Is bullet jump a mechanical or pressure phenomenum?

[Andrew Leigh]

I have agonised over jump. I read of people adjusting their COL with remarkable results, I have tried but simply cannot replicate their experience. I also want my groups to dramatically close up by simply seating differently.

As a Hunter my groups are perfectly acceptable, but that does not stop me from wanting smaller groups to prove better reloading technique and shooting ability.

So my question is then twofold. Is the altering of the jump due to mechanical consideration? Is the jump merely affecting pressure and changing barrel time bring one onto an accuracy node?

Mechanically.
-I understand that as your approach the lands that pressure will experience a non linear increase due to the inertia required to overcome the rifling. As you move away from the land their is a point at which the pressure will increase again.
- I also understand that if you bullet is not seated 100% concentric to the barrel that a longer jump could have an effect on how true the bullet enters the rifling.
- You have the effect of the free bore which is essentially an extension on the jump.
- Then you have the angle at which the leade is cut.
- You have the hardness of the projectile.
- There is the angle of the ogive to be considered.

Pressure
Now of the above there is only one factor that to my mind that is purely mechanical and that is concentricity, the rest ALL result in a pressure change. The pressure change in turn will affect speed and barrel time.

So I have asked if I am not fooling myself. If I am on an OBT node I am in tune with the harmonics of the barrel and therefore any change in jump would require a change in load.

Conversely, If I do not possess ballistic prediction software or a chrony am I simply not unknowingly bringing my rifle onto an accuracy node but altering the jump (sic the pressure and thus the barrel time).

Searching these and other forum you get the school that says jump makes a difference and those that say it does not. Surely we as loaders can get closer to the truth?

By way of example. Although not a great shot I can get 0.6MOA with my 6.5mm. I have changed the jump with out changing the load. I have changed the jump compensating for the load and my groups do NOT change. Neither do they get smaller nor do they get any bigger.

Conversely my .375 has a jump of 0.28". I cannot get any closer as the magazine restricts me. I shoot 1" generally but have managed my best group ever of 0.27MOA? So how important is jump really? My shooting buddy has a 9.3mm and is 0.43" off the lands and shoots sub 0.5MOA, in fact all his rifles are 0.5MOA and better with SAAMI COL's.

Now while I cannot get the jump to have any affect in my shooting I am no way claiming that it does NOT have an effect. In fact I would love it to be true, seat a little more in and groups go from 1MOA to 0.5MOA, hey presto.

I am really keen to get to the bottom of this so anecdotal evidence will be of little value. Has anyone got real data and pic's showing a progression of a load where the change in COL closes and then opens a group?

I can find lots of people that tell me that jump make a difference but have yet to find any proof.

 

[Big7]

Try loading right up to the lands..

No jump and only a slight pressure increase.

 

[Comrade Mike]

What distance are you testing at?

 

[Andrew Leigh]

100m, I can go out to 300m if required.

 

[chuckpro]

Is your barrel factory? I have found on my factory barreled guns the i cant load into the lands. I have been told by a few gunsmiths that it takes a custom cut chamber to load into the lands. I am not very experienced with this so excuse me if i am stating the obvious or i am completely off base.

 

[918v]

You can't fix a crappy load by adjusting OAL.

OAL is a tuning tool. You have to have a good load to begin with. Not every powder and bullet combination will yield good results. The goal is to create a load that exits the bullet out of the barrel at the right time and that has a very small ES. OAL allows you to tune the bullet exit time, but you have to be close.

Most people posting quarter MOA groups on the net are liars.

 

[Andrew Leigh]

Is your barrel factory? I have found on my factory barreled guns the i cant load into the lands. I have been told by a few gunsmiths that it takes a custom cut chamber to load into the lands. I am not very experienced with this so excuse me if i am stating the obvious or i am completely off base.

Both my CZ's (6.5mm and 30-06) I can get into the lands.

 

[918v]

Think of altering the powder charge as a rough adjustment and OAL as a fine adjustment.

 

[Andrew Leigh]

You can't fix a crappy load by adjusting OAL.

