Can someone explain seating depth?

wildfowler.250

Well-Known Member
So assuming I’ve done my load development by powder charge and all at the textbook COAL. I can then tweak the seating depth to hopefully shrink the groups further?

Now I’ve heard that finding/chasing the lands isn’t worthwhile as it changes as the rifle gets used/older? Probably folk will disagree here and that’s great, I’m a relative novice but I think I heard that on panhandle once.

If I have the COAL. How much can I tweak the seating depth by? I’ve seen people talk about 10 thou so 0.01 adjustments? How much of an increase and decrease in seating depth do folk normally go for?


Any good textbooks that discuss this aspect well? Frankly I’ve always been a bit to cautious to go down this route and stuck with COAL as per the textbook.

Cheers!
 

Border

Well-Known Member
Check out Erik Cortinas videos on youtube about seating depth/chasing lands. Or just stick to book c.o.a.l or calibre depth in neck/ seat base of bullet to bottom of case neck.
Short version. He finds jam then comes back 20 thou. He then come back in intervals of 3 thou until he finds a consistent node. All explained in the videos. This can also be done with magazine length minus 20 thou if the cartridge won't fit in the mag.
 

Woodsmoke

Well-Known Member
To be perfectly honest, unless you're seeking the ultimate in accuracy for longer range work I really wouldn't bother. I've reduced COAL length to ensure some particular bullets seat in the cannelure (they didn't with the Lee manual recommended COAL) BUT (and it's a big but) I was loading a good bit below maximum. If you're up around the maximum recommended charge stick to COAL or you run the risk of exceeding the safe pressures. Extending COAL by too much will end up with the potential to jam the bullet in the rifling and perhaps have it pull out of the case (unless you add a crimp. That's just another rabbit hole to fall into though) A few thou' here or there from COAL might make a difference, but then again it might not. Not much help there I know, but so much depends on other factors too. For stalking, if you're happy with your grouping I'd leave it at that :thumb:
 

paul o'

Well-Known Member
Some bullets like to jump some like to jam and some just are blooding rubbish in your rifle ! take the Fox bullets factory coal makes them in my rifle look like iv used buckshot but back them into the case they look like one ragged hole .
 

NigelM

Well-Known Member
The first job I do when working up a load for a rifle is to find out where the lands are. Manufacturers COAL is fine, but when bullet makers recommend a starting jump you're giving your self a disadvantage by ignoring their advice. My general rule is to start lead bullets 30 thou off and copper 80 thou off. Find an accurate node by adjusting charge weight and then if you want to tighten further play with seating depth in 10 thou increments. And don't forget that lands move as the throat wears. Put a thousand rounds down and the lands could move as much as 50 thou depending on the chambering and how hot you run. You will find accuracy drops off and you will need to tweak your load to keep up with the erosion.
 

wildfowler.250

Well-Known Member
Brilliant guys, very helpful! I appreciate I may be over thinking things and it is for a hunting bullet at the end of the day but might be an interesting experiment.

Is it too crude to get a once fired case from my rifle, cover a bullet head with black marker and ‘load’ the bullet to see where it sits to find the lands? Im sure someone has mentioned this before.

I’ve also heard about putting a little dent in the rim of the case so it scratches the bullet and you can gauge where the lands is?

I do like Eric cortinas videos. Will have another look. Honestly I can’t decide if this hobby is great or a curse 😂
 

Dogtra U.K.

Well-Known Member
I have watched Eric Cortina and like the stuff he says. However when he talks about altering the bullet seating depth by 3 thou I find this is the best explanatory video I have seen.

 

baguio

Well-Known Member
Now I’ve heard that finding/chasing the lands isn’t worthwhile as it changes as the rifle gets used/older?
It does indeed change as the rifle gets older but if you find that a bullet is accurate through your rifle when X thou off the lands then you can always load to that by measuring your rifle say once a year and keeping that distance with every new batch of reloads. It works I promise!
 

srvet

Well-Known Member
It does indeed change as the rifle gets older but if you find that a bullet is accurate through your rifle when X thou off the lands then you can always load to that by measuring your rifle say once a year and keeping that distance with every new batch of reloads. It works I promise!
Or when accuracy deteriorates, increase the COAL by 0.003 to 0.005 and shoot a 58959A07-8211-420B-A830-298DFB1A0E92.jpeg group. If it shoots better then you are back in the node. The attached picture shows the result of a 0.010” increase in COAL on the right of the picture To take account of throat erosion
 

Foxyboy43

Well-Known Member
Brilliant guys, very helpful! I appreciate I may be over thinking things and it is for a hunting bullet at the end of the day but might be an interesting experiment.

