Who jams their bullets into the lands, and who jumps them? Has anyone tried both and has experience with better accuracy in either? What does jamming your bullets do regarding muzzle velocity? I know the effects on pressure spikes, but what effect on MV, and is there any benefit on Std Deviation too (similar to crimping?)?
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Who Jams and who Jumps?
- Thread starter punkwood2k
- Start date
Been reading a lot and have learned that you should just about kiss the lands, and then work your way deeper. That way if you have to make seating adjustments you only have one way to go. I always started at .020 off the lands but from now on, I will start like .002 off then work my way back if I have to.
Forget jamming them. If you using this for tactical matches do not jam the bullets. You will have to eject a loaded round and you do not want to run the risk of sticking a bullet in the lands and dumping powder in your action, it SUCKS! On little kernel will jam you system up. Ya I used to jam my bullets for F class, they shot better but when a deer ran across the range and the did a cease fire and unload, my day ended.
Brain litz did a quick interview and the discussed the new hybrids
Bryan Litz Talks ? How to Get Best Results with Berger Hybrids « Daily Bulletin
Brain litz did a quick interview and the discussed the new hybrids
Bryan Litz Talks ? How to Get Best Results with Berger Hybrids « Daily Bulletin
I load my ammo for a .002 jump. I have gotten very good accuracy from my rifles using a little jump, so I have not seen the need to jam.
I jam them .010 with a 308, M24A2 long action.
Have gotten the best accuracy out of my loadings with this 'length' of jam.
Have gotten the best accuracy out of my loadings with this 'length' of jam.
I jump, simply because mag-length won't allow a jam. I also am working with heavy bullets, and they have long bearing surfaces, so they stay straight into the lands with no problem. I don't know a lot about it, but it seems from what I have read that one reason to jam is to help keep the bullet straight as it transitions from the neck of the case into the barrel. One side effect of jamming is that a lower charge weight of powder will yield higher pressure in the chamber in a jam situation. This is because it takes more to get the bullet started moving when it is jammed than it does when it jumps.
Well, specifically, I've read in a couple places that Jamming your rounds will decrease your Standard Deviation, giving you more consistent muzzle velocity, thus the better accuracy.. But also have read about the problem of pulling pullets while unloading, and dumping powder, so they recommend crimping your bullets if you Jam them..
I jam bottleneck cartridges for small groups.
I jump [crimp into the cannelure] straight wall cartridges for small groups.
I jump bottleneck cartridges for hunting reliability.
I jump [crimp into the cannelure] straight wall cartridges for small groups.
I jump bottleneck cartridges for hunting reliability.
Kissing the lands requires consistent bullets. This means you have to sort. And then the lands move away from you over time so you have to monitor throat wear. This is doable on a short throated chamber using bench rest grade bullets, but a pain in the ass on a factory rifle with common bullets from Sierra, Hornady, Nosler, etc. And then you end up with a round with the bullet just barely in the case. It won't fit in your mag. It may fall apart when dropped on the ground. Not worth it.
I'm thinking about changing my loads to a 0.010" jam.. My Savage magazine is huge, and still has room to spare even with bullets that long. I've run about 50 test chamberings with the loads, and havent had any pulled bullets, but I crimp mine pretty decent already for the consistent velocity.. Most of my shooting is going to be 600 yards plus, so I think I'll go for that little bit of accuracy from jamming my 175's.. Bullet is long enough to give me 1/2 caliber depth in the case still (0.18"), and seems pretty firm with the crimp.. I dont know, cant hurt to give it a try.. 
Been jumping for five years now, that will all change when i get away from remington barrels though, or i wont be jumping as much at least.
The barrel tells you what it wants for best accuracy. I shoot 6BR with 107SMK jammed but all of my tactical rifles get jumped because they feed from DBM. As for VLD, there is an explanation of how to find the proper seating depth for your rifle with VLD in the Berger manual. It says they may shoot best anywhere between hard jam and 50 thou off. I have also found that doing an OCW and then fine tuning bullet seating depth makes it easier for me to develop accurate ammo for any rifle.
I agree with Willys and would argue that the small bit of accuracy the you "might" gain from jump is outweighed by being able to unload your rifle without dumping powder all over the place. I know you're thinking that you can work around it but I've tried to operate like that and it's not worth the time. The first time you run your bolt without firing and dump powder all into your action you will want to kick yourself for even considering it.
If you even think that you will participate in a match or get some training with that rifle don't jam your rounds. You will have to unload without firing and it will ruin your day not to mention make you look like an ass out on the line.
If you even think that you will participate in a match or get some training with that rifle don't jam your rounds. You will have to unload without firing and it will ruin your day not to mention make you look like an ass out on the line.
If you gonna jam, or jump, do it at least .005 either side of engagement. .010", even better. There's enough variation in bullets, dies and/or press strokes to make "kissing" consistently a pipe dream. So pick a side and get far enough away from the fence that ya don't get hung up on it...
