Understanding seating depth for my rifle

[BowBaboon]

I apologize if this isn’t the most coherent post, but I’m trying to go about reloading for my rifle without making some ding-dong mistake.

Pertinent information:
Tikka t3x lite 6.5 creedmoor 18” factory barrel
I have about 500 factory rounds shot through the gun
Trying to load Hornady 140 BTHP match

So I followed the steps in the Nosler reloading guide (7) that have you flatten the edge of a case previously fired case left unsized, insert a sharpied bullet to the case, chamber the dummy round, close the chamber to mark the sharpied part of the bullet, remove, and measure the dummy round based on where it touched the lands. I did this 5 times to get an average of 2.864”. However, when I looked at the Hornady manual it states the COL as 2.825” which puts my COL 0.039” longer than book. Even if I bump it back .02 for the “jump,” I’m still .019 longer than book.

This is the first time I’m reloading, so if this is has been covered and I missed it I apologize. How should I proceed on this rifle?

 

[JFK]

Book COL is a suggestion by the bullet mfg. Nothing more. They set it to feed and function in all rifles, but yours may be longer. Check your magazine length as that can often times be a limiting factor as well. Assuming you aren’t jamming bullets into the lands, and they fit the mag, it’s actually a good thing to be able to load them longer as it increases the volume for powder.

Important to also note that the book COL is what the load data was pressure tested at with a given powder. Deviating from it in a big direction + or - can impact pressure. You will notice it faster if you jam the bullets into the lands or have very little jump. Obviously start well below max and work up looking for pressure.

You can also just load some up at book COL and they’ll likely shoot just fine. Measuring max COL the way you did can work, but if the bullet grabs the lands while chambering you can get an artificially long max COL measurement because it seats out a bit when you extract.

 

[codym]

A sammi spec chamber is going to assume a coal of 2.825 for factory ammo. Why do you want to start so close to the lands? It’s not a recommended practice for a hunting or any rifle that requires a high level of functionality to be that close to the lands or jammed, you’re just asking for a stuck bullet and an action full of powder at some point. I usually start 25 thou off the lands find my desired powder charge then do jump testing in 10 thou increments 20-80 thou off. When I find my best shooting jump I will fine tune 5 thou on either side. If your numbers are correct I would start at 2.840 coal. I use CBTO measurement instead of coal unless magazine restriction comes into play otherwise I’m not concerned with coal

 

[glock233]

Just use book COL or close and see how it goes. Beliefs about the alleged importance of seating depth is mostly voodoo caused by insufficient sample sizes.

It probably doesn’t matter for most practical applications in 1-1.25 MOA rifles you are probably shooting.

 

[Yaremkiv]

Seating the bullet shorter or longer than published specs isn't inherently problematic but does deviate from published data so you need to be careful; start well below max and watch for pressure signs. Get yourself a Hornady comparator and a CBTO measuring tool for an easier way to measure max COAL/CBTO. I start with finding the lands just to make sure I'm not jamming the bullet into the rifling. I proceed to load to just under max mag length for charge weight testing. Then, I test for seating depth by loading 5 rounds each reducing seating depth by .005"-.01" from the max mag length. I hate to admit it, but I can't prove that seating depth testing has ever produced a better handload.

 

[17CalFan]

I use a sized case and seat a bullet shallow. Measure COL and then Use the Sharpie, and chamber the round. Verify the COL hasn't changed and look for the lands marks. Seat slightly deeper and repeat until the lands marks just slightly disappear. The COL is a vague number, CBTO is what you really want to know. With out a comparator, you'll pull your hair out because bullet tips lie, especially lead ones. I use this gizmo because they are cheap, fast, repeatable and give you a solid number to record.

https://www.brownells.com/reloading/measuring-tools/bullet-comparators/sinclair-hex-style-bullet-comparators/?sku=749002942

 

[Steve300xcw]

I'd load up a dummy so that it fits in your mag. Sharpie one up, and if the bullet isnt into the lands, consider that part done.

 

[peterk123]

If this is your first time reloading I would not bother with tweaking the OAL based on my rifle. Stick to spec. The impact of tweaking OAL is debated to death, and the conclusion I came to is the impact on accuracy is minimal and I'm just not that good of a shot for it to make a difference.

Additionally, you really need to understand pressure signs if you are going to deviate from standard spec. I've been reloading for fifteen years and I'm still not certain about some of the more subtle signs.

The perfect reload is one hell of a rabbit hole. You can spend a lot of time and money trying to find the perfect round. The variable I found to be the most important is powder consistency. Things like tweaking OAL are tiny fractional improvements at best. Clean, resize, trim and deburr if needed, primer, powder then the bullet. Go shoot.

