Seating depth variation/tolerance?

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Sep 5, 2023
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How much variation is typical when seating bullets?

I am using 2 Hornady Custom dies with micrometer top and 1 Redding Competition Seater die (also micrometer), all in a Hornady Lock-N-Load Classic press. If I set a seating depth and just load 10 or 25 rounds and only check them with the comparator afterward, then I find variations of 1-3 thousandths plus or minus. (More variation when seating compressed loads, less when there's ample space.) I have found those variations with 4 different .243/6mm bullets and at least 3 different .308 bullets. To deal with the variability, I often back the micrometer off by ~2 thousandths before first seating each bullet, then measure BTO and dial 1 to 4 thou more as needed to seat within a half-thousandth of my intended BTO.

So, how much variation is typical when seating a bunch of rounds without measuring every time a round leaves the die?

What +/- tolerance do you require for seating depth (BTO)?
+/- tolerance you accept
 
I have two RCBS presses, an old RS and a RockChucker. I use only the Redding Comp Seater die micrometer for my precision loads and my RS press that is 50 years old. I have RCBS and Hornady dies also. What I have seen is seating very dependant on the bullet. I have tested by measuring bullet base to ogive and with. I see very little variation. Ogive to tip is a different story.
I measure mine loads with RCBS precision mics. I can load 50 rounds and won't have .001 variation with the Redding Comp seater. I just went through and measured 30 of my 260 Rem loads woth a 105gr Sierra Blitzkings and they were +- .0005. I don't think 1 or 2 thousandths makes much difference when you are seating more than .010 off the lands. That is the important measurement is ogive to lands. Where it becomes important is when you get closer than .005. I don't load anything between. 005 to the lands. If I want to be in the lands like I am fireforming a wildcat or AI, I push it well into the lands. If say my brass headspace sizing varies .002 - .003 long and my bullet get seated .002 long then I risk being on the lands and off the lands when on the short side trying to seat at .005 off the lands. On my most very accurate rifle, my seater stem is lapped to the bullet I am using which is usually a Sierra bullet. It gives a little more contact on the bullet. I have multiple seating stems for each die/cartridge for each bullet I am using. I have chronographed a lot of loads and I generally get an increase in velocity closer that .030 off the lands and a spike when you touch them. That is highly dependant on the powder too and if you have a leade of 1°30"or something different.
One caution with Redding dies and Comp seater, you don't use a compressed load or you risk cracking the seater stem. The other important factor I think is more important than 1 or 2 thousandth on the seating depth is runout. This is my thoughts and the way I do my loading.
 
Mine on a cheap Lee press with a redding micrometer seating die are usually +/- .001 base to ogive. No way I would mess with adjusting the micrometer for each individual bullet (or even measure every single CBTO) when I'm loading 200 rounds.

I'm usually loading .030 off the lands, so I wouldn't sweat +/- .002 variance. I'll measure the first few rounds and then every 10 or so after that when I'm doing a big batch to make sure nothing is out of whack.
 
I'm usually loading .030 off the lands, so I wouldn't sweat +/- .002 variance. I'll measure the first few rounds and then every 10 or so after that when I'm doing a big batch to make sure nothing is out of whack.
Yeah. My AR is limited by the mag, and my Ruger 243 and Tikka 308 both have such long throats that getting closer than 30 thou off the lands would mean having well under 1 caliber of bullet shank in the case neck.
 
Seating depth doesn’t matter much to begin with. Variation of a few thou seating depth does not matter at all.
Interesting. Quite a few precision reloaders point to seating depth as one of the largest factors in keeping groups tight. I have not tested that principle, or even done very much seating depth work, and have thought maybe that was why my most of my better load workups are good for 1 to 1.25 MOA (measured over time or in groups of 10-20 shots; those same loads will usually print 5-shot groups about 50% smaller) instead of consistently shooting the 0.5 to 0.75 MOA 10-20 shot groups I am looking for.
 
A variation of + or - .001 from my desired CBTO is typical for me. The variation comes from tolerance in the press mechanism, specifically the pivot pin connecting the ram to the lever. Seating technique is critical. I check every load and can adjust .001 shorter with a bit more force on the lever.

Some of my rifles seem to be sensitive to seating depth and others not. Even in those that seem to be sensitive I don't think a few thousandths makes any difference and my checking every load is probably a waste of time.
 
Bullet manufacturer (consistency of the bullet itself) and variation in your neck tension is likely what’s affecting it (harder tension and the bullet forces further into the seating stem before seating into the neck) not your dies.

I’d say your variance is more on the normal side.

And you’re very likely not shooting a system that precise enough to distinguish a +-.0015 in depth variance to begin with.
 
I'm a believer in seating depth but just like powder charge you should have a window of .003-.006" where the rifle is happy.

As stated above, a large variance of .003 is likely from neck tension or crushing powder.

I'm a bit anal about my loading techniques so I strive for NO more than .001 and even that bothers me a little... ;)

I take an extra step by running my brass through a mandrel die with a dry lube. I believe this is key to consistency.