I do both.......for competition groups I jam.....even bergers..... for speed and semi autos I jump.....
Like others I jam VLDs (in my rifles seem to get better accuracy) and pretty much everything else I jump.
I always jump. Jamming is ok if you're shooting from nice covered firing points but if you're getting mucky and wet out in the field not so good in my experience. If you get any droplets of water on a jammed bullet it is possible, depending on how hot your load is that you are going to get a horrendous pressure spike, tricky extraction and a ruined case.
I did both...
You try loading a 110 Gr. halfjackets into the lands of a chamber cut for 190Gr slugs. LOL
In the end you'll get better results with a slight .002 (give or take) jump.
You try loading a 110 Gr. halfjackets into the lands of a chamber cut for 190Gr slugs. LOL
In the end you'll get better results with a slight .002 (give or take) jump.
I don't even jam benchrest rounds because cleaning powder out of an action at the range sucks. Instead I put a lot of effort into seating depth to get them to just touch - not enough to stick. For everything else, I jump and avoid bullets that will not tolerate that.
Okay, new question.. I just bought a R700 ADL 26" Varmint barrel. The mag length on this is VERY short, and the throat is very long.. so at a minimum, I have to jump 0.015". If you keep the powder charge unchanged, but move from a .010 JAM to a .015 jump (0.025" difference), would the MV stay the same? I understand the physics of pressure will drop going from Jam to Jump, but it will increase slightly because of the 0.025" less case volume from the bullet being seated deeper..
All jump, no jam. Have tried jamming but always get better results jumping. Some rifles even like to REALLY jump. I have a GAP built 338 Edge that likes the 300gr Berger OTM's .120" off the lands.
Tried them both and I have had a mix of all these posts. Your rifle will tell you what it likes after a few groups and looks at the chrono. Don't make yourself too crazy, finding the last spec of accuracy is not as important as laying in the dirt and learning the wind.
I jump the Sierra 155 .035" and the Berger 155.5 FB .070". There is no one size fits all answer. Each rifle will be different.
At one point myself and another shooter had identical Savage 12 FTR's. Shooting same identical brass, bullet, primer and powder we had totally different seating depths for accuracy.
At one point myself and another shooter had identical Savage 12 FTR's. Shooting same identical brass, bullet, primer and powder we had totally different seating depths for accuracy.
Do what your rifle shoots the best while still be reliable. I am expecting my custom 308 back anyday that shot great but you would get a bullet stuck in the bore if you had to eject. That is unacceptable for me of a gun that I will take out in the field. I had it rechambered and should be GTG now. I have kissed the lands starting out with a load. I rough up the bullet with scotchbrite and then seat until I get a mark from the rifling. Then I might turn my Wilson seather just a hair deeper and start from there. I'm looking for 100% field reliability first and foremost. That means mag length and not sticking bullets. Then the load development can begin. I want as much accuracy and I can get from that point. If I don't get what I'm looking for with charge weights I might play with seating depths but first going away from the bore. If still not happy I'll test what jamming will do and if that is the only thing that will make it shoot I probably wont be happy and go abck to the drawing board. F-class/benchrest shooters can be different. LE/Mil, Hunters, and practical match shooters will likely make their day worse by jamming. Maybe a little push isn't bad but you can deff go too far. Good luck
I am trying to under stand the jam jump discussion my self, I got a new gap 300 win mag(haven't shoot yet and going to starting load for it soon) I am going to run HBN coated 208gr. Amax out of it, to the original point will the increased pressure from jamming increase MV or will jumping the bullet with more powder to make the same pressure increase the MV?
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jamb up and jelly tight, or jump the moon. set em where the f*#k they shoot best and don't trash brass.
Jam/jump pressure variations and food for thought, the SAAMI interchangeability warning for the .223/5.56 did not come out until 1979. This is when the military changed over to the M855 round and lengthened the throats on military M16 rifles.
Below pressure variations with the shorter commercial SAAMI throat and the military throat. These pressures were taken with a strain gauge glued to the barrels and calibrated with a cartridge of a known chamber pressure. This method is not the same as direct pressure reading using the copper crusher or transducer methods
Contrary to popular belief the .223 and 5.56 are both loaded to the same rated chamber pressure (52,000 cup or 55,000 psi) and as long as the ammunition is fired in the proper length throat the pressures will be normal. The only reason I'm bringing this up is because the throat on my Savage .223 is "LONGER" than the throats on my two AR15 rifles. This is because today the chamber and throat must be large enough to fit the cartridge "AND" the company lawyer in.
Second I have Quickload and the guesstimates it software spits out, when you seat a bullet deeper than the manual specifies the pressure increases because you have decreased case capacity. As you seat the bullet longer the chamber pressure drops until you get closer to the lands and it starts to increase.
As long as you work up with your seating depth (jump/jam) and watch for excess pressure seating short or long is not a problem. The neck thickness and expander button diameter control neck tension therefore reducing the expander button diameter will keep jammed bullets from being pulled out of the case.