This is probably where I am. Articles I have read intimate that changing the OAL is magic. I think my loads are good to start with and generally am around 12fps std dev. My case prep is thorough and my components generally good. I sort cases by weight and have squeaky clean brass before loading.

My rifles are bedded and I have reasonable glass on them.

So I like to think that my loads are well thought out and very close to nodes. I use QL as a tool and it for this reason that I think I am very close to the nodes and that OAL tuning has minimal impact, hence this post.

My 6.5X55mm is a 0.60MOA rifle and I can get this regularly but not on every outing. My 30-06 is a .75MOA rifle while my .375 is generally a 1MOA rifle. This assuming I am shooting well, so the rifles I know are fine.

OAL is a tuning tool. You have to have a good load to begin with. Not every powder and bullet combination will yield good results. The goal is to create a load that exits the bullet out of the barrel at the right time and that has a very small ES. OAL allows you to tune the bullet exit time, but you have to be close.

This confirms my thinking. On recent 6.5mm loads played again with the OAL, 0.3mm (0.011") change in OAL resulted in 4fps which is a tiny change in barrel time. It was for this reason that I got to thinking about the effect of the OAL.

Most people posting quarter MOA groups on the net are liars.

Sako AV .375H&H Loaded with 300gr. Accubonds at 2 527fps. Std Dev was 5fps for the development routine (normally 14fps) Target set at 100m, shot off a bench, the POA was a 1" sticker.

 

[Andrew Leigh]

Think of altering the powder charge as a rough adjustment and OAL as a fine adjustment.

Are we in agreement that the objective is to meet the node optimum barrel time through these adjustments?

How far off the lands do you start on Accubond type bullets?

 

[fguffey]

I want my bullet to have the running start, I am the fan of bullet jump. I do not want my bullet setting at the lands when the trigger is pulled, I do not want my bullet to hesitate while the powder burns and builds pressure, I am not the fan of the second spike caused by a bullet trying to make up its mind if it is going or not.

I do want to know the length of the throat. I have checked chambers for throat length/free bore, I have pushed bullets of the case before the bullet contacted the rifling. Even then we managed to find something the rifle liked.

And I ask: How could something like this happen? Shrugging, drooping shoulders and receding hair lines is all I got.

F. Guffey

 

[gamestalker]

^^^ this.

I've been chasing lands almost since my inception into reloading 30+ yrs. ago, and with very good success. I don't own but one custom barreled action, so it's my belief and experience, that there is some measurable degree of improvement to be had with most production barrel's when approached correctly.

As 918V said above, it is a means of fine tuning what you have already accomplished with an existing load combination. So it's safe to say, that just because you are getting sub moa with a particular combination, you may not have the same results when landing a bullet with that same combination. In this respect, and experience, there is need for emphases on the powder and projectile utilized.

It was explained and demonstrated to me about 30 yrs. ago by a very experienced BR shooter, friend, and mentor. Now most of us know that BR shooters all have their own claims to various methods and technique's, some are argued as being nothing more than non sense, other's more proven, but landing the bullet is indeed among those methods that have merit. When using the right slow powder as per for a specific cartridge, you get a staging effect when the bullet is up too, or even into the lands. If you try it with a powder having a mid or fast burn rate respectively, the results can cause all kinds of issues, significant pressure spikes, loss of velocity, and ultimately accuracy is often degraded.

A couple of other things to consider that can effect one's success with this. It will most definitely benefit if you are using brass that IS fire formed to the barrel your working with. There is nothing to be gained, and probably loss of accuracy, if trying this with new brass or brass not formed to that chamber. The bullet won't align concentrically with the lands prior & during ignition, for obvious reason. So necked, exclusive fire formed brass is the ticket.

