Whats the point--CBTO and COAL and Magazine limits

[cmoore806]

To all the experienced precision rifle shooters and reloaders:

I fire formed a case, sent it to Hornady to make a custom Modified Case. I cleaned the chamber on my Seekins Havak Bravo 6.5 Creedmoor. I measured the CBTO multiple times and with different bullets. Moral of the story and root of my problem is this: the CBTO measured for my chamber causes a COAL of 2.900" which is 0.10 longer than reloading data OAL and longer than the magazine for the rifle. SO WHATS THE POINT of even measuring these things to get the correct distance from the lands if I will only end up seating to the Max OAL allowed by my magazine anyways?

Also, My CBTO was not identical between two different bullets, I thought that CBTO would be the same for ANY bullet in the same chamber. Is that wrong?

 

[Powder_Burns]

Every bullet has a different bto measurement. Even bullets from the same mfg and lot will have some variance. If you are running a long bullet and obsessing about a consistant jump to the lands, and hampered by magazine lengths then single feeding will be required. When getting a new barrel spec’d out for a specific cartridge your leade can be set for a specific bullet so you can have your desired jump to the lands and be within mag length, but as your throat erodes that length will grow in relation to your chamber’s leade and you may be exceeding mag length at that point. The other workaround is to get a longer action and mag that gives your bullet more leg room to stretch out.

 

[918v]

There are multiple seating depth sweet spots. You can find one that works for you and fits in the mag.

 

[Steel head]

Pick a nice length that reliably feeds then do a jump test working shorter.
Getting up and cozy on the lands is out dated thinking.

 

[Jackomason]

it really doesn't matter. make it fit in your mag and work your way back till you find a good seating depth. its just a reference number that keeps you from blowing yourself up.

 

[cmoore806]

I should have prefaced my original statement by saying my goal is to hit steel targets from 200-1000 yards, not get 1/2" groups. I dont want to spend a lot of time with load development. Just find a load that works without blowing myself up (or my rifle) and can ideally group reliably at 200-1000 yards. And of course, the man behind the gun is the biggest opportunity for reaching this goal, and to focus on myself I want to be confident that the load I use isn't a piece of crap like the man behind the gun at the moment.

If my CBTO is 0.1" longer than my magazine allows then the safest bet from a pressure standpoint for load development would be to have a bullet at MAX OAL possible within MAGAZINE LIMITS and then do ladder test to find the max charge and to find the best group. Unfortunately I am currently limited to 100 yards for load development. But once I find a load that shoots a 100yard group I can live with I figure I could just leave my COAL at 2.800" magazine length or fart around with the seating depth to see if it makes a difference. But then playing with the seating depth will change the load/pressure which starts the cycle over again and IMO is a big waste of time. I cant think of a process that gets me out of the cycle, and this is the reason reloading does not make this endeavor cheaper than just shooting factory ammo.

 

[padom]

To all the experienced precision rifle shooters and reloaders:

I fire formed a case, sent it to Hornady to make a custom Modified Case. I cleaned the chamber on my Seekins Havak Bravo 6.5 Creedmoor. I measured the CBTO multiple times and with different bullets. Moral of the story and root of my problem is this: the CBTO measured for my chamber causes a COAL of 2.900" which is 0.10 longer than reloading data OAL and longer than the magazine for the rifle. SO WHATS THE POINT of even measuring these things to get the correct distance from the lands if I will only end up seating to the Max OAL allowed by my magazine anyways?

Also, My CBTO was not identical between two different bullets, I thought that CBTO would be the same for ANY bullet in the same chamber. Is that wrong?

You need to measure the Distance to the lands using your Overall Length Gauge for EVERY bullet you want to reload for that rifle and record it in your log for that specific rifle/barrel.. Then you are set till you rebarrel that rifle..

First lets clear up so terminology as not to confuse people trying to follow this. CBTO (Cartridge Base to Ogive) measurements are when we measure a loaded round from base of the case to Ogive of the bullet. This is measured using the Hornady Bullet Comparator tool in a set of calipers... This is the correct way to measure your reloaded rounds.

Hornady Lock-N-Load Bullet Comparator Basic Set 6 Inserts

The Hornady Lock-N-Load Bullet Comparator is perfect for the reloader looking to quickly and precisely measure seating depth of loaded ammunition from...

www.midwayusa.com

.

What you are doing is measuring distance to the lands in your rifle with the Hornady Overall Length Gauge I assume...

Hornady Lock-N-Load Overall Length Ga Bolt Action

The Hornady Lock-N-Load Overall Length Gauge is one of the best kept secrets to accurate reloading. This tool allows reloaders to quickly and easily...

www.midwayusa.com

.

