Why does decreasing the jump across the throat have an effect on accuracy?

[ADClope]

I see this mentioned when discussing bolt stuff mainly and where you set your COL at, but I'm curious why it matters. Logically, to me, once the bullet engages the rifling, it should pretty much be locked in for the length of the barrel before exiting... I don't understand how a slight "jump" at the breach would make a difference at all once it has traveled 16" - 18" - 20" - 24" or however long the barrel is. Or is this kind of like splitting hairs in regards to the effect it creates?

[Doyle]

It all has to do with barrel harmonics. Your barrel is not a perfectly straight "pipe" during the firing. It actually moves like a snake - but in tiny, microscopic amounts. The more consistently you can get the bullet to exit the muzzle at the same "vibration" position, the better your gun will group. The amount of jump affects the pressure spikes (there are actually two pressure peaks if I recall) which in turn affects how that barrel is going to move as the bullet travels down it.

Now, some really knowledgeable folks are likely to come along and give you a proper scientific explanation.

[Dufus]

The farther the bullet is from the lands, the lower the initial pressure spike is just after ignition, within reason. The closer the bullet is to the lands, the higher this initial pressure spike is.

That is the principle that Roy developed his Weatherby calibers on. Without the jump created by the freebore in his rifles, he would never have been able to create the velocities that he did at the time of development. With all the newer powders available now, it ain't a problem without freebore.

The most accurate rifle that I own has a 0.204" jump to the lands.

[JeepHammer]

Harmonics is real, the reason for heavy barrels is to reduce harmonic movement of the muzzle during the firing process.

The second part of 'Jump' (Freebore) is how the bullet is effected when it crashes into the throat/rifling.
The bullet is squeezed down, and it gets slammed sideways, no case aligns with the bore perfectly.

Some throats taper differently than others, velocity changes will work differently with those changes.

When people say a barrel is 'Shot Out', often times the rifling is fine, the throat has eroded one way or the other causing problems.
In these cases setting the barrel back and rechambering, cutting a new, even throat taper/freebore brings accuracy right back...

No matter what people say, throats don't erode evenly, one side is going to take more of a beating leading to deformed bullets.
Just 'Touching' the throat with a finish reamer shows off set/out of round/lopp sided throats right away. Not hard to figure that one out...

Off set throats/tapers 'Bend' the bullet into the chamber, and a lop sided bullet is never a good idea...
Some of that off set can be overcome by velocity, 'Jump' from casing to rifling helps to 'Resize' the bullet to form to barrel.
Other times it works against you, fracturing the bullet or lop-siding the jacket.

This is why we run range tests, to find out what works best & what doesn't.

[Wildernesshunter]

Great question and one I have been thinking a lot about lately as I try to dial in several reloads on several rifles.

I usually prep a piece of brass completely, barely seat a bullet in the neck and then carefully load and lock it in the chamber. Once extracted, I measure....and that gives me the exact measurement to the lands.

Is this how others do it as well?

Now to my real question..... how far off that measurement generally produces the best accuracy? Is that an impossible question and it just depends upon the gun, the bullet, the load, etc....????

To find the most accurate load, does everyone simply shoot 5-6 rounds with various seat depths?

Or....is there a magic, pixy dust document floating out there somewhere that could be really, really close and eliminate a lot of guessing and shooting?

[5whiskey]

Now to my real question..... how far off that measurement generally produces the best accuracy? Is that an impossible question and it just depends upon the gun, the bullet, the load, etc....????

Yes it is an impossible question, yes it does depend upon the gun/bullet/load/etc. I will say that in general, I believe most rifles will do better with a very short freebore. Keep in mind that shorter freebore will raise pressure. Some here load to a zero freebore, i.e. the projectile is touching lands when chambered. I do not believe this to be prudent, personally, but I also do not like over-pressure rounds. I will not seat closer than .02 from the lands. I've found this to be generally the most accurate across several rifles I own, but with a safety margin added by having a little freebore. I may decrease group size by 1/10th moa if I seated at the lands, or much closer... but I'm not a bench competitor and to me the reward is not worth the risk. YMMV

To find the most accurate load, does everyone simply shoot 5-6 rounds with various seat depths?

When working up a brand new load, I load up about 25 rounds of each load and average five, five shot groups. I'm also looking at group consistency (as measured by standard deviation). You personally do not have to load this many if you don't want, but I feel that 5-6 rounds is not enough of a sample to get any kind of accurate reading of "which load (or seating depth) is best." I have tweaked a variable on a load before and I only loaded 15 rounds (three, five round groups) per load, but that was just a minor change such as new primer brand, etc.

Or....is there a magic, pixy dust document floating out there somewhere that could be really, really close and eliminate a lot of guessing and shooting?

That would take the fun out of reloading. Heck, half the fun making some different loads and testing them. But no, there isn't any pixy dust other than the limits set by a good reloading manual. Just trial and error.

[Ifishsum]

At one time I experimented a fair bit with different seating depth for my loads and I didn't find it to be a major accuracy factor in most cases. When it did it tended to be with bullets having a longer front taper shape (secant ogive or VLD style) but as always its individual to the rifle. Sometimes things shoot well even though the stats say they shouldn't, and vice versa. For the most part I start at magazine length or .015-.020 away from the lands and only start changing OAL if I am not happy with my results and really want a to make a specific combo work. More often I simply try a different powder or a different bullet to get the result I want.

By nature most of us reloaders like to experiment, myself included. But I'm convinced there are other factors that usually make a much more noticeable difference than seating depth.

[44 AMP]

Is that an impossible question and it just depends upon the gun, the bullet, the load, etc....????

Pretty much.

[ADClope]

Thanks everyone for your responses. This forum is great, so many knowledgeable folks