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gravityfan
01-24-2007, 07:09 PM
Please bear with me, I have only been reloading a few weeks. Love the technicality of it all, but am confused by the discussions of Bullet Jump.

Where I can't get to grips with it is that I read people saying that .020 of Bullet Jump is best, some say even down to zero bullet jump. Some say certain bullets need a longer Jump. Folks talk about adjusting the Jump up or down by .005, and so on.

However, if you look at the reloading tables in, say, the Lyman 48th. The COAL for different weight bullets may be different by up to 1/4 inch.

So, if I use one of the lighter bullets, and the recommended COAL for that bullet, am I not introducing a bullet jump change of an extra 250 thou? How can that be right when people are talking about 20 thou or less?

If I seat a lighter bullet anywhere near the COAL for a heavier bullet, then it's hardly in the neck and obviously unusable, so that's not an alternative.

I had found by trying different load recipes that my rifle "appeared" to prefer a heavier bullet. Now I'm wondering if it just prefers a longer bullet giving shorter Bullet Jump. Maybe I need to find a long, light bullet to try?

It almost seems to me that if you use a shorter bullet, you should case trim to a longer length, so you could have a consistant bullet seating, and a consistant Bullet Jump. That's to say having different length brass to suit differing length bullets (whether to the tip or the ogive, whichever). However, this is not the case, so I'm missing the point somewhere.

See what I mean? If my case TTL is 2.212, and that gives me optimal seating and Jump for a 180 Gr bullet; if I want to use a 20 thou shorter 125 Gr bullet, wouldn't using a case TTL of 2.232 give me identical seating and Jump? Like I say, I must be missing something, otherwise why is this not what one does?

OK, enough drivel, I hope I've explained my dilemma. I'd appreciate the views of those with far more reloading experience.

Cheers, all,
gravityfan

versifier
01-24-2007, 09:04 PM
COAL is a "standard" to (hopefully) insure that your ammo will function through most any action chambered for the cartridge in question. But, with single shots and many bolt actions, one can often seat the bullet way up close to the origin of the lands. The limiting factor with bolt actions is that the max length cartridge may not feed through the magazine, so one must either settle for maximum parctical (functional) length, or shoot it as a single shot. Obviously, there are physical limits, especially with short, lightweight bullets. Then, too, with some short throated chambers, there is insufficient room to seat long, heavy bullets all the way out, or an extremely long throat forces you to use the longest possible bullets to get any accuracy at all.

Now that I have you thoroughly confused :mrgreen:, if it is possible with your action, chamber/throat, and the bullet you have chosen, you will generally find the best accuracy with a bullet jump of .002". Or, your particular rifle may like .004", or seating to touch the lands. The only way to find out what yours likes best is by actually experimenting, because every rifle is different.

gravityfan
01-24-2007, 09:47 PM
Thanks so much for taking the time to reply. Much appreciated.

OK, you're saying that a Bullet Jump of between 2 and 4 thou is best - that's TINY!

It's also impossible with almost all the published recipes in the reloading manuals, except the heaviest bullets, thus my theory that it is length not weight that my rifle prefers.

So would you advise that I use the longest (therefore probably heaviest) bullet in the manuals and try moving the COAL up to almost touching the rifling?

I'm not planning to use some expensive piece of kit to do the measurement, just a dowel down the bore and compare a standard COAL chambered round with a bullet gently held in place against the throat with a pencil.

So I'm guessing you think that if I do that and find that the difference is significant, that I seat my bullets less deep and try again? Naturally, I will work up the powder loads all over again to check for overpressures.

Am I on the right lines?

Cheers.

gravityfan
01-24-2007, 09:49 PM
BTW - single shot loading is no problem, I don't care at all about how the round fits in the mag. I tend to single load anyway so as to ensure no tip damage from recoil.