I also use LE Wilson inline with an arbor press or Redding Micro adjustable dies.
FYI, if you go down the arbor press and inline die rabbit hole, you'll love them.
 
Interesting. Quite a few precision reloaders point to seating depth as one of the largest factors in keeping groups tight. I have not tested that principle, or even done very much seating depth work, and have thought maybe that was why my most of my better load workups are good for 1 to 1.25 MOA (measured over time or in groups of 10-20 shots; those same loads will usually print 5-shot groups about 50% smaller) instead of consistently shooting the 0.5 to 0.75 MOA 10-20 shot groups I am looking for.
Do you have a very high end custom gun? Best of everything? Do you use the most precise and expensive components and dies available? A .5-.75 moa group for 10-20 shots is entirely unrealistic for all but maybe the top 1% of precision rifles and at the hands of only the very best shooters. It’s also entirely unnecessary. 1-1.5 moa will do all you ever need out to 4 digit yardage.

Seating depth is just one of those things that doesn’t matter much. It might make a 1 moa group .75 with perfect tuning, but that simply does not matter. A .25 moa difference in groups will not increase your hit rate one bit and will burn up a lot of time and components trying to achieve that meaningless improvement.

No change in seating depth is going to make a bad group good. Barrels either like the powder or the bullet or they don’t. Those are the variables that actually make a difference. If a barrel hates the bullet or the powder, or both, no amount of seating depth tinkering is going to change that.

Spend your time, energy and components on things that matter. Find your 1 - 1.5” moa 10+ shot group and get out of the reloading room. Go shoot!
 
@SDHNTR is 100% correct but I also understand the struggle.

There are a lot of factors that can impact this measurement and most have been hit on here. While I do think a minor variable is your press and die, I don't believe it's make or break. Sizing consistently is important. Powder charge is important and if you're compressed at all, you're going to have some seating depth challenges. Bullet construction and consistency is another one and likely the largest. In a small sample size of 6mm and 25cal bullets between hornady and berger, I've found more variation in the hornady bullets.

The 3 thou you're referencing isn't enough to be concerned about. Go shoot.
 
Interesting. Quite a few precision reloaders point to seating depth as one of the largest factors in keeping groups tight. I have not tested that principle, or even done very much seating depth work, and have thought maybe that was why my most of my better load workups are good for 1 to 1.25 MOA (measured over time or in groups of 10-20 shots; those same loads will usually print 5-shot groups about 50% smaller) instead of consistently shooting the 0.5 to 0.75 MOA 10-20 shot groups I am looking for.
Do you have a very high end custom gun? Best of everything? Do you use the most precise and expensive components and dies available? A .5-.75 moa group for 10-20 shots is entirely unrealistic for all but maybe the top 1% of precision rifles and at the hands of only the very best shooters. It’s also entirely unnecessary. 1-1.5 moa will do all you ever need out to 4 digit yardage.

Seating depth is just one of those things that doesn’t matter much. It might make a 1 moa group .75 with perfect tuning, but that simply does not matter. A .25 moa difference in groups will not increase your hit rate one bit and will burn up a lot of time and components trying to achieve that meaningless improvement.

No change in seating depth is going to make a bad group good. Barrels either like the powder or the bullet or they don’t. Those are the variables that actually make a difference. If a barrel hates the bullet or the powder, or both, no amount of seating depth tinkering is going to change that.

Spend your time, energy and components on things that matter. Find your 1 - 1.5” moa 10+ shot group and get out of the reloading room. Go shoot!
@SDHNTR is 100% correct but I also understand the struggle.

There are a lot of factors that can impact this measurement and most have been hit on here. While I do think a minor variable is your press and die, I don't believe it's make or break. Sizing consistently is important. Powder charge is important and if you're compressed at all, you're going to have some seating depth challenges. Bullet construction and consistency is another one and likely the largest. In a small sample size of 6mm and 25cal bullets between hornady and berger, I've found more variation in the hornady bullets.

The 3 thou you're referencing isn't enough to be concerned about. Go shoot.
I agree with those answers.

To answer the OP question though, you'll always have a little bit of variance because even a VLD seating stem isn't seating off the ogive. With an RCBS Rock Chucker press and Hornady Custom Grade seating die with a VLD stem I can keep it down to about +/- .0005" on Mitutoyo calipers if I fully seat, rotate 180°, and run the ram up all the way again. Probably around +/- .001-2" if I just seat them. Now I don't even measure loaded rounds that come out of a set die, maybe the first one from a new lot of bullets.
 
I agree with those answers.

To answer the OP question though, you'll always have a little bit of variance because even a VLD seating stem isn't seating off the ogive. With an RCBS Rock Chucker press I can keep it down to about +/- .0005" on Mitutoyo calipers if I fully seat, rotate 180°, and run the ram up all the way again. Probably around +/- .001-2" if I just seat them. Now I don't even measure loaded rounds that come out of a set die, maybe the first one from a new lot of bullets.

