Jump or jam? bullet seating for accuracy

guninhand

guninhand

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I always thought one never let the bullet of a hand loaded cartridge touch the lands due pressure spikes. Recently I've read on other forums that some shooters get good accuracy by jamming rounds into the lands as much as .001 inch or more. How many long range prescision rifle shooters on this site use the " jam" method? How many the "jump" method (bullet seated deep enough to not touch the lands)? If using the jam method, what process do you use to avoid over pressure?
 
Some of this debate also depends on the caliber, shape of bullet and even powder type. Over the years I have found that in my target .308 using 168 gr SMK, the bullets just touching the rifling gave me better accuracy than either a 0.010' jump or a jam into the rifling. However, in my .223 target rifle with 77 gr SMK, I get better results with 0.020" jump. I generally don't like to jam them because slight variations in COL cause more pressure variations that I'd like. I am not so worried about "dangerous pressure spikes", but more accuracy affecting pressure variations. I hope this helps.
 
guninhand said:
I always thought one never let the bullet of a hand loaded cartridge touch the lands due pressure spikes. Recently I've read on other forums that some shooters get good accuracy by jamming rounds into the lands as much as .001 inch or more. How many long range prescision rifle shooters on this site use the " jam" method? How many the "jump" method (bullet seated deep enough to not touch the lands)? If using the jam method, what process do you use to avoid over pressure?
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Every rifle is different , you cannot arbitrarily say that jamming or jumping is the way to go, one has to check with each rifle.
My 260 Fclass rig liked a .010 jump, but the Ruger No.1Fclass i got from Ed begg, needed them to be jammed .005 to get the accuracy .
My Palma gun likes the bullets just kissing.
Cat
 
I don't fit in either the "long range or precision shooter" categories...but I shoot at least 3000 rnds from bolt action centerfires every year. Today I shot 3 five shot groups from a 6.5 X 47L using Berger bullets jammed .010. All the groups could be covered by a dime which is what I expect from this rifle. When "jumping" the same bullets, I could not find a load that gave me the same consistency. I generally start a couple of grains under Max when developing loads for cartridges of this volume.
Most of the rifles I have "prefer" a small jump of .007 to .010 with most bullets. The Bergers I use seem to like to be jammed. I never jam a bullet that I am using to hunt!
 
Been shooting a wide variety of match bullets from all common brands. AS a personal policy, I dont jam but set just off the lands or whatever the mag will allow.

So far, don't jam Berger VLD's and not had a problem.

By tuning with powder charges, been able to get the rifles dialed in.

Not right or wrong, just my tech.

Info on my website if interested.

Jerry
 
ANd yet, I jam Berger VLD 6mm bullets up to 35 thou past first land contact , 20 thou for 6.5's.

I have NEVER used a VLD bullet that liked anything BUT being jammed, and even the tech article sent by Berger that suggesting jumping by as much as 120 thou produced terrible results.

Sierras all seem to like to kiss the lands.

Every gun is different, but after many years of Berger VLD experience, I would advocate 15 thou jam past first land contact as the starting point for load development.

Jamming is a pain sometimes, especially in cartridges that wear throats quickly. Having to load 250 rounds of ammo for a match on a 1000 round barrel means seating each batch of 5 rounds longer than the first.
 
The barrel on my primary target rifle shoots every bullet very well at .020" off the lands. I used to load them touching the lands but thought I would try .005", .010", .015", .020" and .025" off the lands. .020" off the lands shoots the best groups. I tried SMK's, Noslers, Bergers, Amax, all liked .020" off the land.
I must say that seems to be what this barrel (Bartlien) likes but with a new Krieger going on this year, I am sure things will change.
Lots of TR shooter here in Ontario are shooting 155.5 Bergers .040" off the lands.
 
Best accuracy results as regards OAL in relationship to lands, as pointed out by catenthehat or whatever his name is, depends on individual rifle and bullet and can only be determined by testing, which we assure, is what top shooters do to get best results.

Regards,

Peter Dobson
Lapua in Canada
 
My 6BR experience with 4 different Berger and Bart's bullets in a Savage LRPV is that a 0.010" jam gives me the best accuracy. That said it is not by much. I'm talking about 0.2" groups instead of 0.3", so very important to the paper puncher guys like me, and not so important to the minute of whitetail folk.

The down side is that especially with a VLD type profile the OAL is long, and may not fit into the magazine of a repeater. The other dirty little secret, that I don't think anyone has mentioned yet, is when you chamber the bullet and then decide to remove it instead of firing. If the neck tension is low, sometimes the lands hold the bullet and out comes the case with powder going everywhere in the action. If it is a fine powder you may be pooched until you take the gun apart and clean it. You also as minimum need a cleaning rod to get the bullet out. So unpleasant when it happens at the range, and down right nasty if you are hunting 3 miles from nowhere.
 
Obtunded and others have considerable experience, but the OP does not refer to Berger. I used to think that jamming was the only way to make the GB491 Lapua shoot. There are now confirmed winning results with jumping to .040" .