Cheers.

lovedogs
01-25-2007, 12:14 AM
Versifier is a real knowledgable guy and can help you a lot. He's real good at long explanations. Better than I am. But I'll give you a little today, also.

If I understood it right, you touched on case length. On this you don't want to let your cases get too long. Keep them trimmed to recommended length to avoid excessive pressures. That's another topic. If you have questions on that I'm sure we can answer them, too.

Versifier is correct in his stating that short jumps into the riflings are usually more accurate. That, however, isn't always the case. And, sometimes, it's not practical. As he mentioned, sometimes magazine length won't permit seating your bullets way out there. Another thing to consider is that such short jumps raise pressures. If you start low you shouldn't have a problem. But, in most cases, such short jumps just aren't practical. This is mostly a benchresters thing to find super accuracy. But with the accuracy levels possible in most sporter rifles it's to no advantage. If you're into super benchrest accuracy you might try "kissing" the riflings with the bullet and then backing up a thousandth at a time to find your best accuracy.

But in most sporters you don't want to be that close. And, in fact, often you can't fit the rounds in the magazine if you do that. You also want them, in most cases, to feed reliably. Usually, I find a good compromise at between .010 to .020 jump into the riflings. Rifles usually feed better with bullets seated to an OAL .020 off the riflings. If I don't get accuracy there then I start seating the bullet out farther to see if it will get more accurate. Most of my rifles shoot fine with a .020 jump. In fact, I have one old 788 .22-.250 that due to magazine constraints has to have the bullets seated way far off the riflings and it's a 1/4 inch gun. Not many factory sporters will shoot that well.

Keep asking questions. That' how we learn. Remember... the only stupid question is the one you didn't ask!

kodiak1
01-25-2007, 12:41 AM
What lovedog said X2
Ken

gravityfan
01-25-2007, 04:20 PM
Thanks again for all the input. Much appreciated and thanks for your time.

I guess I'm still not quite getting it.

My point is: if the position of the bullet ogive relative to the throat is reasonably sensitive, why is the case TTL the same size even when using a much shorter bullet? If the bullet is .25" shorter, wouldn't a TTL .25" longer provide the same COAL and therefore a better fit to both the chamber and the mag?

I'm not worried about cartridges fitting the magazine. In this case, they fit all too easily - they are shorter by a quarter of an inch! They give me a Bullet Jump of over 250 thou. If my cartridge was .25" longer, then the completed round would fit the mag and the chamber, and give me a more normal Bullet Jump, wouldn't that be the case?

I must be missing something.

Cheers.

klausg
01-25-2007, 06:21 PM
gravity-
The case length is also determined by the chamber; SAAMI specs determine how big everything is. If I am understanding your confusion correctly, I think you are confusing the leade or throat area with the neck area of the chamber. They are two different diameters; I wish I had a picture of a chamber cast as it would show this very easily. I'll attempt to explain it anyway, please forgive any bastardization of terms. Think of the chamber as everything forward of the bolt face until you hit the rifling. The first part is to accommodate/support the case; as you get into the neck area of the cartridge, remember that area has to be large enough to accommodate the diameter of the bullet plus the thickness of the brass surrounding it. Then you come to the leade or throat area which only has to be large enough for the bullet itself, so there is a change in diameter. If they cut the chamber without that change in diameter, then the bullet would have that much more space to get itself canted before entering the rifling, (with a corresponding decrease in accuracy). So by increasing the case length you end up in effect crimping that excess length into the bullet, (this equals a major pressure spike), up until you get to the point where it won't even chamber . Hopefully this made some semblance of sense and clears up your confusion.

