Seating into the lands , Pressures & velocities

M

Metal god

New member
Hello all

I often recommend if a reloader plans on adjusting seating depth to fine tune a load . I tell them on initial load development to start at or in the lands and adjust off/back from the lands . The reason is that if the final charge is close to max pressure they still can adjust seating depth because they will be adjusting away from the lands likely reducing start pressures . Rather then if they were to start .040 off the lands and there final charge was again at max pressure . Moving the bullet closer to the lands can cause pressure spikes resulting in higher then recommended pressures . It also means you may need to move the bullet both directions which to me seems like more time may be needed to find your sweet spot .

So I've done this and it seems to work just fine . FWIW I've also seated closer to the lands after finding a good charge but must say I was a bit nervous doing so the closer I got to the lands Knowing the charge was only .5gr less then the manuals max charge . Anyways I'm still here and the gun is still in one piece so that went well I guess and I did find a really good load with the bullet just touching the lands .

That all said I have a couple questions on this method of starting at or even in the lands .

1) Does this have the potential of creating lower velocities across the board with the given components ? Meaning since in theory I'm creating a pressure spike not intended at the beginning of the ignition , does that cause any other pressure issues ( good or bad ) down bore ?

2) Assuming I am raising start pressure buy seating into the lands . When I adjust off the lands and those start pressures presumably drop/lower the further I get from the lands How does that effect pressures down bore as a whole and as it relates if at all to question #1 and that answer ?

3) and maybe most complex goes to harmonics and barrel time . Again assuming as I back off the lands the pressure/s drop will I be loosing velocity as I do so ? Even if I do and the groups improve do I care ? I mean that's the point of this theory of fine tuning , getting smaller groups correct .

4) what if my in the lands final charge give good accuracy but not great and backing off only get worse . I don't think you want to jam deeper to get better groups . What about backing off then upping the charge to get back up to the pressures I had seated into the lands thinking that puts me back in the same barrel time-ish area ?

But wait there's a 5th ) How much pressure and velocity swings should one see when playing with seating depth in the way I'm describing ?

I'm asking all this because I've really let my rifle shooting and reloading take a back seat to handgun loading and shooting for the last few years and I miss rifle shooting . Not just shooting but locking in and shooting for accuracy < ( real shooting not sitting the gun on a rest , pointing it at the target and touching the trigger :p ) My plan is to try a couple new component combos that I've never used before . This is for 308Win , I have 178gr BTHP and 175gr TMK and the powders are AR-comp and IMR-8208xbr . I haven't decided which bullet and powder combo's I'll start with but that's what I'll be using . FWIW with Lapua brass and Win primers . I have Federal match primers but the 400 of the 1k I've used so far did not seem to make a difference in other 308 loads . So I'm not interested in using them only to run out and never be able to replenish them cus they tend to be hard to get here locally and I have several thousand Win primer as well as CCI LR primers .

Well it's late and I feel like I'm rambling

Thanks
MG
 
Last edited:
When I was shooting 1,000 yard F Class with my 6.5x55 match rifle, I went from seating the bullet 0.012" off the lands to seating them 0.020" into the lands with about 0.001" neck tension. I found that reducing my charge weight by 0.9gr got me the same velocity as I got when I was seating the bullet off the lands.

Don
 
In the old Small Arms Firing School, Mid Tompkins told us he soft-seats, meaning the neck is sized so that finger-pressure can still move the bullet in the neck, and he leaves the bullet sticking out long and lets closing the bolt on the chamber finish seating the round. The only issue is you have to be at a range that allows loaded gun muzzles to point up (mine doesn't) because if you have to unload, you have to point the muzzle up to stop powder falling into your action if the lands hang onto the bullet. But the bottom line is that if the load is developed in that configuration, so the throat is setting the start pressure, it can shoot very accurately.

The rule of thumb is that contact with the lands raises peak pressure 20% as compared to being a few hundredths off. Touching the lands does nothing magic. It is just that as the ogive gets closer to the throat less gas can escape the neck and go around the bullet, delaying the pressure rise time a bit. So as you get close to the lands, peak pressure goes up. How quickly that happens will vary with the radius of the bullet ogive, but for common bullet shapes (not VLD's and especially not round-nose, which have a very gradual taper to their ogive) 0.030" off the lands will usually produce normal pressure.

QuickLOAD has you model land contact by adding 7200 psi to start pressure, roughly tripling it. Pretty commonly, though, it varies by powder, reducing the charge weight by 10% roughly compensates for it, though some powders need a little more reduction to be exactly on pressure. Still, the difference is not uncommonly lost in shot-to-shot variation.

So, what does that do in the gun? Again, it depends on the powder, but, basically, with less powder and more start pressure, the powder will light up a bit faster and peak a little earlier, improving ballistic efficiency (the percent powder energy converted to bullet kinetic energy). So the muzzle energy drop is usually a bit less than proportional to the powder charge difference. A 7% drop would not be unusual. Velocity will drop about half that much; 3.5%.