After using this tool in your rifle with a specific bullet, you can then remove it from your rifle and measure it one of two ways. With straight calipers which gives you your COAL (Cartridge Overall Length) to the lands for that bullet or you can use a Bullet Comparator on your calipers to get the CBTO measurement to the lands for that bullet...

Now, if you have a 6.5cm that measuring 2.90" to the lands that is your COAL, not CBTO. You dont say what bullet, but I am going to assume looking at my notes on 6 different rifles all chambered with a 6.5cm SAMMI reamer that its a Berger 140 Hybrid or Hornady 135 A-Tip?? (thats if you have a SAMMI chamber)

Those are the only 2 bullets that measure around 2.90" COAL. My COAL measurements to lands in SAMMI chamber with 140 Hybrids is 2.909" and 135 A-Tip 2.900"... Those are the only 2 bullets that measure anywhere near that COAL...

This is distance to lands COAL in 6.5cm SAMMI chamber

123 Lapua: 2.80"
130 ELD-M: 2.82"
130 AR Hybrid: 2.83"
140 ELD-M: 2.831"
140 Hybrid: 2.909"
135 A-Tip: 2.900"


You have a few options, pick one of the other popular 6.5cm bullets as you can see their COAL to lands is way shorter than the 2 mentioned above..Or pick a different magazine.... You dont say what magazine you're using and what the internal max COAL is... But there are plenty of magazines out there that you can use 140 Hybrids and 135 A-Tips in a 6.5cm...

For reference, her is max magazine COAL for 3 mags I use: They allow either longer than you need, or right at the lands for you, which you shouldnt be loading into the lands anyway so you are good to go.....

ARC Mag - 2.965" Max Internal COAL
MDT 308 with Binder - 2.905" Max Internal COAL
MDT 308 without Binder - 2.98" Max Internal COAL
AICS 308 - 2.884" Max Internal COAL

Now while all these magazines will work with AICS cut stock/chassis/actions.. The MAX COAL can also be limited by the action if its not setup for it... For example, a Rem700 needs the feed ramp notched to handle COAL's longer than the AICS 2.884"...... Something like my Bighorn TL3 can take all of the above magazines and its max COAL's.....

So to answer your question, its either your magazine, rifle, chassis/stock, action or a combination of those things that is limiting you being able get at or near the lands with the bullet in question (unknown right now).... With the right setup, lots of us have no problems getting at or near the lands with any of these bulelts... I measure to the lands with every rifle/barrel I have, and then start load development at .02" off the lands. Once Ive identified my node and selected a charge in the center of that node, I do a seating depth test to find what my rifle likes the best. You will always find one seating depth that stands out from the rest with the group shrinking up tight.. I cant say the optimal seating depth has ever been one jammed into the lands... Its usually .01-.03 off. I can think of one or two that liked .005 off. I dont jam anything and never found a reason to..

Hope this helps and answers your question

 

[padom]

it really doesn't matter. make it fit in your mag and work your way back till you find a good seating depth. its just a reference number that keeps you from blowing yourself up.

This is a very broad ranging statement and depending on the bullet and caliber it does matter. There are bullets that are very jump tolerant and dont care if they jump a mile (SMK's)and there are bullets that shoot the tightest in that .01-.02 range...... It depends on the bullet and rifle... And we dont know what bullet or magazine hes running. The quick/simple fix to get him properly testing seating depth at .01 and working back to find optimal seating depth may just be a different mag with a longer internet COAL....

This is also not say you CANT load any bullet a mile away from the lands, because we all know you can do anything you want. It just might not be optimal for a few reasons, smallest group size, internal case capacity, ES/SD.... For the point of this discussion, if your using a Overall Length Gauge to find distance to the lands like the OP, then you care and are trying to reload the right way...

 

[cmoore806]

Yeah no confusion here on COAL vs CBTO and thanks for sharing that for anyone reading here who is unfamiliar.

Berger #26795--130gr AR Hybrids; CBTO: 2.2855". COAL 2.9055"
Sierra #1742c 142gr HPBT: CBTO: 2.2875". COAL 2.905"

My Point is that all this measuring is a waste of time if the COAL is too long for an AICS mag because I will have to seat the bullet 0.1" shorter which is 0.1" off the lands at the very closest. It seems people have the opinion that seating close to the lands is the "outdated" method. But it is a concern to me to some degree so I dont blow myself up with a round too long (impossible I've learned since I am limited by COAL that fits a 2.800" magazine limit.

 

[padom]

Yeah no confusion here on COAL vs CBTO and thanks for sharing that for anyone reading here who is unfamiliar.