Is it too crude to get a once fired case from my rifle, cover a bullet head with black marker and ‘load’ the bullet to see where it sits to find the lands? Im sure someone has mentioned this before.

I’ve also heard about putting a little dent in the rim of the case so it scratches the bullet and you can gauge where the lands is?

I do like Eric cortinas videos. Will have another look. Honestly I can’t decide if this hobby is great or a curse 😂
Hmm. Get a fired case, f/l resize it and trim to length; then with a small hacksaw cut a slit through the neck from mouth to near shoulder, pinch it between finger and thumb to ensure bullet grip is good, insert bullet (as long as you can make the COAL but ensuring sufficient grip on the bullet), then gently chamber round, gently extract round (you may have to press down on case as it starts to appear to stop ejector affecting length) measure either COAL or to ogive and note length of round. Pull bullet out again as far as it will seat (might have to squeeze neck again - bullet’s not your wife’s!) then repeat chambering, extraction and measuring. Do this 6 times - this will give you the absolute maximum COAL of your rifle WITH THAT BULLET. You can then “chase the lands“ to your hearts content though for general hunting I go for a nice safe 50 thou off providing you are getting calibre depth or near to. I also advocate crimping for my hunting rounds.
Enjoy but be careful - always start with low charges as per recs. and increase in small increments only and remember the general rule - the shorter the COAL the greater the pressure.
🦊🦊
 

Bowland blades

Well-Known Member
So assuming I’ve done my load development by powder charge and all at the textbook COAL. I can then tweak the seating depth to hopefully shrink the groups further?

Now I’ve heard that finding/chasing the lands isn’t worthwhile as it changes as the rifle gets used/older? Probably folk will disagree here and that’s great, I’m a relative novice but I think I heard that on panhandle once.

If I have the COAL. How much can I tweak the seating depth by? I’ve seen people talk about 10 thou so 0.01 adjustments? How much of an increase and decrease in seating depth do folk normally go for?


Any good textbooks that discuss this aspect well? Frankly I’ve always been a bit to cautious to go down this route and stuck with COAL as per the textbook.

Cheers!

First and foremost the closer you get to the rifling the faster the pressure rises . In stalkingbthe ammo could well be wet as might the chamber another pressure riser ! Mud n grit ?
Mostly all hunting rifles are very generously cut in this area as the mass manufacturers hate being sued , think of remmington and the none safe safety etc .
As a result hunting bullets are made to be very jump tolerant, let me give you an example 55 nosler bt in 243 , try and get anyplace near tge rifling and you bullet will fall out the case spilling powder all over.! Now find me std factory 243 win that won't shoot them great , you will struggle ! Just an example of jump tolerant that all .
Unlike the competition shooter , the hunter will genrally carry on in the rain and mostly the gun will be simply over thier shoulder with a sling . Comp shooters have used rain covers but mostly the RO will call a hault if guns are getting wet as with short jumps and jambing it's a very real risk
One more thing manufacturers have been making the magazines on hunting rifles intentionally short of getting regular bullets close , it crept in overbtge last 20 plus years! Again that healthy fear of litigation they have .

Personally I wouldn't bother chasing the rifling on a hunting rifle but I would look to how much of the bullet was held in the neck and how consentic it was get it to 2 tho or under and that's going to be a better round
 

wildfowler.250

Well-Known Member
Cheers guys, so much useful info here!

what I was going to mention was: I’m measuring the coal with electronic calipers.

Some of the rounds will probably vary between each other by 0.003 in coal without changing the rcbs seating die settings. Yet the above would suggest that this number is a measurable difference to change the seating depth of the bullets. How do you keep such a small variation consistent? Seems like next level. The brass would have to be identical length as well?

I’ve read that I should be measuring the bullets to the ogive rather than coal? But then most books will give you a coal to sit at and not really mention ogive under these circumstances?
 

srvet

Well-Known Member
True. The result will be the same. Great shooting BTW!
Thanks, it shoots well for a stalking rifle thanks to the fine work of @Ronin. That was with a 120g TTSX at 3450fps.
I had put off reworking the load on that rifle as I had just loaded a decent quantity when I realised the grouping had deteriorated a bit and I couldn’t be bothered to move the bullets out with a kinetic hammer and reseat them. Not surprisingly the 7 RM isn’t the kindest on throats, and I figured I would move two 5 thou increments in one go to make up for my inattention to detail. I checked zero after a scope change the other week and it grouped exactly the same so I think it is representative for that rifle.
 

srvet

Well-Known Member
Cheers guys, so much useful info here!

what I was going to mention was: I’m measuring the coal with electronic calipers.

