A Question On Seating Depth

M

murphy

Guest
An anomoly in the way seating depth is used to tune factory made rifles has come to my notice. In a recent thread Jackie Schmidt advised a fellow with a factory produced 308 win. to move his seating depth around in .005 increments. Seems that most folks on this forum do likewise, yet I move my bullets in increments 10 times as big and still get good reliable results. In fact my crude equiptment would never allow me changes that small, yet movements of 1/25" or 1mm in my language, get me into the accuracy window. The trick seems to be to know which side of the window you are on as some loads shoot better approaching the lands and some moving away. If our equiptment is not precise we can still get good results if we err in the right direction rather than fall off the edge going the wrong way. My question is this? Am I crazy to move my bullets in such large increments and if it works, why do you guys use .005 or smaller. Am I missing out on something?:confused::confused:
 
OK, Jackie's on the road so's he can't yell at me for several days :D

My guess is that it's been so long since Jackie handled something not a BR rifle that he thinks that .005 is a large move....... when In fact I agree with you, gross movements on the order of ten or twenty thou at a time are more usable for factory stuff.


BTW, you DO have the means to effect small changes if you use a threaded stem seating die. A full revolution of your seater stem will be on the order of .040-.050 depending on your thread pitch. If your seater is a Wilson you can count on 40thou/turn. Take it to the bank. :)

al
 
To alinwa

Many thanks for your reply and I hope I haven't got you into too much trouble. Sometimes this forum just cracks me up.:D:D:D
 
As I recall, instructions on my Hornady OAL guage recommend backing off about 2 to 4 hundredths from the contact length. I have been trying to set my seating die to back off 2 hundredths at least. I measure with a caliper and a "bullet comparator" gadget.

You are suggesting 5 thousanths. This is not much. The OAL guages are not perfect. When I measure with a bullet, I usually try several times, write down the results, and there will often be slight discrepancies at the thousanths level.

In other words, the OAL guage will vary slightly at the thousanths level of precision. We may be getting meaningless precision, that is beyond the instrument's accuracy.

If you are off just a hair, your bullets could be into the lands. If you WANT this, fine. When loading at max levels of powder, this can cause pressure problems.

Your comment is borne out by gun writers, who often suggest that different guns respond differently to the amount of freebore. Some want almost none, some will shoot accurately with quite a bit. For example, Weatherbys have such a long throat, it is almost impossible to put a bullet just off the lands with these rifles.

Maybe someone else has an opinion. No question, many out there know far more than me.
 
Another thing to consider in seating depths for factory rifles is that they frequently use leade angles (the included angle that the reamer cuts the rifling in the throat) that are steeper than most BR leades. I've been told by a gunsmith who turns out some pretty accurate rifles that if the leade angle is steep as in most factory rifles bullets seem to like to be jumped, and 30-60 thousandths jump isn't unreasonable at all.

As Al says if you're using a seating die with a threaded seating stem, just figure out how many threads per inch the stem is threaded. If it's 28 tpi than one turn = 0.036"/turn (1/28"). So 1/8 turn = 0.0045", 1/4 turn = 0.009" etc. If the stem is threaded some other pitch all it requires is a $4 pocket calculator, and maybe a pitch gauge. If you don't have a pitch gauge and a hardware store is close take the seating stem to the store and compare the threads on the stem to various machine screws and the one that matches is the pitch of the seating stem. Nothing wrong with being, uh, thrifty. :D
 
murphy said:
why do you guys use .005 or smaller. Am I missing out on something?:confused::confused:
Click to expand...


Murph, I'm gonna' answer this part of your question just because at heart this IS A BR FORUM. :)

In traditional 100-200yd BR be it Group or Score, there's a LOT OF WORK DONE in that .000 to .005 range..... and in the BR world thousandths are nice big numbers to work with. I regularly go back to rifles/barrels which haven't been fired for perhaps years and within minutes can and will screw a barrel in, grab out a box of bullets, set the sizing die and neck die such that I'm bumping the shoulder LESS THAN A THOU and seating my bullets to easily WITHIN A THOU of where my notes say the gun/barrel was last shooting. I can do all of this with a set of calipers and one or two tools..... a "shoulder setback gizzie" which is a hunk of barrel stub that's been hit with the chamber reamer and a Sinclair ogive "nut." For .22's and 6mm's all's you need is just the NUT cuz you can use it to measure both the seating depth AND the shoulder setback.

THE REASON that BR guys play around in this .000-.005 range is something called "tuning" which simply DOES NOT APPLY to factory rifles. You'll have to take my word for it but what that .000-.005 can do is turn a 5-shot group from an oblong tall hole (NO-winning) shape to a nice round BUG hole (WINNING :)) shape.

"Tuning" only applies to supremely accurate (Bench Rest) rifles.

Now, on the subject of "pressure problems when you maybe inadvertently TOUCH THE LANDS!!!" :eek::eek::eek:

Fuh'GEDDABOUDIT.........

AIN'T gonna' happen.

Pressure inside your case drops about 300psi per TEN thousandths (.010) of freerun to the lands.

To get ANY MEASURABLE or noticeable "pressure problem" or spike you'd have to have a MAX'd load set up for 50 or 100 thou out of the lands and THEN run one out to touch the lands. The result would be a "spike" of 1500 to 3000psi......noticeable but rarely dangerous.

If you're running normal sized cartridges at anywhere near Max pressures you're in the neighborhood of 55,000 to 65,000psi so even a 3000psi spike is only 4-6%.

Now don't get the impression that I think a 6% overload isn't STUPID bordering on criminal :) It IS stupid..... so don't work up a load that's an eighth inch off the lands and then just cram it in..... but going from 5thou out or even TEN thou out to touching the lands won't even register.

In fact, a much larger problem than this is folks playing around at MAX using thrown powder charges instead of weighing them individually. Thrown charges from even the best measures can and absolutely WILL vary by 2-4% charge to charge and as much as 6% day to day IME..... This is huge. Some powders like the ubiquitous H4350 CANNOT BE THROWN to within closer than 2-3% no matter how good you are!!!

You couple sloppy seating depth with charges that vary by 2-3% and you are asking for trouble.

I hope this all makes sense.... :)



al
 
To Alinwa.

Your insight into benchrest methods and techniques is of great interest to me and you may rest assured that I build a margain of safety into my reloads. The question of tuning is a bit of a sticking point as I believe that the underlying principle is the same, wether your rifle is the finest benchrest rifle or a 1942 Lee Enfield. Provided the bedding is sound and the barrel free of fouling, all rifles can be tuned buy incremental changes to seating depth even though the degree of refinement may be entirely different. A coarse war time production rifle may go from a 3" group to a 1" group and a bench gun may get the few thou that are needed to win a match. But heaven help the shooter that does not abide by this principle. :cool:
 
To Lynn

MY moment of enlightenment came about 1 year ago after studying this forum. Since then I have tuned about 7 rifles mostly by seating depth and the results have been outstanding. The thing I like about this method is that I always seem to find multiple loads with different bullet weights that shoot to the same point of impact. I hear you about the size of the seating depth changes and maybe with a couple of my better rifles there may be another dimension of accuracy that I am yet to uncover, in any event I won't find out if I don't try. I feel sorry for people who think that a bullet should be seated in a certain spot and no other as they are missing half of the potential of their rifle.:eek:
 
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