So, to get a matching velocity from a bullet seated into the throat with the same powder does mean increasing the peak pressure about 9–11% and using a charge of something like 97% of the charge you used 0.030" back from the lands would be in the ballpark of doing that with a lot of powders. However, using QuickLOAD is still the best way to dial a particular powder charge difference in for a particular cartridge. QuickLOAD often does not get pressures and velocities exact. Still, it gets the ratios of charge weight changes needed for a particular powder to achieve a particular velocity or to accomplish a pressure percentage shift. Assuming you have achieved that matching velocity without pressure signs, you will have a bullet that gets more of its acceleration from the peak pressure and less of it from muzzle pressure because of that faster and more complete burn. This will mean that it comes out of the peak going faster and gets less of its acceleration later in the barrel, so the barrel time is shorter than you have with the same velocity load of more powder loaded 0.030" off the lands. It also means that if you load to match barrel time, you will have to go for a slightly faster velocity when seating off the lands than you have when you are in contact with them.

The bottom line is that when you seat the bullet deeper to tune accuracy, you are changing pressure, velocity, and barrel time all at once. In effect, you are tuning a detuned with every step. You can use QuickLOAD to come close, based on velocity differences you get, to tuning in matching barrel times. However, I find the seating depth that is best for one load often turns out to be best for others with that same bullet (or sometimes a number of bullets), so I now look for that minimum group first, then try tweaking barrel time. At first, I assumed it would always work best to match barrel time, but for some reason, it isn't always so. The barrel time that is best can shift a bit with bullet seating depth, and I am not sure why. It may be the difference in timing with which the chamber starts to stretch as the bullet enters the lands, initiating a pressure wave pair. All I can do is running a second ladder after finding the best seating depth to see if it finds you a better sweet spot or not.
 
Lets start with soft seating , I've tried it but very little . Mostly because I just wasn't that comfortable doing it for the reasons described above among others , so I wont be doing that .

Thanks Unclenick that was very helpful ,lots of good info to sort through . I do however really like the idea of running a second complete load development once I find the seating depth . I've never done this and quite frankly never even crossed my mind . I may actually do this with a couple loads I have .

This brings up another question of charge increments . When using a scale with a +/- .1gr variance and the charge likely in the 40+gr area . What increment would be optimal to move up in ? When I first started I did .5gr increments but later changed to 1% of max . Meaning if max was in the 40+gr area I'd move up in .4gr increments - 50+gr would be .5gr increments and anything 30gr or less would be .3gr increments . I never went to .2gr increments because a scale that only gives you a +/- of .1gr means those .2gr differences could be .1 , .2 or .3gr differences . That to me seems like you could end up chasing your tale if you're not careful .

When I was shooting 1,000 yard F Class with my 6.5x55 match rifle, I went from seating the bullet 0.012" off the lands to seating them 0.020" into the lands with about 0.001" neck tension. I found that reducing my charge weight by 0.9gr got me the same velocity as I got when I was seating the bullet off the lands.
Click to expand...

This also makes me think I should start below the recommended start charge if I'm starting in the lands ??? Maybe 5 to 10% less the recommended starting charges ?

Example : If min is 39gr and max is 42gr maybe start at 38gr and stop at 41gr ?

Where is the best place to buy Quickloads ? I found a site based out of the UK but not sure if that's the right place to start ?
 
Last edited:
CAUTION: The following post includes loading data not covered by currently published sources of tested data for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.

Since the bullet in the lands raises pressure about like 10% powder charge would, I would start 20% below maximum unless that reduced loading density below 70%, in which case I would start at 70% loading density and work up toward 10% below the listed maximum.

I don't go below 0.3 grain increments using a standard powder scale either. I have scales that see 0.02 grains or less for that.

QuickLOAD's only authorized U.S. distributor is NECO.
 
I would start 20% below maximum
Click to expand...

Am I wrong in calculating 20% of 40 is 8gr . Meaning starting at 34/35-ish grain and work up to 38gr for a load expecting a max of 42gr ? Thanks , that's MUCH lower then I would have thought to start at . I'd really need QL to tell me where/what gr 70% load density was but with the powders I mentioned above I'd think 34gr would be pretty close to 70% .
 
I’m not sure this is what you want to hear, but I start where I want to end up. I generally start at 0.030” off the lands. I find I end up 0.020-0.030” off the lands. It still should be fine tuned, but I usually end up close.
 
MG,

Hodgdon puts a maximum load for a 175-grain bullet at 42.5 grains of IMR 8208 XBR, but that's in a Winchester case. I would assume 42 grains max in your Lapua case until workup shows otherwise. 80% of 42 is 42×0.8=33.6 grains and a jammed bullet max would be 42×0.9=37.8 grains, assuming you can't safely work up further. That gives you 14 steps to play with. If the 175-grain TMK is seated to 2.800 COL, and the bulk density of your lot of powder is 0.92 grams/cc as QuickLOAD shows, 33.6 grains of powder would be at about 78.7% case fill, if I assume case water overflow capacity of 57 grains of water. A COL of 2.970" is then where the boattail would be flush with the case mouth and there be no hold on the bullet, so I know you won't seat it out that far. But if you could, the charge at that bulk density would still have 73.6% case fill. So you should be fine to start down that low.
 