FWIW, I reload 6.5cm for my wife's rifle. Hornady 140 or 143 grain bullet, StaBall 6.5 powder. OAL to spec. It's dead nuts accurate. It matches the Hornady Hunter round results.

 

[Chris in TN]

If I were advising a new reloader in person and they gave me the info you just did about your throat length, I'd tell them to either a) start at book-spec COAL and forget seating depth, or b) start by backing your COAL off by 0.060" and even if that put you seating deeper than book-spec, go with that. There are some fairly decent indicators that something like 0.060" off the lands is a very good trouble-free seating depth over the lifespan of a barrel.

(It's much more complex than that, but that's enough for a new reloader, and then some).

I'd probably just stick with book depth.

Just know that seating longer lowers pressure - to a point. As you get into the lands, pressure increases, due to the force needed to start the bullet moving into the lands from a dead stop. Seating deeper avoids that problem but also effectively reduces capacity. None of this will be a problem if you take to heart the advice repeated in every load manual - start low and work up, for safety. If you have a tight chamber and short throat and thick brass and hotter primers, you could easily end up with max pressure a full grain, or more, before your buddy with the same rifle with a slightly bigger chamber and longer throat and thinner brass (thin brass means more effective capacity) and a standard primer. And all load data - whether found in a manual or online from other people or in a model like QL or GRT - is just a sort of n=1 data point that can't be assumed to be universally safe.

If you have a chronograph, use it. If you start load workup and someone's book max load with a given set of components and barrel length is 2700' and you get above that even with less powder, there's a very good chance you're hitting max pressures, even if you don't see signs. TANSTAAFL. Velocity=pressure, at the end of the day. And you may well hit pressure signs before you hit someone else's published max safe speeds. Life will go on, I assure you. You'll see minor variations in barrel speeds between barrels but nothing major.

But whatever you do, don't start chasing seating depth optimization based on smaller sample sizes. That's a great way to chase your tail and convince yourself you've found 'the load' based on a single 3-shot or even 5-shot cluster of shots.

 

[BowBaboon]

Y’all are the bee’s knees. Thanks for everyone’s input on the topic.

It likely would’ve been helpful for me to put my goals in the initial post. I’m just hoping to reload ammo that will get me 1.5moa at 500 for more economical practice and to learn some new skills along the way.

I will load some at book COL with a lighter charge and slowly work up.

 

[TaperPin]

It brought a smile to my face seeing someone use a traditional method of determining length into the lands. Good job. Marketing has convinced most new reloaders the only way to determine such things is a fancy gizmo, but as you’ve seen the fancy toy wouldn’t have changed anything. The same will be true of shoulder push back.

Not a bad idea to start at published lengths and tinker with length after you’ve settled into what kind of reloading fits your preferences. No rifle on opening morning of the deer season has been put out of commission with standard length rounds, but many newly developed loads .xxx” off the lands were too long to function in magazines, or were some how loaded longer than desired and stuck the bullet in the bore and dumped powder as the case was removed. A good rule of thumb is to cycle a number of rounds through the chamber to not have any surprises before important hunting or range trips.

Just following good basic reloading practices like those described in the book are good to well below MOA accuracy if the rifle is capable. Keep up the good work.

 

[Andouille]

I also have a Tikka 6.5CM and have confirmed through measurements that it has "long throat," and understanding your chamber will be helpful if you decide to tinker with loads to optimize velocity and accuracy. The long throat in Tikka 6.5CM and various bullet loads are discussed in this thread: https://rokslide.com/forums/threads/the-tikka-6-5-creedmoor-reloading-thread.152775/page-4

Here's another great read: https://bergerbullets.com/effects-o...coal-and-cartridge-base-to-ogive-cbto-part-1/

And another article showing that seating depth for some bullets (6mm DTAC and 6.5MM 147 ELD-M) has broad "forgiving" areas where accuracy remains the same: https://precisionrifleblog.com/2020/04/05/bullet-jump-load-development-data/

I believe that seating depth is relevant for two important reasons, despite the common perception that it "makes no difference." I think this is a misconception based on testing of minute increments (0.002-0.010" changes) which are largely a waste of time, but there appear to be measurable changes with large (0.050" jump) increments. The differences I see are:
1. Potential for increased velocity (more powder volume) without additional pressure. Tikka's are already on the "slow" side, so this helps make up a few FPS, especially when using higher quality brass (e.g., Lapua, Peterson, ADG, Alpha) that has a smaller case volume.
2. Finding a "forgiving" seating depth that will allow for throat erosion to never have to "chase the lands" (see article above), and
3. Fine-tuning of accuracy, but I'm taking about 0.030-0.050" changes with 10-round group tests, rather than tedious tiny 0.002" jump ladders to look for "nodes" or other statistically questionable changes.

