competition for over a 100 years now.

For instance, golden target 130 grain, a VLD bullet many have experience with as i think berger has the same bullet (norma). I load it at 77.5mm COAL and i hit lands at around 82mm.
I have 0 neck grip then.

What gives me better results, regarding even velocities and accuracy, is bullet grip. a well accepted and well tried recommendation is 6-6.5mm of bullet seating depth. Anything more then that just reduce charge weights.

The closes bullet i have been able to get to the lands are the ELD-M 147, and it still had a fair bit of jump. I managed 25mm over 25 shots at 100 meters, that might not sound impressive but the best bench rest shooter that day had 18mm. And this with a Sauer 101 made for huting and 56cm barrel (v0 around 720msec).

[kmon]

Seating depth I play with but in several of my rifles I can seat no where near the lands and still get a round that will function through action and fit in the mag. one of those with Berger 140gr VLDs is easily sub half MOA .12 off the lands. If I want to shoot jamed into the lands it is still a half MOA shooting single shot.

i never owned or loaded for a Weatherby rifle that a bullet loaded to kiss the lands could fit in the mag and some bullets like stated above cannot be seated out far enough to reach the lands. Berger even suggest with their VLD bullets to seat to the lands but if that is not an option start .040 off the lands and increase by multiples or 040 until you find a sweet spot.

I have loaded for a Remington 700 in 257 wBY that blows primers on factory Weatherby ammo fairly often and sticky bolt lift because it is a little short throated and not a lot of free bore like the Weatherby rifles are.

[Bobke]

This is first post on topic that should run for a bit. Great site to see where pros are spending their time, effort and dollars to improve long range results.

https://precisionrifleblog.com/2020/03/21/bullet-jump-and-seating-depth-reloading-best-practices/

[grayfox]

Something that occurs to me, is how do we know that the set of lands for a given barrel, has contact points that are exactly equal in distance from the breech face -- so that the bullet in striking those lands has a perfectly perpendicular plane (to axial travel) of resistance from the lands at that moment? If one or more lands has a slightly longer or shorter (and I'm talking 0.001" or maybe 0.005"... but not perfect -- neighborhood for distances) breech-face distance, then as the bullet strikes those lands, it will tend to get off-set away from perfectly axial bore lineup.
And even if those lands are cut perfectly at first, how do we know that they will always equally erode after each 25 or 50 shots (or whatever delta we should use for this question). Bottom line, for all except the highest price barrels, we probably do not have that level of perfection in the formation of lands at the end of the throat. And I doubt if anyone could guarantee equally-level erosion over time. But this is all as a question, I don't have any proof one way or the other.
I just wonder.

[Lemonaid]

Grayfox, I have observed the throat of some of my rifles with a bore scope and it looks like they do indeed wear unevenly.
There is a possible temporary fix with fire lapping. At some point it effects accuracy enough that it warrants a new barrel or face it off and rechamber the old.
But if you can find the sweet spot for a long-ish bullet jump you can keep shooting the same barrel for a lot longer

[Bobke]

Not a lot to be done to correct and/or control barrel throat erosion-short of setting a barrel back often enough to think you've got it under control-if only for a moment, and even if so, how about how runout affects bullet alignment to a perfect bore axis/throat? Not sure if i'll ever be able to actually shoot the difference in either control group, but keep trying to eliminate RO in loaded ammo. Recently picked up a bore scope and am astounded at how well a particular 6.5GR AR barrel shoots-consistently-with substantial erosion in the gas port. Lots of variables to control, and we do so, some with effect, others of little circumstance.

[grayfox]

Roger to both, and I guess it's my point, at least for non-OCD guys like me, we probably won't ever be in a position to have to worry about/control for these particular variables. It is true that a barrel, as Kelm puts it, is a consumable (which to me implies replace like 1x or more per year), but for my shooting style it's more like a depreciable asset over a few yrs, so the variables I tend to run into are brass life, primer mfr, and powder lot variations. I don't (and probably won't ever) seat to ogive length so bullet to bullet variations will tend to be +/-0.003 for good ones and +/-0.010 for lead-nosed ones, all of which, for a coal that I settle on (within +/-0.003, say) have been able to find nodes that give 1.0 moa and sometimes ~0.75 or so if I'm really on my game. Thus thinking about distance-to-lands of +/-0.001 or 0.002 is just a variable lost in the weeds for me.
So one session the cartridge might be coal at 2.265, the next time I set them up it might be 2.268 or 2.264, but still looking at consistent, good-hunting-ammo style accuracy. So I know the mechanical systems are gtg, just have to focus on the shooter part of the equation.
It is interesting to read these guys however, and see what they're doing.


Edit to add: 1 lb of powder yields 175-200 +/- loads (depending on caliber), so after that, if I have to break into a different lot, then will go thru my MV test and seating test again to optimize. It usually turns out close to the load and MV of the previous lot, but this exercise also would by nature adjust for any variations due to land/throat erosion and account for any delta in jump distance over that last 200 rounds.

[A5BLASTER]

With the 120 gold dot. I have seen that being to close to the lands is a bad thing.

I first worked this load up at 2.290 inch's and 28.6 grains of ar-comp. Load shoot great in two rifles but beat the crude out of the brass in my 16 inch rifle.

Backed the seating depth down too 2.270 inches and dropped down and reworked the load. Ended up right back at 28.6 grains but groups much tighter almost the same speed but the brass fired from the 16 inch came out looking much much better.

So my thoughts are in that 16 inch barrel the seating depth of 2.290 was just to close and spiking pressure.

Here is the kicker at 2.290 coal the jump was still like 40 thousands to the lands.

So in my limited experience, I may start out longer but most my loads now end up depending on bullet being 20 to 30 thousands shorter then where I started the work up.

[lazyengineer]

Interesting. I've always figured closer is better. With most bullets you don't have much choice since magazine length limits you anyway to shorter then the lands. But I will say, with Grendel I have found some bullets with some barrels require being shorter then mag-length or they will indeed hit the lands; making this article that much more relevant.

Interesting finding, thanks for the post.

[Bobke]

http://bulletin.accurateshooter.com/2019/06/tech-tip-how-to-reduce-run-out-with-seating-dies/