Seating Depth

steelrat · April 3, 2008, 02:23 PM

How close can I safely get to the lands? Can I have the bullet touching or do I need to back it off a bit? I'm loading .243 winchester with Hornady 100gn btsp.

brickeyee · April 3, 2008, 02:27 PM

You can jam the bullet into the rifling IF you started that way doing load development.
If you already have a load, back it off if you are moving the seating depth around, especially if it is near a max load.

steelrat · April 3, 2008, 02:46 PM

I am developing this load, so is it better to start on the lands or off to start testing?

amamnn · April 3, 2008, 03:35 PM

A lot of people want to use BR techniques on factory guns. They are not always appropriate and sometimes possibly dangerous----so:

Jam seating is a technique used by a lot of BR shooters lately and one best left to them. Load data that you find in manuals and on the www published by bullet and powder makers is done at the COAL that they publish. The safety margins for pressure are not as wide as a lot of people think. The closer to the lands you seat your bullet, the more you increase start pressure. if you began at a minimum powder charge as noted in the above post, and proceeded along keeping a sharp eye out for the TRUE signs of incipient overpressure, you could work up a jam seated load---if your rifle has a tight enough chamber to jam the bullet into the lands and still keep enough bearing surface in the case neck to function properly. These days, that's not likely in a factory rifle. Start pressure does not increase linearly as you approach the lands. Powders burn differently under different pressures. What was a safe charge at .010" off the lands might be dangerous in a given chamber at jam seating.

Jam seating is not necessarily the best method for accuracy even in BR rifles. I have one that likes a jammed load and another of a different caliber that likes the loads to be .020" off the lands. Most casual shooters misunderstand the reason that jam seating can be of use in the appropriate chamber. There is no real benefit beyond control of the start pressure, making it--while quite higher than standard seating--more uniform, therefore more predictable. Gas seepage past the bullet is also controlled to an extent if boat tailed bullets are used. Bullets do not "drop" between ignition and the point in time where the ogive contacts the lands.

Time spent on breath control, wind reading, practicing follow through--yes there is follow through--even on the bench-- will do a lot more for accuracy than feeling around for the lands on a factory rifle. Learning about neck tension and possibly investing in a good neck sizing die would be more advisable for controlling start pressure in a load for a factory rifle.

If you have just recently gotten into BR shooting or long distance varminting after purchasing a custom gun, then jam seating might be for you-- or not. You can surely find lots of information on the subject in the archives on benchrest.com, where Jackie Schmidt is a jam seating guru. 6mmbr.com is another good source for archived material on the subject. If you are truly interested in getting the best accuracy from a custom gun, you might also want to subscribe to Precision Shooting magazine.

SL1 · April 3, 2008, 03:47 PM

If you are using a regular jacketed bullet, try about 0.02 - 0.03" off the lands to start. And, try moving away as well as closer.

But, if you are using a solid copper bullet like the Barnes Triple Shock, then start at least 0.05" away and do not try to get the bullet all the way to the rifling. Really thick-jacketed bullets like Noslers probably should not be started against the rifling, either.

Also, realize that the best seating position is not necessarily independent of the powder charge. Changing powders, or even changing the charge weight with the same powder, may lead to a different optimum seating depth.

You probably do not want to try to vary seating depth with the lightest "start" charges, because they are often not the most accurate. I would select a seating depth that is about what the bullet manufacturer calls for, work up my load with my chosen powder until I find the most accurate one, then vary my seating depth A LITTLE around that. If things keep getting more accurate as I approached the rifling and my charge weight is near max, I would reduce the charge a little and see if that reduces accuracy any, before deciding whether to go forward. If I found a sweet spot in my seating depth, I would then recheck for a different sweet spot in charge weight, both a little more and a little less.

In all cases, be sensitive to signs of over-pressure. If you have a strong bolt-action rifle, you should not get into safety problems if you are sensitive to pressure signs and make only small changes at a time. But, if you get a load that you like that is really too hot for your gun, you will wear-out your barrel and maybe your action too fast by shooting it repeatedly.

SL1

brickeyee · April 3, 2008, 06:58 PM

I would stay off the lands for a typical factory gun.
0.020 is a good starting point.

ZeroJunk · April 3, 2008, 07:34 PM

I'm curious about the potential difference in accuracy between, for instance, being .06 or .006 off the lands. Is it potentially .1 MOA , 1 MOA or what? Years ago for hunting and casual target shooting I would find the lands and back off 1 turn of the die. Later on, I would try to get within a few thousandths. Now, I may just load to the OAL listed in the manual.

To be honest, I don't see much difference. Of course, I am far from being a competitive bench rest shooter.

April 3, 2008, 02:23 PM	#1
steelrat Member Join Date: February 8, 2007 Location: Utah Posts: 66	Seating Depth How close can I safely get to the lands? Can I have the bullet touching or do I need to back it off a bit? I'm loading .243 winchester with Hornady 100gn btsp.

