Bullet Jump and its affect regarding Accuracy

[treeman]

Originally Posted by A-R
The rifle will still be able to safely shoot 458 Lott and 458 Win Mag, although some long-range accuracy may be lost due to increased bullet jump with the shorter cases.




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We have debated this before, but let me ask any way, do you still believe jump affects accuracy?
What is the verdict these days?
I say jump is just a way of raising and lowering chamber/cartridge pressure with change in COL being measured by distance from the lands.
My .270 has an 0.3 mm jump load and is accurate, it also has a 10 mm jump load that is accurate.
My mate has matching number Musgrave that has a 11 mm jump, I had thread that asked about it back in 2012/13 abouts.

[Toxxyc]

Some people believe chasing the lands is the answer. I don't. I've seen first-hand how accuracy increases while increasing the jump.

And yes, bullet jump does affect accuracy, definitely. But like you said, it is used to affect chamber pressure, which is what makes the difference. It's only part of it though. Neck tension also changes and that can also increase accuracy. A bullet barely being held on by the case won't be stable or straight going into the barrel, which could also easily affect accuracy.

IMO - load what works. Forget about chasing lands. You should be able to come relatively close to under 1 MOA groups without worrying about the jump in your rifle just by playing with the powder charge, case prep and bullet choice.

[shooty]

Here you go from a competitive professional shooter.

As a note. On my Sako 85 222. Im seating the bullet to magazine length. According to listed specs. Its like 5mm further out. If I seat that way or spec depth. It still groups equally well.

[shooty]

Best way to increase accuracy. Adjust your trigger if possible. Or get a good trigger.
If you can a Bix n Andy. Others are good. But this is superb.

Then Bump size Cases.

Get a Hornady Concentracity tool. As its the only one that can actually correct Concentricity. And not just tell you its out or not.

And in all of these consistency is key.

[Toxxyc]

I installed a new trigger on my rifle a week or two ago. I haven't had the chance to go shoot it yet, but I simply can't wait. The factory trigger on the rifle was so bad, I could see the crosshairs creep on the target as I tried to squeeze off the shot, so I know it played a role in my accuracy. Now just need to book the time on a range.

[shooty]

As a note. On Sako 85's and Tikka's. They have great triggers and are adjustable.

However you can open them and replace the small spring. And you can get the trigger much better.

YoDave sells these at $10-00 or you can use a parker pen spring.

[tcbc]

Here you go from a competitive professional shooter.

As a note. On my Sako 85 222. Im seating the bullet to magazine length. According to listed specs. Its like 5mm further out. If I seat that way or spec depth. It still groups equally well.

This is excellent video. I wish I had seen it when I was still shooting TR Bisley


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[Toxxyc]

As a note. On Sako 85's and Tikka's. They have great triggers and are adjustable.

However you can open them and replace the small spring. And you can get the trigger much better.

YoDave sells these at $10-00 or you can use a parker pen spring.

I have a Rem 700. With that crappy adjustable trigger. I tuned it to the limit of it's safety and it still wouldn't break properly. Once I had a challenge with my FIL and we tried to see who could shoot the most small clays at 100m, offhand. I took the first shot and broke a clay, and handed him the rifle. He loaded up, took aim and eventually lowered the gun and asked me why I gave it to him with the safety on.

Safety wasn't on. It's just that sometimes that trigger decides it wants to break and 11 metric tonnes, and sometimes it's not too bad. Inconsistency is a nightmare though.

[Messor]

Many people have a very one dimensional approach to accuracy.
Accuracy itself is complicated and it differs from application to application.

You can shoot pmp ammo at 100m and have a VERY good grouping with even large ES values.
Does this represent accuracy, well yes and no, that ammo won’t group at distance.
Take this topic, jump, will jamming a bullet into the lands create a more consistent shot start pressure, yes, but people don’t name that aspect directly, they will only focus on concentricity.
Will jamming a bullet into the lands cause a faster rise in pressure, and a faster initial burn rate, yes, and that is one of the core principles of OBT(quickload) etc.

In real life you’ll find many people that do something they believe to increase accuracy, and in reality it does, they just don’t know the specific function behind their own actions.

I would say this, if you load your bullets with as little as possible runout, and on an accuracy node, and with as low as possible ES and SD values, those bullets will shoot.
Getting there just requires a bit of experience and ballistic knowledge.

[A-R]

There are lots of things that affect accuracy and precision, and lots of people who don't even know the difference between the 2...

The biggest single factor affecting accuracy is still the nut behind the trigger. Some of them are very hard to adjust.

I think this thread focusses solely on one aspect of accuracy, and that is it's relation to bullet jump. So I will try to stick to that.

From personal experience across several of my own rifles, I have made some empirical observations. One of the correlations I found is that a rifle will generally group well (precision) when the length of the bullet's bearing surface exceeds the "jump", measured from the case mouth to the place where the bullet jams into the rifling. My theory is that, if the bullet enters the rifling before leaving the case mouth, it will be positively supported at all times until it leaves the muzzle. As soon as the "jump" gets longer than the bearing surface, there will be a brief period where the bullet sits unsupported in the middle of a high pressure, high temperature gas flame mixed with hot burning powder particles blasting it on all sides. This condition can't be good for consistent alignment of the bullet with the barrel throat, so it likely enters slightly skew and won't turn into proper alignment after being gripped by the rifling. Just my theory from my limited experience, as I am not inclined to peek in there while all this is happening.

One particular example is a Weatherby rifle with a typical long Weatherby throat. The free bore is a full 15mm on this one. It will not group under 2MOA with any normal boat-tailed bullets I have tried. Change to a flat base, which has a longer bearing surface, and it groups under 1MOA. Whether I seat them close to the lands or deep in the case makes little difference to the groups. It only changes the MV (and pressure).

Another example is a 243 rifle. It shot sub-0.4MOA all day long with 87gn V-Max bullets up to a round count of around 1500. Then, suddenly, the precision went to pot. Accuracy too. At this point I could not seat the bullets out far enough to touch the rifling anymore. (Not that I loaded it this way, I just tested for this condition to know.) I then switched to 100gn Sierra PH bullets with a flat base and much longer bearing surface. Groups were small again. As soon as they opened up again around the 2200 round mark, I tried to measure the throat erosion by seating a bullet out to touch the lands. It would not contact the lands while still touching the case. So the throat was longer than the bearing surface. Time to make a plan. The gunsmith cut the chamber off and reamed a new one. It shoots groups again.

There are a few more examples, but I'm not going to bore you with all those sad stories of rifles not wanting to shoot well.

My point is that if the distance ("jump"), measured from the case mouth to the lands is longer than the bullet's bearing surface, accuracy (and precision) will deteriorate significantly.

If a precision rifle suddenly shoots 3MOA groups, it is junk. But if an elephant rifle shoots 3MOA it is probably just perfect. So to see what happens to your 458Lott, 3" Exp or 450Watts when feeding it factory 458 Win Mag ammo, test it. The target will tell the story. Then you can decide if the performance is good enough for a brain shot on an ele or heart or spine shot on an angry buffalo at less than 10m.