I found my max cartridge overall length on both those bullets years ago

178gr bthp 2.890
175gr TMK 2.946 but that does not sound right . My notes are not in my handwriting which means I was measuring while my dad was writing down what I told him they were. I’m not sure 175TMK numbers are correct .
 
It does sound long, but not impossible. The TMK is a secant ogive bullet with a large ogive radius, so it will, indeed, stick into the throat further before the throat meets its contact diameter than many tangent ogive bullets like the 178-grain Hornady match bullet will. Hornady's 178-grain ELD and ELD-X are another story, and would seat long, like the TMK.
 
Some bullets like being seated into the lands or just off the lands where others like a little more distance from the lands to give optimum accuracy. You have to find the sweat spot of your rifle.
 
Unfortunately I don't have quick access to the rifle but here's the bullet seated to 2.946

PElXwi.jpg


Plenty of bullet hold as we can see so I'm thinking that number is correct . I also have the comparator measurement and it's only .001 off . I doubt we got both measurements wrong and they both also match up .

Some bullets like being seated into the lands or just off the lands where others like a little more distance from the lands to give optimum accuracy. You have to find the sweat spot of your rifle.
Click to expand...

Thanks I agree , I'm now trying to figure out how to do that safely ;)
 
I got to thinking that maybe .005 jam isn’t enough to be a consistent seating depth . I remember one load development I tried where I had the bullet just touching the lands and I had what may be the worst ES/SD’s I’ve ever had . I have always figured that the variance in case head space , base to ogive measurements etc. contributed to the issue . I’m not sure how much a .004 or .005 variance would be , meaning maybe sometimes the bullet was .002 or .003 off the lands while other times it was .002 or .003 in the lands???

I’m thinking maybe I should do at least .010 jam ??
 
That variation was possibly due to the distances from your case shoulders to your bullet ogive, the thing that controls how far the bullet sticks into the throat when a case headspaces on the chamber shoulder, was not perfectly uniform. Plus there is usually some bullet-to-bullet variation as to how far the bullet's ogive contact point with the lands is from the point where the seating die contacts it is. So you probably had some rounds with a small gap and some jammed a little bit into the throat and some just flush with the throat. 0.010" jam into the throat is a common target value for bullets seated that way.
 
I got to thinking that maybe .005 jam isn’t enough to be a consistent seating depth .
Click to expand...

You are thinking right. Bullet ogives, even match bullets, can vary greatly. That's why I seated 0.020" into the lands and never closer than 0.010" when seating off the lands.

Don
 
You might also try Gordon’s Reloading Tool, GRT. I have been trying it and for powders it has the results seem quite good and comparable to QuickLoad. Some powder data needs more calibrating.
Both QuickLoad and GRT allow you to change the start pressure and seating length and see what happens to pressure curve and velocity. They won’t be exact until you measure velocity and adjust parameters to fine tune.

GRT has IMR 8208XBR data and it’s a free tool so why not try while waiting for your QuickLoad CD to arrive :-)

https://grtools.de/


Andrew - Lancaster, CA
NRA Life Member, CRPA member, Calguns.net contributor, CGF / SAF / FPC / CCRKBA / GOA / NAGR / NRA-ILA contributor, USCCA member - Support your defenders!
 
I've noticed 308 Winchester powder charges more than 10% below typical maximum don't fully expand cases to chamber dimensions.

As the case shoulder is set back a few thousandths from firing pin impact before peak pressure is at hand, excessive reduced loads don't have enough peak pressure to expand the case to chamber dimensions and push both ends of the case to the chamber limits.

Correctly measure case head to shoulder dimension before then after firing empty primed cases. I was amazed at up to .008" difference.
 
Unclenick said:
That variation was possibly due to the distances from your case shoulders to your bullet ogive, the thing that controls how far the bullet sticks into the throat when a case headspaces on the chamber shoulder, was not perfectly uniform.
Click to expand...
Thanks for explaining the reality of what controls bullet position relative to the throat when the round is fired. Case heads are typically not touching bolt faces when fired.

I will add the fact that the throat contact point diameter is typically a few thousandths less than bullet diameter. If your gauge used to measure bullet ogive to some place on the case has a touch point diameter smaller than the barrel's bore diameter, it's too small and very misleading because that point never touches the rifling.
 
Last edited:
Thanks Bart . I agree with the issue with reduced loads but in this case by jamming the bullet into the lands I raise expected pressures . This in theory should raise these reduced load pressures to standard load pressures. However I’ll bring my calipers , gauges and chrono to help understand whats going on .

As for seating depth, you/our observations are why I’m going to seat even deeper into the lands then the .005 I loaded at . This should help guarantee a more consistent start pressure results in a more consistent burn as a whole . At least that’s the working theory .
 
You must log in or register to reply here.
Back
Top