I start my landed loads with no less than a mid table charge of a good slow powder for that cartridge and bullet combination. To reiterate, the slower the powder, the better. Once I'm into the lands slightly, I chamber a round with the bolt, then when I remove it I check for the turn stripe left by the bullet getting rotated during bolt function, this lets you know you are decently into the lands, which IMO, and as related to me, that you have good contact sufficient to stage the bullet during initial pressure build. If the strip is only visible on one side of the olgive, then oal needs to be longer, until you have

I don't claim to have, nor do I have any in depth knowledge of internal ballistics. But the art of this type loading has been very successful for me with just about every barrel I've every loaded for. One such example, is a load I just worked up for a good friend for a 700 SPS chambered in 300 WM. He couldn't get that rifle to group with anything he fed it, and he went through a lot of reloads and factory, changed triggers, had a good builder lap and float the barrel, and the best he could get from it was 2" @ 100 yds.. This last week he brought it to me and asked if I could maybe give it a try. So I used IMR-7828, fire formed brass, and a cheap ol Speer 165 gr. Hot Core as my projectile. He shot it yesterday and was completely blown away with the groups he got, which were all under an inch. I loaded him 5 rounds at 60%, table and increased the charge at each 5 round batch, all grouped under an inch, with the higher end batch (85% of table) grouping right around 1/2".

GS

 

[Andrew Leigh]

Thanks for all the replies thus far. I appreciate the response.

At GS.

A couple of other things to consider that can effect one's success with this. It will most definitely benefit if you are using brass that is not fire formed to the barrel your working with. There is nothing to be gained, and probably loss of accuracy, if trying this with new brass or brass not formed to that chamber. The bullet won't align concentrically with the lands prior & during ignition, for obvious reason. So necked, exclusive fire formed brass is the ticket.

A little lost with this statement as it contradict the last sentence. The bold section, should that not read "brass that IS fireformed tot the barrel you are working with."

It has even got to the point that I get my shooting buddy the shoot the identical loads so we have an average of two shooters results.

Maybe I am overthinking this, my application is only hunting but it is nice to improve.

 

[gamestalker]

Thanks Andrew for catching that, I did in fact intend to write "Brass that IS fire formed for the action your loading for".

Sorry about that, old age catching up to me I guess.

GS

 

[taliv]

it depends on the bullet you are using. some are sensitive to it, others are not. as you stated in the OP, the angle of the lands and the angle of the ogive make a difference.


also, keep in mind what happens when the firing sequence. i don't know if this is true, but i've always thought at least one of those 'stops' was intuitively obvious. The mike referenced below is mike rock of rock creek barrels.

When Mike worked at Aberdeen proving grounds, the Army used high speed bore videos with mirrors, thermal imaging and computers to analyze any and everything that happens when the firing pin hits the primer and the round goes off. When the primer ignites there is enough pressure to move the bullet forward into the lands. The bullet then stops. As the primer ignites the powder, more pressure builds moving the bullet forward where it can stop again. Once there is enough pressure from the round going off, the bullet is moved down out the barrel. All of this happens in nanoseconds (billionths of a second). Your bullet starts and stops at least twice and sometimes three times before it leaves the barrel. This is fact.

http://www.snipercountry.com/articles/barrel_breakin_II.asp

assuming it's true, then people who seat INTO the lands are just eliminating one of those stops.

they are also changing the alignment because the bullet is still supported when it contacts the lands. consider your brass neck is probably at least 3 thou and sometimes 5-8 thou smaller than the chamber. so basically, after you've closed the bolt, the brass is sort of laying in the bottom of the chamber.

 

[Comrade Mike]

100m, I can go out to 300m if required.

You're not going to see the difference with COAL adjustment at 100m. At 500m it will become more obvious on the target.

As a previous member stated. Think of powder charge as your rough adjustment and COAL and concentricity as your fiend adjustments.

 

[moxie]

Fantastic!

This is a really advanced reloading thread.

 

[taliv]

when the bullet comes out of the barrel, there are basically only 2 things that matter: its speed and direction.

any group size issue that can be seen at long range can also be seen at 100 yards. the benchrest guys do that all the time. it helps to have one of their little measuring doohickeys if you're shooting in the 3s or less.

changes in velocity can be seen at the muzzle with a chronograph, or at long distance without.

maybe i'm missing something, but i don't think moving to 500 yards will show you anything about COL that you can't see at 100. i'd like to hear what those of you saying otherwise think it will show and why

 

[Comrade Mike]

Because it's going to take less precise measurement to see the difference at 500 than it would at 100.

