Bullet seating depth experimentation...

SEHunter · December 22, 2009, 09:38 PM

This is kind of a general thought but came about with working up loads for my 22-250 with a 50gr C/T ballistic silver tip bullet. I realize ultimately that trial and error is what determines the depth for a particular gun/load but to seat my bullet out to .10 or .20 from the lands in my Rem. 700, there is not near enough bullet left in the case neck if the "width of bullet diameter" method is used to ensure proper bullet alignment. That being the case, i decided to make the base of the bullet even with where the neck joins the shoulder which allows bullet-to-brass contact to begin just up from there a little since it is a boat tail. I was just wondering if the base being out of the shoulder would some how help create some type of even pressure distribution to better launch the bullet. Dont know if that makes any sence to you all but basically, i was wondering if any of you used any type of analogy like that when trying to be a certain measurement off the lands/rifling was not an option.

Win1892 · December 22, 2009, 10:06 PM

Get a longer projectile. I have a number of "bench" guns that I try to seat within 5-6 thousandths of the rifling, sometimes lightly touching the lands. 223, 308, 270, 6.5-284, 338 Lapua, etc.

When using a lighter projectile I try to keep at least a third of the neck full of bullet. Don't hunt with these! They are prone to damage, or just plain falling apart.

**Unclenick** · December 22, 2009, 10:08 PM

No to the OP's general thought (if I've got it right?). If you analyze pressure against a boattail you can separate it into pairs of force vectors that have a forward component and an inward (toward the bullet axis) component. When you add up the forward vectors and add them to the forward force on the flat part of the base, the net result is the same as the force applied to a flat base bullet. The same will be true regardless of where the bullet is seated because inward pressure toward the bullet axis produces no net propulsion force.

The normal advice is to seat the bearing surface of the bullet one caliber into the case. The bearing surface is the cylindrical groove diameter portion of the bullet. That means the boattail doesn't count as part of the seating depth for that purpose. It does, however count for increasing pressure over what you would get from the same charge with a flat base bullet of the same weight whose bearing surface was seated to the same depth. That is because the boattail eats up room in the case. The exception would be if seating the flat base to that same depth caused its ogive to get enough closer to the throat to cut gas bypass at bullet release, thus raising pressure by that different mechanism.

Another thing to keep in mind: One of the chapter authors in the Precision Shooting Reloading Guide mentioned how he one day accidentally turned the micrometer on his seating die the wrong way to make a final adjustment. As a result, he wound up with 50 rounds seated 0.050" off the lands instead of 0.020" off the lands as he had intended. He debated pulling the bullets, but decided to shoot them for practice instead. When he did, to his amazement, the gun got the tightest groups it had ever shot. Tighter than with bullets 0.020" off the lands. So, the lesson is the old one that your gun isn't the other guy's gun and you will have to find what it actually likes? There is no magic number for seating depth except the one that works in your particular gun.

SEHunter · December 23, 2009, 07:00 AM

Thanks guys. I had a feeling i was not on to anything but i just put that out there anyway. I wont be able to use larger grain bullets to allow mw to be close to the rifling because my barrel is 1:14 twist and i have read it most likely will not stabilize heavier bullets than 55gr. I will just seat it maybe a little less than one diameter and go from there.

**Unclenick** · December 24, 2009, 03:23 PM

The main advantage of seating up close is to get the tip of the bullet in the throat before it gets going too fast to self-center in the throat's taper. If you seat deeper, but get the bullet straight into a straight case, then that makes a lot less difference. The Forster and Redding sliding sleeve competition seaters help with that. Maximizing bullet bearing surface also helps. That means flat base, not a boat tail. I recommend trying the flat base 53 grain Sierra MatchKing for that reason. It is a tack driver in my .222, which also has a 14" twist, but which has never liked boat tails much. To keep the case straight, the inexpensive Lee Collet Die for neck sizing only is just about impossible to beat at any price.

gearheadpyro · December 24, 2009, 05:52 PM

I figure out optimum seating depth as a part of my load development. I determine powder charge first, then load a few shots of progressively longer lengths. The one that groups best gets tweaked until I'm happy with it.

I wrote an article about my load development, (http://www.rifles-shooting-reloading...velopment.html).

Hope that helps.