The books/booklets that used to be available from Sinclairs stress trying varying OAL as a key component to top accuracy for the particular rifle-bullet.

Regards,

Peter
 
Denis L., an amazing F-Class Shooter and ex-BR shooter and I have had this discussion many times and the reality is there are no rules. I always load develop by finding an optimal powder load first and then optimal seating depth, There are empiracle starting seating depth points for different bullet types. I start slightly jammed with VLD's and invariably work deeper. I start by kissing the lands with non-VLD-s and work in back from there.

Load development is a semi-voodoo art. I have chambers cut with the same reamer, in barrels made by the same maker, and even having sequential serial numbers, and one likes one type of bullet, and the next will not shoot them and like another.
 
guninhand said:
I always thought one never let the bullet of a hand loaded cartridge touch the lands due pressure spikes... If using the jam method, what process do you use to avoid over pressure?
Click to expand...

The most I have seated into the lands is 0.015-0.020". And by seating into the lands I'm using the zero point as just touching the lands. And, I do not confirm how much the bullet really does go into the lands. I just seat for example 0.010" long. In reality the bullet does go into the lands, but also since I only use 0.001" neck tension I suspect the bullet slips back into the case as well. Those who get real serious about this, use markers on their bullets and measure the mark the lands makes on the bullet. They have various targets of square marks or rectangular one way or the other.

I can't be bothered going to that extent. I just record how much long or short I seat and measure the group sizes I get. The group sizes tell me whether it is good or bad. Simple as that.

On pressure, my assumption is that if velocity goes up then I'm seeing more pressure. A buddy had a whoops and seated about 0.050" into the lands with I suspect a pretty stiff fit in the neck, and he saw lots of extra velocity and I'm sure pressure. But for a lighter neck fit with 0.015 into the lands I see almost nothing for extra pressure. Perhaps 10-15 fps out of 3400 fps. I consider that negligible.

The one place I do avoid is just touching the lands. While I can believe that is a good spot to seat at, I don't believe it can be done reliably, unless every bullet is measured to the ogive, and I'm not going to do that. Too time consuming. If one does load that way then you risk bullets having a mix of slightly off, right on, and slightly jammed. I can't see that being a good thing.
 
Obtunded said:
ANd yet, I jam Berger VLD 6mm bullets up to 35 thou past first land contact , 20 thou for 6.5's.

I have NEVER used a VLD bullet that liked anything BUT being jammed, and even the tech article sent by Berger that suggesting jumping by as much as 120 thou produced terrible results.

Sierras all seem to like to kiss the lands.

Every gun is different, but after many years of Berger VLD experience, I would advocate 15 thou jam past first land contact as the starting point for load development.

Jamming is a pain sometimes, especially in cartridges that wear throats quickly. Having to load 250 rounds of ammo for a match on a 1000 round barrel means seating each batch of 5 rounds longer than the first.
Click to expand...

If you're jamming .020 - .030" don't the bullets push back in to the case?
 
Depends on the neck tension it takes to keep them there.

One of the things I experimented with quite extensively, was annealing. This uniforms neck tension, and makes the amount of jam consistent from one case to the next.

Some guys jam seat 6mm bullets.... using 1 thou neck tension, and then use the bolt to seat the bullets. It works well for many scenarios, but if the neck tension varies, as is certainly the case with non-annealed necks, who really knows where the heck your bullets end up in relation to the lands?

Since annealing, I have upped the amount of neck tension I use to seat bullets to 3 thou. (from 1 thou) and while I cannot vouch for the amount the bullets may be pushed into the case, neither are the bullets remaining behind in the lands if a case must be ejected.
 
I seem to pick a place somewhere between zero (just on the lands) and 5 to 10 thou jump for most of my loads. As already mentioned, I also figured being just off would give better consistancy based on uniformity of the bullets.

I am shooting 155 berger VLDs currently in my 308 and maybe I should try seating deeper. But do you have problems with bolt closer with 30 thou stuck into the lands?
 
I shoot more MKs and Scenars in my target/varmint rifles than anything else, so I can only speak to them. IMHO, when the ogive of the bullet contacts the lands that effectively uniforms the bullet pull weight. If a bullet is jammed into the lands however, there is no guarantee the bullet enters the lands concentrically, but there are other factors at play as well; such as is the base of the bullet solidly against the powder column, or can the bullet push back into the case? Jamming seems to introduce more variables than it solves. I have not personally seen any radical improvement to accuracy by introducing a jump to the lands, even with bullets that recommend it.
 
warrenb said:
I seem to pick a place somewhere between zero (just on the lands) and 5 to 10 thou jump for most of my loads. As already mentioned, I also figured being just off would give better consistancy based on uniformity of the bullets.

I am shooting 155 berger VLDs currently in my 308 and maybe I should try seating deeper. But do you have problems with bolt closer with 30 thou stuck into the lands?
Click to expand...

The only time I have ever pulled a bullet when ejecting an unfired round is with a very light bullet with a very short shank and very minimal neck to shank contact, and it has never happened to me with a cartridge larger than 6mm.
 
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