Seating Depth

[Stuart C.]

Hi,
As I might've mentioned, I've loaded shotshells for as long as I can remember, but I'm just getting into the centerfire reloads. I'm pretty much good to go and have all the fun gadgetry on the bench, but I'm still not completely clear on seating depth. 

I'm reloading for 7mm Mauser and want to avoid any excessive bullet jump before hitting the lands (which could cause throat erosion down the road) that can occur when loading the smaller (~140 or less grain) bullets. 

Correct me if I'm wrong but ideally I should be ~ .002 off the lands (or even just touching the lands), right?   How can I be positive how far off the lands I am?  Should I make up a dummy cartridge by slowly moving the bullet down until the cartrige is just short enough overall to chamber, and measure that dummy round and keep the seater die set at that depth?

Any seating depth info for this trainee is greatly appreciated.
Thanks.
 

[PA-Joe]

Yes, make up a dummy round. Hornady also sells gauges you can buy both bullet omparators and headspace.

There are several way to measure: you can take a case and cut a slit down the neck. Adjust the tension and then put you bullet in that and chamber the round. Take several measuremetns until they are consistent. Ogive measurements are the best using a bullet comparator.   

[dave375hh]

I don't know where you got the idea that short bullet jump to the lands will prevent throat erosion. Throat erosion is caused by powder gas and powder kernels hitting the throat after the bullet passes the throat. Seating your bullets at .002 is shorter than I seat varmint loads. Hunting loads such as your 7MM should be .020 to .030 in a big game load, and .040 to .060 for Barnes X or TSX bullets.My varmint/target loads run from .003 to .010 and you need serious equipment to even measure the actual distance. If your measuring to the bullet tip you'll have .005 to .010 variation due to the tips being inconsistent from the factory. To measure these short jumps you need to measure to the ogive or the place on the bullet where it becomes full diameter because that's what actually hits the lands first. splitting a case neck and chambering a dummy round is not accurate enough for seating a bullet that close. I use a Sinclair bullet comparator and measure to the ogive of my loads, as long as you measure with the comparator you will be able to seat the bullets that close. As I stated above hunting loads should not need to be that close, and will not affect throat erosion. Some benchresters actually seat into the lands from .001 to .005 but you still need to be able to measure accuratly.

[beemanbeme]

Uuuhhhhh, there is no relation between bullet jump and throat erosion.  Weatherby was able to achieve his high velocities with the use high intensity loads and long throated chambers.  The .2 and touching the lands comes from a bench shooting concept that the less a bullet travels before engaging the lands, the less it can go askew. A sound principle but of little use in the hunting field.  In your 7x57, unless it is custom chambered, I doubt if you can come close to the lands with a 14ogr bullet. 
Throat erosion comes from heat.  Nothing else.  So use some self disclipline in your rate of fire and you'll be shooting you rifle for a long time.

[Stuart C.]

Ok.  I got it on the throat erosion.  Big misunderstanding on my part. Mea culpa. 

Back to my question:  How should I correctly identify the proper seating depth for a bullet?

Thanks again!

[beemanbeme]

The rule of thumb (and it ain't writ in stone) is the bullet should sit in the neck 1 calibre or more. ie, your 7mm bullet should be seated at least .284" into the neck. The .300WM and .300Savage being notable exceptions to that rule.
 Often it is the length of the magazine that determines how far out you can seat your bullets. 
Its been my experience that changing seating depth will not make a good load out of a bad one.  So I would suggest working fairly close to book values until you fine a good load and then try to improve it by tweaking your seating depth a bit. 

[PA-Joe]

beeman makes some good points. You should have at least one bullet diameter inside the case neck. Your COL will also be limited by your magazine. Once you have deteremined the COL you would like to use then you have to make certain that it will feed consistently from your magazine. Unless you are going to use it single shot. 

[Stuart C.]