Berger #26795--130gr AR Hybrids; CBTO: 2.2855". COAL 2.9055"
Sierra #1742c 142gr HPBT: CBTO: 2.2875". COAL 2.905"

My Point is that all this measuring is a waste of time if the COAL is too long for an AICS mag because I will have to seat the bullet 0.1" shorter which is 0.1" off the lands at the very closest. It seems people have the opinion that seating close to the lands is the "outdated" method. But it is a concern to me to some degree so I dont blow myself up with a round too long (impossible I've learned since I am limited by COAL that fits a 2.800" magazine limit.

Ok so if your COAL for a 130 Hybrid is 2.90" then you dont have a SAMMI 6.5cm chamber.. Thats what I would call a factory lawyer chamber. Super long throat. Your an outlier, and the only fix for that is a new chamber/barrel. Now Im confused, you arent loading 0.1" off the lands if your COAL to lands with that 130 Hybrid is 2.9055" and the max COAL of an AICS magazine is 2.884". Thats a jump of .025" to be touching with .005" left in the mag.. You can back that off to .035 and Im sure they will shoot great AND fit in your AICS mag. The AR Hybrids like to jump.

And you arent going to blow yourself up if you are reloading properly.

Step A) (which you have done) is find distance to the lands with said bullet and start .01-.02 off that. Some even start kissing the lands as this finds max pressure then

(B) Do an OCW load workup in .2-.3gr starting 10% below max and working up looking for pressure and STOPPING if you have pressure signs.....

If you follow this you arent blowing yourself up. Its the guy that has zero data, loads some long ass bullet jammed in the lands with a super hot charge that is taking a risk.....


But your original gripe, your situation is unique, its your chamber/rifle that is causing your issues with the bullets in question. Those bullets have TONS of room in a AICS mag fired out of a SAMMI 6.5cm chamber... The issue youre running into was a big complaint with the 260rem chamber....Not being able to load the 140gr and long 130gr bullets near the lands because it didnt fit in a mag.... Thats when 6.5cm came out and allowed you to use these long bullets in a magazine... This is an issue with your chamber/gun not 6.5cm....

Easy cheap fix, grabbed some MDT AICS pattern mags and you can get to the lands and some..

 

[308pirate]

There are two CBTO measurements: CBTO with the ogive touching the lands and CBTO if your loaded rounds.

Unless you really know what you're doing CBTOL has to greater than CBTO of your loaded ammo. How much smaller does CBTO needs to be depends on several things. Primarily it depends on whether you intend the rifle to be a repeater or a single shot. Then refine from there based on group size.

Remember that as CBTO decreases, so does case volume and you can't forget to watch for pressure signs.

 

[Dthomas3523]

If you know you will be using mag length as your max seating depth, then no, you don’t need to measure to the lands, as that measurement doesn’t matter since you can’t load further than mag length.

However, you might not know this until after you take the measurement.

And no bto will not be the same for every bullet. It’s only a comparison measurement. Also, when using calipers and other common tools for this stuff, you can easily induce a few thou of error multiple ways. So don’t get caught up if you’re a thou or two different here and there.

 

[cmoore806]

Ok so if your COAL for a 130 Hybrid is 2.90" then you dont have a SAMMI 6.5cm chamber.. Thats what I would call a factory lawyer chamber. Super long throat. Your an outlier, and the only fix for that is a new chamber/barrel. Now Im confused, you arent loading 0.1" off the lands if your COAL to lands with that 130 Hybrid is 2.9055" and the max COAL of an AICS magazine is 2.884". Thats a jump of .025" to be touching with .005" left in the mag.. You can back that off to .035 and Im sure they will shoot great AND fit in your AICS mag. The AR Hybrids like to jump.

And you arent going to blow yourself up if you are reloading properly.

Step A) (which you have done) is find distance to the lands with said bullet and start .01-.02 off that. Some even start kissing the lands as this finds max pressure then

(B) Do an OCW load workup in .2-.3gr starting 10% below max and working up looking for pressure and STOPPING if you have pressure signs.....

If you follow this you arent blowing yourself up. Its the guy that has zero data, loads some long ass bullet jammed in the lands with a super hot charge that is taking a risk.....


But your original gripe, your situation is unique, its your chamber/rifle that is causing your issues with the bullets in question. Those bullets have TONS of room in a AICS mag fired out of a SAMMI 6.5cm chamber... The issue youre running into was a big complaint with the 260rem chamber....Not being able to load the 140gr and long 130gr bullets near the lands because it didnt fit in a mag.... Thats when 6.5cm came out and allowed you to use these long bullets in a magazine... This is an issue with your chamber/gun not 6.5cm....