That's a good trick that I use with compressed loads. Forgot to mention that.
 
You can easily check for yourself if it makes a difference by sorting loaded rounds by seating depth. You don’t even need to fire separate groups - color long bullets red with a sharpie, short ones blue, and leave those in the middle uncolored. On paper if there is a difference in your rifles it should be easy to spot if it’s significant.

Many seating stems are quite rough so changes in the force required to seat the bullet force the stem onto the bullet varying amounts. I’m a fan of sacrificing a few bullets to use them for lapping the inside of the stem to match the bullet shape. Just chuck one end of the bullet in a drill, add some 600 grit lapping compound and work it until there’s a nice smooth area of contact. (I actually carefully chuck the stem and run it on the points of loaded rounds, making sure to not screw up the threads and to clean the lapping compound well, but people read that and do it all wrong so I lead with the technique that is harder to screw up.)

Some cartridges, like the 338 wm, are almost always shot with compressed loads, so I’ve never considered it something to stay away from, but there is a difference in technique. Getting a good settling of the powder is important, if that’s tapping by hand, long drop tubes, or mounting a small motor with offset weight to create a vibration on the loading bench. An old school square orbital sander without sandpaper often had a felt base that loaded cases could be lightly touched against, but it’s more trouble than it’s worth and tapping the case with a bullet is strangely therapeutic.

The next most important thing with compressed loads is to get that seating stem smoothed out. If the bullet is sticking in the stem and the shell holder is actually having to pull the loaded round off the stem, it needs more work because that can be enough to slightly pull the bullet.

Leaving the loaded round in the seating die with press cammed over for a few seconds can make a big difference. I seem to get the best results with one second for some, 5 seconds for others, but have had to use 10 seconds before. The less neck tension, the more problems you’ll have. If you’re using bushing dies with low neck compression, bullets will get pushed out more easily. Same with excess neck lubrication.

Compressed loads aren’t perfect, so I cull the longest and mark them with a sharpie for use as fouler shots or practice seated or standing where accuracy doesn’t matter as much. We laugh at the number of times we can ring the 300 yard gong offhand with fouler shots when we first pull up, and how hard we have to concentrate to do it with good ammo later in the shooting session.
 
Some or most of the variation you are seeing is differences in the bullet tip itself, especially if loading lead tipped bullets. Be consistent in your press stroke and look at picking up a Hornady Bullet Comparator set. I have come to really like using them. Bullets sometimes vary at the tip lot to lot a little so my process is this: When I first build a load I will measure around 25 bullets for length and find the average bullets. (tend to vary +/- .005" with a bell curve). I then use the average length bullets to set my seating die. Once they are the length I am after I measure them with the bullet comparator and write this number down. Then every time I reload that load I use the Comparator and it's notated length setting to set my seating die. Using the comparator will show you much more consistent depths than you realize you are getting. Mine tend to run out less than .002" at very most, and generally exactly the same length.
 
Some or most of the variation you are seeing is differences in the bullet tip itself, especially if loading lead tipped bullets. Be consistent in your press stroke and look at picking up a Hornady Bullet Comparator set. I have come to really like using them. Bullets sometimes vary at the tip lot to lot a little so my process is this: When I first build a load I will measure around 25 bullets for length and find the average bullets. (tend to vary +/- .005" with a bell curve). I then use the average length bullets to set my seating die. Once they are the length I am after I measure them with the bullet comparator and write this number down. Then every time I reload that load I use the Comparator and it's notated length setting to set my seating die. Using the comparator will show you much more consistent depths than you realize you are getting. Mine tend to run out less than .002" at very most, and generally exactly the same length.
OP literally specified measuring Base To Ogive with a comparator.
 
OP literally specified measuring Base To Ogive with a comparator.
Dang it I read right over that, sorry guys! Only thing I can add then is press stroke. I have noticed that if I just run the stroke down to stop and back up I get small variations. If I give a little extra pressure at the end of stroke every time into the stop my depths are more consistent.
Also lite neck tension can be a culprit do don't be afraid to play with neck bushing size if needed.
Happy Reloading
 
I'm usually getting 0.003 inch total variation.
I've noticed significantly higher variations on compressed loads. Double. If I have a choice, I shoot for non-compressed loads.

I don't know if there are any bullet seating dies that actually press on the ogive. That's the only way you could isolate out bullet variations in the process. I found some batches of nosler ballistic tips that are extremely consistent like +/- 0.0015 and on those I can just measure overall length. Some expensive tipped bullets are all over the map for bullet length. And yes I know it doesn't matter.

Basically, I'm not a good enough shooter to pick out this variation from normal noise. Still shooting sub 3 inch groups at 500 yards under still conditions with what some of you might call a musket (308) I'm more worried about variable wind then I am about 0.002 on seating depth. Seating depth matters in a lot of rifles. But not that small variation at my skill level.
 
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