COL and Accuracy

[Andrew Leigh]

Could someone please explain to me if there is any science behind the COL / Jump you load to and accuracy (read tightness of groups)?

On local forums I repeatedly read of someone finding an accurate load and then tweaking the jump until the tightest group is achieved. Comments like "altering the jump will cause the group to miraculously close" I have played with the jump with no apparent change in group size. What am I missing here.

I can comprehend how different bullet ogives may require different seating depths due to the change in start pressure created on entering the lands by a more obtuse ogive shape.

Who can honestly testify to the fact that they have achieved noticeable results by altering COL and why?

 

[gamestalker]

I have been working with OAL, or jump to the lands, as it were, for about 30 or so years, and with impressive results, IMO. Most of the cartridges, rifles, and bullets I've worked with have grouped significantly better when up close to the lands. The cartridges and bolt actions I've worked with include .243 win, 6mm Rem, .270 win, .280 Rem, 7mm RM, 30-06, 308, and 300 WM.

The way I do it, is I work up my powder chargel at zero off the lands, and then if I don't achieve acceptable grouping at that oal, I begin working back in .010" increments until I've found the sweet spot for that bullet, rifle, and powder charge. I also work with only one exclusive head stamp at a time, sorted by weight, and fire formed in that chamber.

Not all barrels will produce improved results when seated close to the lands. One of the exceptions that comes to mind right off the bat, is two M77's I've loaded for, one was a 30-06, the other a .280 rem, and with both of those, the bullet needed a running start.

I've also had mixed results with Berger bullets, some have performed better when given a running start, others just the opposite. About the only way to really evaluate your results is to first make sure you have a good shooting system, so that you can eliminate as much of the human factor as possible.

Another element I've noticed, but it could be just coincidence, is faster burning powders don't seem to do well when up close to the lands.

GS

 

[Blue68f100]

I do it a little different. I do a normal load development. Then I work the range around the best node in smaller increments to make sure I'm in the middle. Then I start stepping toward the lands. I have had my best groups with Sierra Matchkings. I've had some guns that it just doesn't matter where your at on OAL. Mainly do to very generous chambers. Meaning your bullet will be forced to jump no matter what. I normally end up in the 0.015"-0.030" off lands.

Some BE shooter actually use very little neck tension and load long. Then use the chamber to push the bullet back into the case, setting at Zero jump. The only bad thing about this is that you have to load 1 round at a time. And you run the change of pulling the bullet if your forced to remove the round without firing it. Most of these shooters are using custom built guns.

 

[Andrew Leigh]

Thanks.

My experience is that the closer you get to the land the more pressure and the deeper you seat the bullet the more pressure. On a 7.62 Israeli Mauser recently I started at 0.012" off the lands and then incrementally I increased the jump by 0.012" (odd number but makes metric sense 0.30mm). The reduction in pressure at the lands would appear to have been offset by the increase in pressure in the case the average velocity moved a mere 17fps and this for a 0.08" move in COL. No change in group pattern or size.

I too load from new cases, I statistically cull by weight which normally results in only 35 cases from a batch of 50 remaining. Cases are ultrasonically cleaned every time, I trim every time, one or two will always need a little cleaning up. I double weigh charges during load development. I seat carefully and normally get std deviations of around 12-14fps. I use QuickLOAD as a load development tool

On my .375 I cannot get closer than .280" to the lands and this with 300gr. Accubonds yet she shoots MOA and better, have wrung a .24MOA group out of her. My 30-06 is 0.7MOA and my 6.5mm is .6MOA both with SGK's .08" off the lands (when I am driving them correctly that is).

My shooting buddy loads everything to SAAMI COL and most his rifles shoot well sub MOA and most .5MOA.

So it is with great interest that I follow this theory as if I am able to close my existing groups up by merely altering the jump I can get significant gains.

I believe the proponents of altering jump are experiencing real results but I simply cannot replicate this in my range outings. Why would that be?

 

[gamestalker]

I'm just spit balling here, but maybe it has something to do with the sequence. As I stated earlier, I start at zero off the lands, and rather than start with a minimum charge, I start with something around mid table, maybe a tad heavier. I feel like this provides the pressure necessary the expand the brass a bit quicker, thus providing better cartridge to chamber alignment, which I figure reduces stutter or chattering as the bullet starts. This is just a guess though, but what ever the reason I have experienced excellent results with my approach.

As well, I also feel slower burning powders help to facilitate improved accuracy when choosing to seat at zero, or there in. Again, I feel it is some how related to bullet start, and case expansion sequence. Maybe I'm way off base here and missing some other very pertinent details, but this is my thoughts on it.

GS

 

[Andrew Leigh]

Frankly I have always been a little too chicken to start off the lands. But now that I have QL I can relate that back to a pressure. So maybe one day.

If it works it works, however I am one of those odd people, I like a reason, a reason lets me apply this to all calibres. My experience is that Sierra's have a more obtuse ogive and therefore require more of a "running jump" at the lands while my hunting bullet the Accubond requires less as the ogive has a longer taper. Recently I was doing some reading an realised that rifle manufacturers all have a different angle taper on the leade, this also affects how easily the bullet gains access to the rifling.

Perhaps I do not experience a big change because I do a lot of research all other loads and COL's for the particular bullet and rifle combination. So it may be that I am starting with information that is good.