April 3, 2008, 03:35 PM	#4
amamnn Senior Member Join Date: May 13, 2006 Location: WA, the left armpit of the USA Posts: 1,323	A lot of people want to use BR techniques on factory guns. They are not always appropriate and sometimes possibly dangerous----so: Jam seating is a technique used by a lot of BR shooters lately and one best left to them. Load data that you find in manuals and on the www published by bullet and powder makers is done at the COAL that they publish. The safety margins for pressure are not as wide as a lot of people think. The closer to the lands you seat your bullet, the more you increase start pressure. if you began at a minimum powder charge as noted in the above post, and proceeded along keeping a sharp eye out for the TRUE signs of incipient overpressure, you could work up a jam seated load---if your rifle has a tight enough chamber to jam the bullet into the lands and still keep enough bearing surface in the case neck to function properly. These days, that's not likely in a factory rifle. Start pressure does not increase linearly as you approach the lands. Powders burn differently under different pressures. What was a safe charge at .010" off the lands might be dangerous in a given chamber at jam seating. Jam seating is not necessarily the best method for accuracy even in BR rifles. I have one that likes a jammed load and another of a different caliber that likes the loads to be .020" off the lands. Most casual shooters misunderstand the reason that jam seating can be of use in the appropriate chamber. There is no real benefit beyond control of the start pressure, making it--while quite higher than standard seating--more uniform, therefore more predictable. Gas seepage past the bullet is also controlled to an extent if boat tailed bullets are used. Bullets do not "drop" between ignition and the point in time where the ogive contacts the lands. Time spent on breath control, wind reading, practicing follow through--yes there is follow through--even on the bench-- will do a lot more for accuracy than feeling around for the lands on a factory rifle. Learning about neck tension and possibly investing in a good neck sizing die would be more advisable for controlling start pressure in a load for a factory rifle. If you have just recently gotten into BR shooting or long distance varminting after purchasing a custom gun, then jam seating might be for you-- or not. You can surely find lots of information on the subject in the archives on benchrest.com, where Jackie Schmidt is a jam seating guru. 6mmbr.com is another good source for archived material on the subject. If you are truly interested in getting the best accuracy from a custom gun, you might also want to subscribe to Precision Shooting magazine. __________________ "If the enemy is in range, so are you." - Infantry Journal

April 3, 2008, 02:27 PM	#2
brickeyee Senior Member Join Date: December 29, 2004 Posts: 3,351	You can jam the bullet into the rifling IF you started that way doing load development. If you already have a load, back it off if you are moving the seating depth around, especially if it is near a max load.

April 3, 2008, 02:46 PM	#3
steelrat Member Join Date: February 8, 2007 Location: Utah Posts: 66	I am developing this load, so is it better to start on the lands or off to start testing?

April 3, 2008, 03:47 PM	#5
SL1 Senior Member Join Date: November 8, 2007 Posts: 2,001	If you are using a regular jacketed bullet, try about 0.02 - 0.03" off the lands to start. And, try moving away as well as closer. But, if you are using a solid copper bullet like the Barnes Triple Shock, then start at least 0.05" away and do not try to get the bullet all the way to the rifling. Really thick-jacketed bullets like Noslers probably should not be started against the rifling, either. Also, realize that the best seating position is not necessarily independent of the powder charge. Changing powders, or even changing the charge weight with the same powder, may lead to a different optimum seating depth. You probably do not want to try to vary seating depth with the lightest "start" charges, because they are often not the most accurate. I would select a seating depth that is about what the bullet manufacturer calls for, work up my load with my chosen powder until I find the most accurate one, then vary my seating depth A LITTLE around that. If things keep getting more accurate as I approached the rifling and my charge weight is near max, I would reduce the charge a little and see if that reduces accuracy any, before deciding whether to go forward. If I found a sweet spot in my seating depth, I would then recheck for a different sweet spot in charge weight, both a little more and a little less. In all cases, be sensitive to signs of over-pressure. If you have a strong bolt-action rifle, you should not get into safety problems if you are sensitive to pressure signs and make only small changes at a time. But, if you get a load that you like that is really too hot for your gun, you will wear-out your barrel and maybe your action too fast by shooting it repeatedly. SL1

April 3, 2008, 06:58 PM	#6
brickeyee Senior Member Join Date: December 29, 2004 Posts: 3,351	I would stay off the lands for a typical factory gun. 0.020 is a good starting point.

April 3, 2008, 07:34 PM	#7
ZeroJunk Senior Member Join Date: August 14, 2006 Location: Browns Summit NC Posts: 2,589	I'm curious about the potential difference in accuracy between, for instance, being .06 or .006 off the lands. Is it potentially .1 MOA , 1 MOA or what? Years ago for hunting and casual target shooting I would find the lands and back off 1 turn of the die. Later on, I would try to get within a few thousandths. Now, I may just load to the OAL listed in the manual. To be honest, I don't see much difference. Of course, I am far from being a competitive bench rest shooter.