Let's work through an example of how seating depth affected my Tikka Roughtech 6.5 CM 20" ELD-X loads.

For starters, Hornady Precision Hunter 143 ELD-X COAL is 2.814" and max CBTO is that will chamber is 2.916", so the PH load is jumping ~0.102. Velocity was 2,586 fps and group size right around 1-1.2" at 100 yds.

I loaded 143 ELD-X at 0.050" off the lands (COAL of 2.886") in Lapua brass (1.5gr smaller capacity that Hornady) and achieved a velocity of 2,666 FPS and group size of 0.70" for 14 shots. Pressure signs were minimal and Gordon's Reloading Tool estimated 60,641 PSI, so a window below the SAAMI max of 63,091 that I'm comfortable with, and the increase in velocity and accuracy is appreciable given that my 20" barrel is no speed demon.

For comparison, loading ELD-X in Lapua brass with the same charge at the Factory COAL of 2.814 is estimated to have a velocity of 2,675 FPS and an increase in pressure to 61,210 PSI, leaving a smaller margin for PSI with a fractional gain in velocity.

To conclude, start at Book COAL if you want to focus on a simple first load. If you want to tinker more without introducing too much tedious testing, measure your jump, and then try 0.050" closer to the lands. Maintain at least 0.030" jump from the lands to account for any measurement error, and to accommodate throat erosion, though that is pretty slow in a 6.5 CM. Also keep in mind that brass volume matters; larger volume brass allows for a greater charge and velocity at lower pressure than the same load in smaller volume brass.

 

[Andouille]

Here's a fairly foolproof "loctite" method of measuring jump without the Hornady OAL tool if you're not in a hurry.
1. Use a fired case (de-primed), bend the neck in slightly so a bullet is held moderately tight (just tight enough that it holds when pressed with one finger.
2. add a little Loctite to the inside of the case neck.
3. Insert bullet very LONG in the case.
4. Wipe a little One-Shot or light oil on the bullet ogive.
5. insert round into gun chambering slowly, let sit 24 hrs.
6. slowly remove round. The bullet should be Loctite'd in place at the max COAL/CBTO. The lands shouldn't pull the bullet out.
7. Measure bullet CBTO and COAL. Subtract 0.005" for bullet JAM in the lands to be safe.
7.5. Sharpie on the bullet ogive and rack it back into the chamber, remove, and inspect for marks from the lands- there should be some if the bullet is lightly "jammed." This tells you that the bullet didn't pull out of the case while extracting.
8. Remove bullet by heating case with a lighter and twisting bullet.
9. Run a patch with cleaner/degreaser through the throat of your chamber.

 

[SDHNTR]

I’ve found that method of denting the case mouth to be very imprecise. If you don’t want to use a Hornady OAL gauge then seat a bullet in a dummy case way too long. Remove your firing pin from your bolt. Attempt to chamber the round gently, but do not force it at all. Seat .005”- .010” deeper at a time until it easily chambers, meaning the bolt pretty much falls on its own.

Then measure jump from there (assuming magazine fit).

You could also just load a dummy round to mag length, check for smooth feeding, then load at that length.

 

[Terrapin]

Another interesting way to find COAL that I used back in the day.

Put a flat tipped jag or brass screw in the end of your cleaning rod that is smaller than the bore diameter. With the bolt installed, slide the cleaning rod in from the muzzle end until it hits the bolt (yes, I know putting a cleaning rod in from the muzzle is a no no… be a little careful). Make a mark on the rod.

Remove the bolt and drop a bullet into the bore. Slide the rod up until it hits the bullet tip. Measure from the crown of the barrel to the mark. That is your COAL when the bullet is touching the lands. It helps to slide a pencil or dowel pin in from the bolt side to push against the bullet. You can really feel when the bullet starts to engage if you have the cleaning rod and dowel pin moving the bullet back and forth into the lands. I went so far as to make a set of ground shaft collars to make measuring easier, but it wasn’t as accurate or as easy as the Hornady gage.

As far as which seating depth, I almost always find a seating depth that is more accurate. I’ve had it vary from just off the lands to >.100” off the lands… a laborious and time consuming process. Anymore, I usually just use .015 off the lands or maximum magazine length. Then find a bullet that shoots well at that length at the pressure/ velocity I want to achieve.


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