Easy cheap fix, grabbed some MDT AICS pattern mags and you can get to the lands and some..

I read up on the MDT AICS pattern mags, looks like they could hold the OAL I am looking for, but as you said my action/chamber would then be a problem and I bet I'd have to cut into my action feed ramps to accommodate. Looks like I need to just load the ladder and get shooting!

 

[Bwhntr53]

There are two CBTO measurements: CBTO with the ogive touching the lands and CBTO if your loaded rounds.

Unless you really know what you're doing CBTOL has to greater than CBTO of your loaded ammo. How much smaller does CBTO needs to be depends on several things. Primarily it depends on whether you intend the rifle to be a repeater or a single shot. Then refine from there based on group size.

Remember that as CBTO decreases, so does case volume and you can't forget to watch for pressure signs.

Ok. I am going to be reloading as soon as I can get everything set up. Using my bolt action can I set my case and bullet in my reloader, of course with no powder or primer, seat the bullet enough for it to set well in the case. Then carefully insert the case with bullet into my rifle. Slowly close the bolt action all the way closed. Take the case out and using my caliper measure the length of the case and bullet. Will this put the bullet touching the lands? Then back
off from that to the published length of overall cartridge. And work towards the lands for the best accuracy. Sorry this is so long. Something that I read in the past but can't remember where.
Thanks
Bwhntr53

 

[308pirate]

Ok. I am going to be reloading as soon as I can get everything set up. Using my bolt action can I set my case and bullet in my reloader, of course with no powder or primer, seat the bullet enough for it to set well in the case. Then carefully insert the case with bullet into my rifle. Slowly close the bolt action all the way closed. Take the case out and using my caliper measure the length of the case and bullet. Will this put the bullet touching the lands? Then back
off from that to the published length of overall cartridge. And work towards the lands for the best accuracy. Sorry this is so long. Something that I read in the past but can't remember where.
Thanks
Bwhntr53

If you don't care about the cartridge fitting in the magazine, you can ignore published OAL..you'll be far longer than that unless your rifle was short chambered by mistake.

If you want to feed from the magazine you only need to check OAL to the lands just to confirm that it is longer than OAL that fits in the magazine.

 

[hollywood1981]

diffrent bullet sizes require diffrent sesting depths ie you would want to get your cbto measurements for that. COAL is more for magazine feeding, i usually measure my length of my mag, and make sure my cartridge will feed/fit.

 

[Ape_Factory]

I started test loads for a new AR15 precision build tonight under guidance of an experienced reloader. I'm a novice. It's a 223 Wylde chamber, 16" barrel, 1:8 twist.. I have a buffet of various bullets to test, from 50gr. Hornady Vmax to Nosler 52gr CC, Hornady 73gr ELD-M, Sierra 77gr, SMK and a few others. Just want to see what the barrel generally shoot well and then develop hunting, target and reasonably accurate/precise cost-effective plinking rounds for training.

Anyway...Started out with the 52gr. Noslers, Custom Comp HPBT, nothing fancy, along with Hornady 55gr. Vmax. They're cheap and I've already prepped 1500 cases.

First things first, I tapped a bullet into the casing and dropped the bolt about 1/3 of the way back, dropping it on the dud round. Did this four times, pulling the bullet each time and taking the overall average measurement. I end up with 1.931. Once I seat the bullets to this measurement using the Hornady comparator gauge and digital calipers, the OAL is now too long to be magazine fed.

So now it looks like I'm having to test with two variables, powder grains and then OAL. Looks like this'll be a two-round test? First test, everything at mag length, vary the powder. Once I fine-tune the powder charge, I test seating depth? Am I missing something? Seems like if I do it to mag length, I have a pretty long jump to the lands. Like the OP, just wondering what the point of even measuring the chamber is when your mag is the limiting factor.

 

[padom]

I started test loads for a new AR15 precision build tonight under guidance of an experienced reloader. I'm a novice. It's a 223 Wylde chamber, 16" barrel, 1:8 twist.. I have a buffet of various bullets to test, from 50gr. Hornady Vmax to Nosler 52gr CC, Hornady 73gr ELD-M, Sierra 77gr, SMK and a few others. Just want to see what the barrel generally shoot well and then develop hunting, target and reasonably accurate/precise cost-effective plinking rounds for training.

Anyway...Started out with the 52gr. Noslers, Custom Comp HPBT, nothing fancy, along with Hornady 55gr. Vmax. They're cheap and I've already prepped 1500 cases.

