Can anyone explain bullet jump and its role in pressure and accuracy. I've read some on it, but don't understand everything I know about it.
9 replies [Last post]
Tue, 2006-08-01 23:18
Wed, 2006-08-02 11:14#1
I think that your refering to the movement of the bullet from the case to the lands befor the bullet starts down the barrel? If that's it, it's a kinda open question of how much is to much. what is sure is that if the bullet is jambed into the lands or touching, pressure will jump. Wish I could tell you why but I'm not sure. Some people go ahead and load ammo that way anyway and just back off the powder chg to reduce pressure. The theory is that with the bullet into the lands, you get better alingment of the bullet to the bore which insures that the bullet is not cocked even the tiniest bit as it starts into the rifled barrel. It's an accuracy thing that some people swear by.
Here's the thing tho. You really don't need a rifle that shoots well below 1/2" for anything other than competation or bragging rights! If that's what you want then it might be worth a try. I say worth a try as different individual rifles may not like it. In a competation rifle, when the bolt is closed on a round, the bullet is going down the barrel. At the rifle range, when the bolt is closed on a round, no matter what, it can be shot downrange. But if your in the hunting field and you decide to open your action and clear it for any reason and the bullet is stuck in the lands, you risk pulling the bullet and dumping the powder in the action, not a good thing. Or, if you use a crimp and the bullet is stuck in the lands, you may not be able to open the action at all in the field. This to gain accuracy you don't even need.
As for what makes to much jump, that's open to debate also. Different rifles shoot better or worse depending on the distance off the lands the bullet is seated (jump). Weatherby cartridges have extra jump built in. It's called freebore and what it does is increases case capacity without increasing the size of the case. When the Weatherby round is fired, the bullet moves up to the rifling and stops until sufficient pressure is generated to move the bullet down the barrel. Even with the freebore (jump) they have, I understand Weatherby rifles shoot very well.
I had one time a 264 Win Mag to work with. The owner was upset because the guy loading for him was giving him ammo that was to hot. Well he went to a gunsmith several times complaining of excessive pressure and the last time the gunsmith fixed his problem. A 140gr bullet could not be seated out far enough to react the lands. The gunsmith cleaned out the lands way up the barrel. Turns out, the guy that was loading for him was using H335 powder and to much at that. The powder burned to fast so peaked pressure way to soon, so the gunsmith gave him a lot of freebore that increased caes capacity to the point that he probally could have just dipped the case full of bullseye and been ok. The barrel was of course ruined!
Another time I had a 7mm Mag of my own cleaned out to take a 160gr bullet seated to the base of the neck, long throated. That was years ago and what I found was that I was able to increase my powder charge a good bit without a preassure spike. I could have still had the same charge without seating out the bullet because the long throat in fact increased case capacity.
The best way I've found to load is to make sure that all the bullets are seated a bit OFF the lands resulting in a small amount of jump to the lands. One thing you really don't want in hunting ammo is ammo that doesn't feed and function flawlessly. And should you be loading close to max, you sure don't want to find out that the small bit of jump that most the shells have is missing on ONE and have that one spike pressure because of zero jump.
Thu, 2006-08-03 04:52#2
The rule I go by is seating bullets 62/1000th off lands when using slow burning powder(H4831) and 31/1000th if useing fast burning powder, which I seldom use now. I read this in an article on Benchrest shooting many years ago, and Never had a problem with it
The greater the distance the bullet travels unrestricted before it goes down the pipe, the greater the pressure,the greater the pressure the greater thejump.
No idea if there is a formula for calculating this, but now I'm curious.
Thu, 2006-08-03 21:40#3
Thanks guys. I've been Pressing my bullets to the suggested OCL. I seen a gadget at Sportsmans Warehouse to measure the space with, but its nearly $50
Sun, 2012-06-24 19:53#4
Red Lip Stick
winknjack wrote:Thanks guys. I've been Pressing my bullets to the suggested OCL. I seen a gadget at Sportsmans Warehouse to measure the space with, but its nearly $50
I do thiis thing I read about in the American Rifleman long ago.
Take a fired case, spent primer and all, put your bullet just barely in the case, leave it sticking out, put the lipstick on the bullet, this keeps the bullet from sticking in the gun, put it in the action and close it, open it very carefully and remove the cartridge, measure the total length, this is your new oal. True some guns got so much distance, you need a bus ticket to go from the case to the barrel lands, I got one such gun. But that ole Remington should work. Hope this is what yur talkin about.
Fri, 2006-08-04 00:55#5
Every rifle shoots differently.
What I like to do is put a bullet in an unresized case leaving the bullet to stick out far enough for it to reach the lands. Coat the bullet with a marker so you will be able to see the marks from the lands and slowly push the bullet into the barrel and lock the action. This will give you the max oal to the lands. Set the seating die to .07 less than that oal and keep shortening it until you find the oal that work best in that rifle.
I think I explained that right. It's been a long day.
Tue, 2006-10-03 14:48#6
Fuzzybear and Don
You both seem very knowledgable and I have a problem that maybe you can help with. I have a Remington 700 model 264 that was made in 1962 according to the Remington code markings on the barrel. I have heard that the early model 264 barrels have a tendancy to wear out quickly and lose their accuracy. Mine may be to that point, but I am not sure. At 100 yards with the same hand loaded rounds it will shoot a spread of 4 - 6 inches primarily right of center, but not always. After 4 or 5 shots, the group gets a bit tighter so after several rounds, it does not look so bad, but you never know where the first shot is going to go.
Could this problem be caused by seating bullets too short or too long, barrel fatigue or some other reason? I can actully rule out most of the shooter error since I shoot a 300 Win Mag first and after that, the recoil from the 264 seems petty. I purchased the 300 win mag to replace the 264, but would like to keep it as a back up rifle if I can fix the inaccurate shot pattern.
Any thoughts or ideas?
Fri, 2011-12-02 08:56#7
your pesky 264
That kind of pattern is indicative of scope issues. Yes, check out the stock and I would also recomend using a quality copper solvent to clean any copper fouling out of the bore.
But next, put on a known good scope. I had a pesky Leoupold that would do that to any rifle I put it on, it would just open up the groups. I have it in a box ready to send back for service now.
As to throat erosion, you can see that for your self with a good bore light. But unless you really shot it allot, I doubt that is your issue.
Tue, 2006-10-03 21:06#8
When they get some years on them. Some stocks will get a bit of a warp and/or swelling in the barrel channel.
Make sure the stock is not putting any side pressure on the barrel.
If it was a free floated barrel. The barrel should not touch the stock at all. You can check it by sliding a dollar bill between the barrel and the fore stock. It should be clear to about and inch of the action.
If you have a slight warp. A quick fix would be some sandpaper and 6" or so piece of wooden dowel. The dowel should be a bit smaller in diameter than the barrel channel. Wrap the sandpaper around the dowel and work it up and down the channel. Refit the barrel. Tighten the back screw(s) first. Run the dollar bill again to check for clearance.
Another way to check if barrel restriction is the problem. Remove the barreled action. Lay a single layer of a match book cover in the channel where the front of the action seats. Reset barrel. This temporarily removes any restrictions.
If it shoots better. The barrel was being restricted and the channel in the stock will need to be opened up.
Some Remington are built with a pressure point, at the forward of the barrel channel. If it shoots better when you free floated it. Remove the pressure point and open the channel.
Fri, 2011-12-02 10:25#9
Any/each time you remove action & barrel from stock, (wood , synthetic, don't matter), make sure you retorque the screws to the same place or in/lbs. I have a rifle, Pillar blocked, that will double the group size if screws are not torqued evenly to the proper tightness.