Some of the rounds will probably vary between each other by 0.003 in coal without changing the rcbs seating die settings. Yet the above would suggest that this number is a measurable difference to change the seating depth of the bullets. How do you keep such a small variation consistent? Seems like next level. The brass would have to be identical length as well?

I’ve read that I should be measuring the bullets to the ogive rather than coal? But then most books will give you a coal to sit at and not really mention ogive under these circumstances?
To measure to the ogive you need a comparator. I find the Sinclair Hex nut comparator to be much more consistent than the Hornady one with the insert. The trouble with comparators is that they all vary which is why the loading manuals list COAL
 

Foxyboy43

Well-Known Member
Cheers guys, so much useful info here!

what I was going to mention was: I’m measuring the coal with electronic calipers.

Some of the rounds will probably vary between each other by 0.003 in coal without changing the rcbs seating die settings. Yet the above would suggest that this number is a measurable difference to change the seating depth of the bullets. How do you keep such a small variation consistent? Seems like next level. The brass would have to be identical length as well?

I’ve read that I should be measuring the bullets to the ogive rather than coal? But then most books will give you a coal to sit at and not really mention ogive under these circumstances?
For hunting bullets, especially with a lead (my gawd - he used the ”l” word!) tip you will get a variation in length both in bullet and final loaded round, try and measure a handful before you put them in the case - so base to tip COAL is always going to be a bit of a moveable feast but generally for hunting purposes this is not a problem, unlike target shooting where it could be an issue but as you don’t use lead tips for target shooting the consistency of bullet length should be much better. If you can determine your ideal base to tip length then you can always remeasure to the ogive for future loads, like SR above I use the Sinclair nut, more out of interest than necessity - this latter measurement should always be consistent assuming you are using the same bullet of course.
Personally and for 30 years plus I have only occasionally measured to the ogive but with my standard jump of 50 thou I have never had any issues. In the overall scheme of things and in hunting bullets 3 thou difference is nothing to get concerned about, you will probably find that unless you have spent a lot of money, your callipers may not even work to that accuracy anyways!
Many folk also advocate loading to the length of your magazine but in a number of my guns the bullet would simply fall out of the case! However if it works for you….
Stay safe.
🦊🦊
 

Edinburgh Rifles

Well-Known Member
No factory rifle has ever needed land busting loads to print groups better than you can demonstrate.

Anyone attempting to tell you they accurately measure to sub 10 thou, the secant ogive angular surface of the bullet in relation to a multi point abraded surface also an angular junction (the points of the lands) with a £6 set of calipers from Aldi is deluding themselves....

Load to factory length
if you must mess with OAL to tweak, use a dummy round and set the die once you prove that load on paper.
The actual measurement is largely irrelevant but the ability to replicate the seating depth.

couldnt tell you where my lands are
can tell you how long the rounds are to the tip though..
 

Bowland blades

Well-Known Member
No factory rifle has ever needed land busting loads to print groups better than you can demonstrate.

Anyone attempting to tell you they accurately measure to sub 10 thou, the secant ogive angular surface of the bullet in relation to a multi point abraded surface also an angular junction (the points of the lands) with a £6 set of calipers from Aldi is deluding themselves....

Load to factory length
if you must mess with OAL to tweak, use a dummy round and set the die once you prove that load on paper.
The actual measurement is largely irrelevant but the ability to replicate the seating depth.

couldnt tell you where my lands are
can tell you how long the rounds are to the tip though..

We should measure at a fixed diameter of the bullet shoulder curve as tips ain't so reliable . Dummy round all tge way though I keep one for every load for faster die set up !
I only seat using LE Wilson chamber type hand dies adding or removing home made shims from a coke / beer can allows fine weeks a few tho a time
Verniers are really not that precise but a die correctly set up with axworking load definitely is repeatable till they move further away through wear and erosion
 

caberslash

Well-Known Member
No factory rifle has ever needed land busting loads to print groups better than you can demonstrate.

Anyone attempting to tell you they accurately measure to sub 10 thou, the secant ogive angular surface of the bullet in relation to a multi point abraded surface also an angular junction (the points of the lands) with a £6 set of calipers from Aldi is deluding themselves....

Load to factory length
if you must mess with OAL to tweak, use a dummy round and set the die once you prove that load on paper.
The actual measurement is largely irrelevant but the ability to replicate the seating depth.

couldnt tell you where my lands are
can tell you how long the rounds are to the tip though..

This.

Handloading snobbery is rife on here... wonder if they spend more time in the field or in the shed?
 

HandB

Well-Known Member
I use 75thou of jump for all my full bore rifles (7x57 and two 308s) and 15 thou jump for my .222. They all shoot decently tight groups (approx 1MOA or less on a good day).
 
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