The gun I'm loading for is in fact a single shot.  I want to get the seating depth set correctly on my die.  I'l go back and re-read the instructions.  But I just wanted to know from the more experienced if there is some better way to lock in the seating depth on your die KNOWING it puts the bullet just off the lands (for optimal accuracy and what I mistakenly thought was wear that can come from smaller bullets that are farther away from the throat than other larger bullets).

Anyway, the old manual I have for the RCBS dies seems kind of vague in describing this.  I'm thinking of stopping on the way home and getting another manual.  Good excuse to pick up some more stuff I don't need.

Thanks!

[jgalar]

To check for COL with a specific bullet I use a cleaning rod and 2 cleaning rod stops. I have finished the cleaning rod stops so they are exactly the same size.

Close the bolt with an empty chamber.
Put a rod stop loosely on the cleaning rod and insert it down the muzzle so it is resting on the closed bolt face and tighten the stop at the muzzle.
Remove the rod.
Open the bolt and insert the bullet (not cartridge) into the chamber and lightly push it to engage the rifling.
Install the other rod stop loosely on the cleaning rod and insert the rod down the muzzle. Gently allow the rod to rest on the bullet and tighten the rod stop at the muzzle.
Remove the cleaning rod and measure the outside of the cleaning rod stops. This measurement, minus the size of one rod stop, is the COL for that bullet in that rifle. 
Don't forget to remove the bullet from the rifling.

Cast bullets work well with the bullet seated into the rifling. I wouldn't seat a jacketed into the rifling.

[Catfish]

I have been playing around with center fire rifle since 1966 and have always loaded my ammo. In that time I have learned that every gun is different. I have had guns that liked the bullets .030 to .040 off the lands and other that shot best with the bullet jammed into the lands. If you really want to know where your gun shoots the best forget about all the measurements and seat % bullets to what the book say is max. oal for your round. Fire a group with them. Then load 5 more .010 shorter. If the group is better load 5 more another .010 shorter until the group starts to open up. If the groups get worse after the first load load 5 round .010 longer and try they. If you start touching the lands pressure will go up so you have to watch when you go over oal, but factory gun are usually long throughed for safety reasons and are still safe something over max.sammi aol.

[LaOtto222]

I use a Stony Point seating depth gauge. It is made for this specific reason. I can change bullets by using the Stony Point and have my new measurements in minutes and be sure of what I got. I then use micrometer type bullet seater to seat them exactly where I want them. As has been said before, no two rifles are alike. Some rifles shoot best .020 - .040 off of the lands, that is a fact. However, over all I found that seating the bullets to just touch or a few thousands into the lands give me the best over all accuracy. I try to develop my loads to this criteria. If I do not get satisfactory results, I then start backing away from the lands. I am not talking hunting loads here. I shoot single shots (including bolts) and they are varmint type loads. For a hunting rifle, you need to be cognizant of magazine length and bullet friction in the neck, some times even a crimp. I do not worry about bullet grip, to a point. I have the lands to hold back the bullet and I do not have to worry about recoil changing the OAL or pulling bullets completely out of the case. As long as they do not move around in the neck with reasonable handling they are good to go.

[pagris]

Stuart C:  My variation on determining the maximum overall length for a specific bullet is to make two slits, at a 180 degree angle, in the neck of fired, deprimed, case, using a Dremel tool.  I then lightly polish around the slits to remove any burrs (to avoid scratching the chamber) and then, using a toothpick, I put two small dabs of epoxy inside the neck at a 90 degree angle from the slits.  I then start a bullet into the case and chamber it in the rifle.  Chambering the round forces the bullet into the lands of the barrel and pushes it into the case. After allowing it to sit overnight, I can eject the case the next day and I have a permanent record of the maximum length for that particular bullet/case combination.  The overall length can then be measured or, better yet, an ogive length can be determined using the proper comparatorr tool. I then label the case and use the resulting measurement to determine the overall lengths I want to try for my reloading - I usually try a range from 0.010 to 0.030" off the lands at first.

At times I find that altering the COL has made a significant difference in a load's performance - at other times, the impact seems minimal.