The accuracy part comes more into play when the bullet is supported only by the throat and the rear of the case by the bolt. This reduces case misalignment with the body of the case and neck when full length resizing. Adjusting seating depth is nothing more than fine tuning the load and hitting the node at the proper point.
The Rifleman's Journal
Checking Seating Depth
by: Germán A. Salazar
The Rifleman's Journal: Basics: Seating Depth
"A good place to begin (and frankly to stay unless you're already at the Master level in NRA Highpower) is to jump 0.020" for conventional tangent ogive type bullets such as the Sierra MatchKing line or Berger's new designs and to jam 0.015" for VLD type bullets such as Berger VLD, Hornady AMAX, JLK VLD, and others of the secant ogive design"
The Rifleman's Journal
Reloading: Partial Neck Sizing
by: Germán A. Salazar
The Rifleman's Journal: Reloading: Partial Neck Sizing
"a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway.
In conclusion, I believe that allowing the bullet to find a relatively stress-free alignment in the throat by full length sizing (including the neck) and turning necks to enhance concentricity gives the bullet the best probability of a well-aligned start into the rifling"
The comment below was also stated at "The Rifleman's Journal"
The late Jim Hull of the Sierra ballistics lab always joked that "the cartridge should fit the chamber like a rat turd in a violin case".
Below pressure variations with the shorter commercial SAAMI throat and the military throat. These pressures were taken with a strain gauge glued to the barrels and calibrated with a cartridge of a known chamber pressure. This method is not the same as direct pressure reading using the copper crusher or transducer methods
Contrary to popular belief the .223 and 5.56 are both loaded to the same rated chamber pressure (52,000 cup or 55,000 psi) and as long as the ammunition is fired in the proper length throat the pressures will be normal. The only reason I'm bringing this up is because the throat on my Savage .223 is "LONGER" than the throats on my two AR15 rifles. This is because today the chamber and throat must be large enough to fit the cartridge "AND" the company lawyer in.
Second I have Quickload and the guesstimates it software spits out, when you seat a bullet deeper than the manual specifies the pressure increases because you have decreased case capacity. As you seat the bullet longer the chamber pressure drops until you get closer to the lands and it starts to increase.
As long as you work up with your seating depth (jump/jam) and watch for excess pressure seating short or long is not a problem. The neck thickness and expander button diameter control neck tension therefore reducing the expander button diameter will keep jammed bullets from being pulled out of the case.
The accuracy part comes more into play when the bullet is supported only by the throat and the rear of the case by the bolt. This reduces case misalignment with the body of the case and neck when full length resizing. Adjusting seating depth is nothing more than fine tuning the load and hitting the node at the proper point.
The Rifleman's Journal
Checking Seating Depth
by: Germán A. Salazar
The Rifleman's Journal: Basics: Seating Depth
"A good place to begin (and frankly to stay unless you're already at the Master level in NRA Highpower) is to jump 0.020" for conventional tangent ogive type bullets such as the Sierra MatchKing line or Berger's new designs and to jam 0.015" for VLD type bullets such as Berger VLD, Hornady AMAX, JLK VLD, and others of the secant ogive design"
The Rifleman's Journal
Reloading: Partial Neck Sizing
by: Germán A. Salazar
The Rifleman's Journal: Reloading: Partial Neck Sizing
"a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway.
In conclusion, I believe that allowing the bullet to find a relatively stress-free alignment in the throat by full length sizing (including the neck) and turning necks to enhance concentricity gives the bullet the best probability of a well-aligned start into the rifling"
The comment below was also stated at "The Rifleman's Journal"
The late Jim Hull of the Sierra ballistics lab always joked that "the cartridge should fit the chamber like a rat turd in a violin case".
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After finding my node, I tested both jamming and jumping on my 308 and 338. In both cases, jump led to slightly larger groups but way higher SD's. With my bullets jammed into the lands, my ES is generally in the single digits with 5 rounds. (I have repeated this test many times over and over and with different chrono's). With my rounds jumped, my ES was 20-40. This still produces a "reasonable" SD according to many people, but why would I give up sub 5 SD's.
Some people mention unloading and emptying powder in their rifle.... I'm sure it does happen to them, but I have personally never experienced it , and I have loaded and unloaded many times. On top of that I barely have any neck tension on my 308 rounds, so you would figure it would happen to me more often than most. I'm not sure what separates my experience with theirs (maybe they have done it thousands upon thousands of times to get that malfunction?), but if it's something that happens 1 in 10,000 rounds I can live with that for better accuracy.
Some people mention unloading and emptying powder in their rifle.... I'm sure it does happen to them, but I have personally never experienced it , and I have loaded and unloaded many times. On top of that I barely have any neck tension on my 308 rounds, so you would figure it would happen to me more often than most. I'm not sure what separates my experience with theirs (maybe they have done it thousands upon thousands of times to get that malfunction?), but if it's something that happens 1 in 10,000 rounds I can live with that for better accuracy.