There are currently two plausible explanations for me on jump. The one is that if your bullets are not seated concentrically, the further you are from the lands the more chance the bullet has to go off course and to enter the barrel a slight angle, this means it exits at a slight angle with obvious consequences.

The other theory is that altering the jump is really a fine adjustment of the velocity which allows one to come onto a node. I am not a great believer in this theory though but the first theory for me makes sense.

Why the first theory makes sense for me is that in doing load development on my .375 I had bought some flat base 270gr. Hornady's (NEVER again). On seating I was merrily seating away at a reasonable speed. Well getting to the range I was all over the place. After some thought I realised that perhaps more care needed to be taken when seating a flat base bullet as entry to the case is a little more tricky. I altered my seating process to super slow, gently "feeling" the bullet into the neck. The results for the same load were truly chalk and cheese. I now seat all bullets with the utmost of care and this has been probably one of the biggest step functions in my drive for accuracy.

More recently I needed to re-crown a 7.62mm so I thought I would take the opportunity to drive a bullet into the muzzle and measure the bore. I proved that one can dive a bullet in the barrel at an angle and that the bullet will maintain that angle, it does NOT right itself in the barrel.

I think too that is why so many BR boys shoot off the lands, this automatically centres the round.

 

[gamestalker]

I agree with your theory 100%, and it's actually a pretty well established fact, as you said, ask your BR shooters why they do it, and I'm sure most will support your thoughts on this. Which is why I also relate my results to bullet start, it needs to get started straight to maintain consistent alignment as it travels down the barrel.

The same is said for how you seat your bullets, I too am a stickler for making sure my bullets get seated with the utmost care, which the RCBS competition seating die has accomplished quite well for me. So plain and simple, if it doesn't get a straight start into the barrel, it sure isn't going to fly very consistent once it leaves the barrel, thus producing unpredictable external ballistics.

And to help assure I am not messing up neck alignment during resizing, I always make sure the necks are adequately lubed inside, and out to reduce drag and chatter when withdrawing it. I use to think it was a bunch of hog wash that the ex pander button couldn't pull the shoulders up, or the necks off center, that is until I started checking my brass after sizing, wow! And another little trick I've been doing for a long time, is I leave the expander rod a little bit loose on top of the resizing die so it can float when I bring the ram back down.

GS

 

[jmr40]

I load to just fit the magazine. If it chambers without issue I shoot them, if not I seat the bullet a little deeper until it does chamber. If accuracy is good I'm good. If accuracy isn't what I want I experiment with shorter, but in most cases there is no need. The Barnes bullets are the only exception that shot better when seated deeper. I rarely even measure the length.

I'm a hunter and casual range shooter, not a serious target shooter. If it puts 3 shots under 1" I'm happy. I've never had any trouble finding a load that wouldn't do that. Often closer to 1/2".

 

[Andrew Leigh]

And to help assure I am not messing up neck alignment during resizing, I always make sure the necks are adequately lubed inside, and out to reduce drag and chatter when withdrawing it. I use to think it was a bunch of hog wash that the ex pander button couldn't pull the shoulders up, or the necks off center, that is until I started checking my brass after sizing, wow! And another little trick I've been doing for a long time, is I leave the expander rod a little bit loose on top of the resizing die so it can float when I bring the ram back down.

Mmmmmm, I see a problem in my process. I have been getting judder on some cases on movement over the expa button on the way back. I was given a set of RCBS dies in .308 which I am using to set up a load for a friend, due to being currently unemployed, I am unable to purchase the brand dies I want (this for a couple of reloads on a calibre I do not own).

I find the expanda button design on the RCBS dies to be of particularly poor design, the angles are rather aggressive so the friction I believe is counter productive. I am amazed that this is not a ground surface but a machined one. I am considering machining a new one, are the hardened fully or just case hardened. Any idea of what the correct diameter should be? Is the ball diameter .308 and the spring back causes the tension?

I like the idea of allowing the expanda button to self centre through the slight play that will be introduced.

I have a calibre sized nylon brush screwed into the mounting hole for the primer arm on the Rockchucker. This gets lube but obviously not enough. Cotton wool ear buds tend to disintergrate too rapidly to be a lasting solution. Any ideas?

A final point, the inside of my cases are squeaky clean before seating, how does this friction affect the neck and tension?

 

[spitballer]

An interesting thread and worth reading. It does seem to me that there's potential for slop with standard neck sizes, also.

In my case I have a standard .255" neck for a .224" bullet. Do the math: with .012" neck walls I get a total of .248". This leaves .007" for gases to blast by and throw the bullet out of whack if it's not centered by the lands.

 

[climbnjump]

I've also had mixed results with Berger bullets, some have performed better when given a running start, others just the opposite.

For those who like to read, Berger has a series of articles that address this on their website:

Getting the Best Precision and Accuracy from VLD bullets in Your Rifle

Effects of Cartridge Over All Length (COAL) and Cartridge Base To Ogive (CBTO) – Part 1

Effects of Cartridge Over All Length (COAL) and Cartridge Base To Ogive (CBTO) – Part 2


The first link gives their method for determining the best jump for their VLD bullets - a "sweet spot" that can very from in the lands to .150 off the lands.

Their hybrid or "classic" hunting bullets are by design less sensitive to jump and work well further off the lands than the regular VLD designs.