-Klaus

versifier
01-25-2007, 06:30 PM
Klaus, you put your finger right on it.
The chamber is cut to the specs of the case of a specific neck length. (SAAMI was formed to standardize modern cartridge/chamber dimensions.) If your TTL is too long, when a loaded round is chambered (with difficulty!), it compresses the top of the case's neck into the sides of the bullet because there is a step in the transitional area at the beginning of the throat. (Kind of like the crimping shoulder in a seating die, but a greater distance from the head of the case.) If you examine a good illustration of a rifle's chamber, you can see it clearly. A too long TTL inhibits the bullet exiting from the case because the brass cannot move outward to release it properly, dramatically increasing chamber pressure. Max case length is where the neck of the case almost contacts the step, and TTL is a safety factor designed to accomodate the lengthening of the case that occurrs in resizing for a number of loadings before trimming is again necessary. Max case length is an absolute for reasons of safety. In theory, it might be nice if the neck and throat of the chamber were a straight line enabling you to adjust the case neck length to suit each individual bullet, but in practice it would increase the instability of the bullet as it made the jump to the origin of the lands, like one would see in a rifle with a severely eroded throat.

You do make a valid point about varying the neck length to suit the bullet, though, but it does not apply to sporting rifles such as you and I would be shooting and reloading for. Many benchrest rifles are custom chambered so that the max length is specific to the chosen bullet, and in these rifles, the fit of case to chamber is so precise that the neck thickness is very carefully arrived at by turning or reaming the excess brass. These necks have just the barest minimum of room to expand for the exiting bullet, are super carefully neck sized with ultra precise specialty dies and may last for hundreds of firings. You may not be able to use anything other than that one specific bullet in the custom chamber, however. In a BR chamber, there is minimum expansion of the case body upon firing also, so the brass case body need not stretch appreciably to fill the slightly over size chamber like that in a sporting arm, and does not need to be resized. (Hence the long case life.) If you tried to set up a sporting rifle that way, you would have a very difficult time at best loading and feeding, not to mention an hour or more spent on fitting each case to the chamber initially.

gravityfan
01-25-2007, 07:17 PM
Guys, thank you. The mist parts, albeit slowly.

I'm visualising now, and I see what you mean. The cartridge has to fit the first part of the chamber, and cannot be any longer as it would be too wide for the throat.

What I was not seeing in my mind is that my "argument" is with the throat. The throat has to be long enough to accomodate the longest allowable bullet. So if you use a shorter bullet, you have no choice but to have a longer Bullet Jump. The brass has to end at the Max Length point, and cannot be any longer or you would have chambering and/or pressure problems.

So short bullets are always going to be a pain. If I am to try different weight bullets, I need to find ones that are not hugely shorter even if lighter.

At last - great explanations and my sincere thanks.

One step further in my understanding.

Cheers.

klausg
01-25-2007, 09:24 PM
gravity-

So short bullets are always going to be a pain. If I am to try different weight bullets, I need to find ones that are not hugely shorter even if lighter.


Not always... I think one of versifier's favorite phrases on this forum is "...every weapon is different..." or words to that effect, (v-please excuse any error in that quote). It all depends on how your chamber is cut. For instance I have a .222/20 ga Savage M24 with a very long throat. I get around that by seating my bullets out as far as possible, as in just enough that they don't fall out. I am WAY over the COAL as listed in any manual, I get away with it because: a) it's a single shot, and b) it is SAFE in MY WEAPON. You ask why I don't just use a longer bullet, say a 70 grain? The twist won't support that bullet length, (mine is a 1 in 14", I wouldn't even try those longer bullets in anything slower than say 1 in 10"). Years ago, I remember reading an article on getting a .30-06 chamber re-cut with a longer throat to enable seating bullets out further. The idea was to increase the available powder capacity on the 220 grainers. So, you see there are a lot of different variables out there; the only one who can tell you what's going to work best is YOUR WEAPON. Be safe about it, start low & work up; when you get stuck on something or have any questions, post here and generally someone has already been there and will help you out. Take care

-Klaus

versifier
01-25-2007, 10:47 PM
Interestingly, some rifles don't care and seem to shoot everything well, even short bullets, regardless of how far they have to jump. You can only find out by experimenting.