First things first, I tapped a bullet into the casing and dropped the bolt about 1/3 of the way back, dropping it on the dud round. Did this four times, pulling the bullet each time and taking the overall average measurement. I end up with 1.931. Once I seat the bullets to this measurement using the Hornady comparator gauge and digital calipers, the OAL is now too long to be magazine fed.

So now it looks like I'm having to test with two variables, powder grains and then OAL. Looks like this'll be a two-round test? First test, everything at mag length, vary the powder. Once I fine-tune the powder charge, I test seating depth? Am I missing something? Seems like if I do it to mag length, I have a pretty long jump to the lands. Like the OP, just wondering what the point of even measuring the chamber is when your mag is the limiting factor.

In an AR you aren't getting near the lands and feed from a mag....start with magazine length rounds like 2.25 with 77smk and find your node...then do seating test within mag length to find smallest groupings....you can get ASC SS mags to give longer coal up to 2.31...77smk, and 55 vmax you will find optimal in the 2.245-2.26 range

 

[david8989]

So I'm in the boat of loading to magazine length so max COAL/CBTO doesn't matter to me a ton as far as my final seating depth. But being a comp shooter, I burn barrels up yearly or sooner. So knowing the max COAL at certain round intervals helps me track throat erosion and barrel life. Typically I'll take measurements at 0, 1000, 2000, 2250, 2500, 2750, etc and once I find that the throat erosion is speeding up compared to by baseline earlier in the barrels life then I better have another one on hand. Hope this gives you other reasoning for knowing your measurements

 

[Ape_Factory]

Got it, thanks padom and David 8989! David, I'm assuming throat erosion is increased by your firing rate and the subsequent heat generated? Should I typically check every 2000 rounds or so on a non-comp rifle?

 

[david8989]

Yes throat erosion is directly related to your caliber choice, and the heat generated by your rifle through firing quickly, long strings, and repeated strings when the rifle hasn't fully cooled. If this were a non-match rifle, I would base my intervals on the caliber choice. Every rifle I'd check at 0 & 1000rds for that baseline. For a 6.5 or 30cal non-magnum I'd check again at 2k and every 500rds after that. For a 6mm or magnum I'd start checking every 500rds after 1k. Doesn't cost anything to keep an eye on throat erosion but It could cost you an important shot by not tracking it

 

[Steel head]

In an AR you aren't getting near the lands and feed from a mag....start with magazine length rounds like 2.25 with 77smk and find your node...then do seating test within mag length to find smallest groupings....you can get ASC SS mags to give longer coal up to 2.31...77smk, and 55 vmax you will find optimal in the 2.245-2.26 range

It’s as simple as this.

 

[Shootin Stuff]

You might not be able to reach the lands at mag length, but now you KNOW you can’t reach the lands at mag length. Otherwise you would be guessing and so it isn’t a waste at all.

Comparators NEVER sit directly on the ogive/bearing surface intersection so they will read differently between projectile types that have a different ogive shape, tip to ogive (and COAL) will be different even though they might all have the same (or similar, depending where on the ogive your comparator makes contact) CBTO length.

If you start hand loading for other cartridges in future, just slit a case neck with a thin cutoff wheel, seat a bullet long and close the bolt. The neck tension will be light enough to set the bullet back after touching the lands and tight enough to hold it there whilst you gently eject and measure your CBTO.

As others have said, seat them to max mag length and do a ladder/OCW test and fuck around with shorter CBTO’s once you have found a powder node.

If you have a range of seating depths that group similarly then load to the longer end of that range, this will ensure that as your lands erode you will stay “in tune” for as long as possible. Once it goes out then just move your seating depth forward by the length of your window and it should come right back in again.

 

[upsdownsideways]

Not to dig up an old post, but I am in a similar boat when it comes to mag sized ammo,

I am not quite following the procedure here. My stock will be the MPA matrix, with the appropriate mags with it.

Should I just go with the max length and work my way down from there? Shooting 140 hybrid Bergers which are known not to be jump sensitive.

Where does the Ogive and maximum mag length coincide together?

 

[Powder_Burns]

Start just under max mag length, you don’t want the meplat up against the front of the magazine. Work down in increments until you arrive at a seating depth that shoots the best groups. Take into consideration that as your throat erodes you may be seating progressively longer to keep a consistant jump, and if your oal is too close to max mag length you may not be able to seat bullets much further out in the future.

 

[upsdownsideways]

Start just under max mag length, you don’t want the meplat up against the front of the magazine. Work down in increments until you arrive at a seating depth that shoots the best groups. Take into consideration that as your throat erodes you may be seating progressively longer to keep a consistant jump, and if your oal is too close to max mag length you may not be able to seat bullets much further out in the future.

Thanks. I guess when things reach that point I'll hope it won't effect the grouping that severe.