SEHunter · December 24, 2009, 09:52 PM

Unclenick, are the matchking bullets for hunting or target only? I have heard many good things about them but thought most bullets that were termed "match" were not intended for use on game. (although with varmits, its not like any bullet thats traveling 3600+ fps would not yeild the desired result) Im using the 50gr ballistic silver tip now but i weighed a few and between the lightest and heaviest, they were about a half grain difference in weight. I dont know if that is considered a significant difference or if it will affect accuracy, but i may have to change brands if it does. I disassembled a factory hornady round with a v-max and it weighed 50gr on the dot.

Gearheadpyro, i understand what you mean but i guess i have always thought seating depth alone would determine if any given bullet/powder combo would be at its best. I guess thats why i have always been intimidated by finding the right depth because i hate to think i cross off an excellent load when the only reason it performs poorly is because i dont have a good seating depth, you know? I mean, how do you know, because if .10" makes or breaks the results then it would take forever (or good luck) to not just toss that load from thinking its not a good combo. The onlything i know to do is pick the best performing load of several that have the same depth, then test the depth on that one load. As it sits now, to have decent bearing surface in the case, i am about .45" away from making contact with the lands.

wncchester · December 25, 2009, 12:30 PM

"i was wondering if any of you used any type of analogy like that when trying to be a certain measurement off the lands/rifling was not an option."

No analogies need apply. Nor seating one caliber deep, nor using boat tails in most instances.

The one caliber deep "rule" thing isn't meant to align bullets, as such. It's just a nice way to insure that normal handling doesn't dislodge a bullet. Nor is seating into, or even close to the lands, an automatic aid to accuracy. If the bullet is loaded straight and the rifle is chambered straight it will enter the rifling straight.

Boat tailed bullets are worth trying for accuracy but they offer no ballistic advantage until the velocity drops below the speed of sound, about 1,200 fps. Most people obtain better accuracy with flat bases except at very long ranges. Any two bullets with the same BC and speed will have virtually the same trajectory at any practicle range, no matter the shape of the base.

jiw275 · December 27, 2009, 10:33 PM

A great source for cartridge information.

Mike

December 22, 2009, 09:38 PM	#1
SEHunter Senior Member Join Date: October 6, 2009 Posts: 332	Bullet seating depth experimentation... This is kind of a general thought but came about with working up loads for my 22-250 with a 50gr C/T ballistic silver tip bullet. I realize ultimately that trial and error is what determines the depth for a particular gun/load but to seat my bullet out to .10 or .20 from the lands in my Rem. 700, there is not near enough bullet left in the case neck if the "width of bullet diameter" method is used to ensure proper bullet alignment. That being the case, i decided to make the base of the bullet even with where the neck joins the shoulder which allows bullet-to-brass contact to begin just up from there a little since it is a boat tail. I was just wondering if the base being out of the shoulder would some how help create some type of even pressure distribution to better launch the bullet. Dont know if that makes any sence to you all but basically, i was wondering if any of you used any type of analogy like that when trying to be a certain measurement off the lands/rifling was not an option.

December 22, 2009, 10:08 PM	#3
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,060	No to the OP's general thought (if I've got it right?). If you analyze pressure against a boattail you can separate it into pairs of force vectors that have a forward component and an inward (toward the bullet axis) component. When you add up the forward vectors and add them to the forward force on the flat part of the base, the net result is the same as the force applied to a flat base bullet. The same will be true regardless of where the bullet is seated because inward pressure toward the bullet axis produces no net propulsion force. The normal advice is to seat the bearing surface of the bullet one caliber into the case. The bearing surface is the cylindrical groove diameter portion of the bullet. That means the boattail doesn't count as part of the seating depth for that purpose. It does, however count for increasing pressure over what you would get from the same charge with a flat base bullet of the same weight whose bearing surface was seated to the same depth. That is because the boattail eats up room in the case. The exception would be if seating the flat base to that same depth caused its ogive to get enough closer to the throat to cut gas bypass at bullet release, thus raising pressure by that different mechanism. Another thing to keep in mind: One of the chapter authors in the Precision Shooting Reloading Guide mentioned how he one day accidentally turned the micrometer on his seating die the wrong way to make a final adjustment. As a result, he wound up with 50 rounds seated 0.050" off the lands instead of 0.020" off the lands as he had intended. He debated pulling the bullets, but decided to shoot them for practice instead. When he did, to his amazement, the gun got the tightest groups it had ever shot. Tighter than with bullets 0.020" off the lands. So, the lesson is the old one that your gun isn't the other guy's gun and you will have to find what it actually likes? There is no magic number for seating depth except the one that works in your particular gun. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