 

[taliv]

that's true

but, you also have to account for things like wind at 500 that you didn't have to account for at 100. so i'd generally prefer to solve the precise measurement problem than isolate environmentals

i.e. "hey joe can i borrow your calipers" instead of "well these shots are definitely 4" apart, but i think the wind shifted 3 times while i was shooting my group"

 

[Walkalong]

I like to analyze at 100 or 200 yards. The farther you go out, the more wind is a factor. It is best to use wind flags when tuning loads. At least pay attention to the wind, and mirage.

I see all kinds of "tests" posted where people shoot one group with each charge weight and make decisions from that. One group means very little. The load has to repeat its self, over and over.

Some applications like the bullet into the lands, while some like the jump. Only way to know is to try.

I seated 6PPC bullets into the lands, but did many other things to go along with that. I seat the 168 Gr SMK in .308 to fit my mag, which means they have a pretty good jump, and they shoot very well.

There is a great deal of match ammo out there that has to fit any firearm chambered for it, and it shoots great in guns up to the challenge.

 

[gamestalker]

Agreed, once you get out past 200 yds. wind alone, even a light breeze, or gusts becomes a major variable, much harder to determine what's going on IMO.

GS

 

[918v]

Are we in agreement that the objective is to meet the node optimum barrel time through these adjustments?

How far off the lands do you start on Accubond type bullets?

I don't chase lands.

I start at an OAL where I have a solid cartridge, not some fragile piece of crap barely hanging on to the bullet. And it has to fit in the magazine.

I then use QL to get me to where the node is supposed to be (per the barrel node time chart) and load .5grs under and .5 grs over in .1gr increments. I pick the load with the smallest ES closest to the node.

I get pretty good results that way. It's usually within .2grs of QL's prediction. Adjusting QL to the particular powder lot is the biggest challenge.

 

[Centurian22]

You can look back in my posts and see that I recently debated whether to chase lands or not. So glad I did and I absolutely owe credit to gamestalker for the method of doing a powder work up close to the lands. Previously at 2.8" COAL my best was 0.8" with 40.9gr and 1.1" (@100yrds) with 43.6gr imr4895 in .308 under 168gr SMK's. After loading 0.010" off my lands and redoing a powder workup I got this:



0.375" center to center 4 shot group in the upper right with 41.2gr. As to mechanical vs pressure: I have no clue but awesome question and good read on the replies.

 

[Andrew Leigh]

I don't chase lands.

Generally neither have I as I have always wanted a round that will always chamber when hunting.

I start at an OAL where I have a solid cartridge, not some fragile piece of crap barely hanging on to the bullet. And it has to fit in the magazine.

I do this too.

I then use QL to get me to where the node is supposed to be (per the barrel node time chart) and load .5grs under and .5 grs over in .1gr increments. I pick the load with the smallest ES closest to the node.

Here we differ a little. I work out two loads in QL at the two fastest and safest OBT nodes. I then download 3 rounds with a mild load as foulers and 5ea of the anticipated nodes. I head off to the range and pop them over a chrony. I then take the average speeds recorded and add back the loss in velocity from muzzle to chrony and arrive at my "real" MV.

I then tweak and change the burn rate for each load such that the calculated MV in QL matches the measured velocity. Once that is done I then tweak the charge weight of powder to get be back to the OBT node that I want. This then becomes my load. It is from this load that further jumps changes etc. appear to make little difference. I do not change this load, I may change nodes but I trust QL (perhaps too much).

One quirk with gun powders appears to be that the same powder will produce different burn rates depending on a variety of factors;
Calibrev(burn rates for the same powder differ from my 6.5 to 30-06)
Case % fill / Pressure / Seating depth.
As the pressure increases so to does the burn rate.

For this reason I have found it necessary to have a burn rate for each load / set of components.