December 24, 2009, 03:23 PM	#5
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,060	The main advantage of seating up close is to get the tip of the bullet in the throat before it gets going too fast to self-center in the throat's taper. If you seat deeper, but get the bullet straight into a straight case, then that makes a lot less difference. The Forster and Redding sliding sleeve competition seaters help with that. Maximizing bullet bearing surface also helps. That means flat base, not a boat tail. I recommend trying the flat base 53 grain Sierra MatchKing for that reason. It is a tack driver in my .222, which also has a 14" twist, but which has never liked boat tails much. To keep the case straight, the inexpensive Lee Collet Die for neck sizing only is just about impossible to beat at any price. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

December 24, 2009, 05:52 PM	#6
gearheadpyro Senior Member Join Date: September 3, 2009 Location: Charleston, SC Posts: 169	I figure out optimum seating depth as a part of my load development. I determine powder charge first, then load a few shots of progressively longer lengths. The one that groups best gets tweaked until I'm happy with it. I wrote an article about my load development, (http://www.rifles-shooting-reloading...velopment.html). Hope that helps. __________________ www.southernmarksman.com Owner of & Chief Instructor for The Southern Marksman, LLC NRA Life Member, NRA CRSO, Multi-Discipline NRA Certified Instructor

December 27, 2009, 10:33 PM	#9
jiw275 Junior Member Join Date: December 11, 2009 Location: Michigan Posts: 3	SAAMI.ORG A great source for cartridge information. Mike

December 22, 2009, 10:06 PM	#2
Win1892 Junior Member Join Date: September 4, 2008 Location: NW Florida Posts: 9	Get a longer projectile. I have a number of "bench" guns that I try to seat within 5-6 thousandths of the rifling, sometimes lightly touching the lands. 223, 308, 270, 6.5-284, 338 Lapua, etc. When using a lighter projectile I try to keep at least a third of the neck full of bullet. Don't hunt with these! They are prone to damage, or just plain falling apart.

December 23, 2009, 07:00 AM	#4
SEHunter Senior Member Join Date: October 6, 2009 Posts: 332	Thanks guys. I had a feeling i was not on to anything but i just put that out there anyway. I wont be able to use larger grain bullets to allow mw to be close to the rifling because my barrel is 1:14 twist and i have read it most likely will not stabilize heavier bullets than 55gr. I will just seat it maybe a little less than one diameter and go from there.

December 24, 2009, 09:52 PM	#7
SEHunter Senior Member Join Date: October 6, 2009 Posts: 332	Unclenick, are the matchking bullets for hunting or target only? I have heard many good things about them but thought most bullets that were termed "match" were not intended for use on game. (although with varmits, its not like any bullet thats traveling 3600+ fps would not yeild the desired result) Im using the 50gr ballistic silver tip now but i weighed a few and between the lightest and heaviest, they were about a half grain difference in weight. I dont know if that is considered a significant difference or if it will affect accuracy, but i may have to change brands if it does. I disassembled a factory hornady round with a v-max and it weighed 50gr on the dot. Gearheadpyro, i understand what you mean but i guess i have always thought seating depth alone would determine if any given bullet/powder combo would be at its best. I guess thats why i have always been intimidated by finding the right depth because i hate to think i cross off an excellent load when the only reason it performs poorly is because i dont have a good seating depth, you know? I mean, how do you know, because if .10" makes or breaks the results then it would take forever (or good luck) to not just toss that load from thinking its not a good combo. The onlything i know to do is pick the best performing load of several that have the same depth, then test the depth on that one load. As it sits now, to have decent bearing surface in the case, i am about .45" away from making contact with the lands.

December 25, 2009, 12:30 PM	#8
wncchester Senior Member Join Date: December 1, 2002 Posts: 2,832	"i was wondering if any of you used any type of analogy like that when trying to be a certain measurement off the lands/rifling was not an option." No analogies need apply. Nor seating one caliber deep, nor using boat tails in most instances. The one caliber deep "rule" thing isn't meant to align bullets, as such. It's just a nice way to insure that normal handling doesn't dislodge a bullet. Nor is seating into, or even close to the lands, an automatic aid to accuracy. If the bullet is loaded straight and the rifle is chambered straight it will enter the rifling straight. Boat tailed bullets are worth trying for accuracy but they offer no ballistic advantage until the velocity drops below the speed of sound, about 1,200 fps. Most people obtain better accuracy with flat bases except at very long ranges. Any two bullets with the same BC and speed will have virtually the same trajectory at any practicle range, no matter the shape of the base.