Measuring CBTO for Real and Cartridge COAL

[Ernest T]

I received my modified cartridge from Hornady today so I'm able to measure my Savage's headspace. I had to do it multiple times because I wasn't sure how hard to seat the case in the chamber or the bullet to the lands. The case doesn't "click" against the shoulder of the chamber, its more like the shoulders just starts squeezing the case. Same for the bullet.

My first attempt yielded 2..272 as did my second. I figured if my third attempt was the same, it would be an accurate measurement and I was done. The next one was 2.264 and the four subsequent attempts were above and below that up to .004. I finally hit on 2.260/1 several times in a row and I'm bottoming out the case against the shoulders without cramming it in there, and the bullet is on the lands with just enough pressure that it requires a good rap on the butt of the gun to get it out. I'm going to attempt this again tomorrow until I'm confident this is a repeatable number.

With a CBTO of 2.260, the COAL is 2.887 with a Hornady ELD Match 140 grain bullet. The maximum COAL for that round is listed as 2.800. Even allowing some space for backing off the lands, this is still going to be a long bullet. As I understand the effects of loading long, velocity, and possibly accuracy, are going to suffer at normal loads because there is extra empty space in the case. A cartridge this long won't fit in my AICS 10 round magazine, but I've heard a 5 round magazine will take a bullet up to 2.900 so that is an option. Is there anything else to consider before I start working up loads with these measurements (assuming the CBTO really is 2.260)?

 

[varget204]

I received my modified cartridge from Hornady today so I'm able to measure my Savage's headspace. I had to do it multiple times because I wasn't sure how hard to seat the case in the chamber or the bullet to the lands. The case doesn't "click" against the shoulder of the chamber, D more like the shoulders just starts squeezing the case. Same for the bullet.

My first attempt yielded 2..272 as did my second. I figured if my third attempt was the same, it would be an accurate measurement and I was done. The next one was 2.264 and the four subsequent attempts were above and below that up to .004. I finally hit on 2.260/1 several times in a row and I'm bottoming out the case against the shoulders without cramming it in there, and the bullet is on the lands with just enough pressure that it requires a good rap on the butt of the gun to get it out. I'm going to attempt this again tomorrow until I'm confident this is a repeatable number.

With a CBTO of 2.260, the COAL is 2.887 with a Hornady ELD Match 140 grain bullet. The maximum COAL for that round is listed as 2.800. Even allowing some space for backing off the lands, this is still going to be a long bullet. As I understand the effects of loading long, velocity, and possibly accuracy, are going to suffer at normal loads because there is extra empty space in the case. A cartridge this long won't fit in my AICS 10 round magazine, but I've heard a 5 round magazine will take a bullet up to 2.900 so that is an option. Is there anything else to consider before I start working up loads with these measurements (assuming the CBTO really is 2.260)?

Did you remove extrater & ejector for accurate measurements ??

 

[Ernest T]

Did you remove extrater & ejector for accurate measurements ??

I'm not sure what you're referring to. The bolt is out of the gun when making these measurements.

 

[Straightshooter1]

I received my modified cartridge from Hornady today so I'm able to measure my Savage's headspace. I had to do it multiple times because I wasn't sure how hard to seat the case in the chamber or the bullet to the lands. The case doesn't "click" against the shoulder of the chamber, its more like the shoulders just starts squeezing the case. Same for the bullet.

My first attempt yielded 2..272 as did my second. I figured if my third attempt was the same, it would be an accurate measurement and I was done. The next one was 2.264 and the four subsequent attempts were above and below that up to .004. I finally hit on 2.260/1 several times in a row and I'm bottoming out the case against the shoulders without cramming it in there, and the bullet is on the lands with just enough pressure that it requires a good rap on the butt of the gun to get it out. I'm going to attempt this again tomorrow until I'm confident this is a repeatable number.

With a CBTO of 2.260, the COAL is 2.887 with a Hornady ELD Match 140 grain bullet. The maximum COAL for that round is listed as 2.800. Even allowing some space for backing off the lands, this is still going to be a long bullet. As I understand the effects of loading long, velocity, and possibly accuracy, are going to suffer at normal loads because there is extra empty space in the case. A cartridge this long won't fit in my AICS 10 round magazine, but I've heard a 5 round magazine will take a bullet up to 2.900 so that is an option. Is there anything else to consider before I start working up loads with these measurements (assuming the CBTO really is 2.260)?

It takes a little practice to develop a feel for getting consistent measurements. This is what I did to help get some consistency. . .

I push the tool in firmly, but not hard, with a certain feel so the case is making good contact as I hold the locking screw. Then I lightly push the bullet with the rod to the lands then with firm light tapping on the rod to be sure the bullet in touching but not jamming. After the light tapping, I'll keep my finger on the rod and tighten the lock screw.

It still takes a little practice, but once you're get the feel for it, you'll get more consistent results.

 

[Ned Ludd]

If I understand you correctly, you'll be loading to mag length, but COAL with bullets seated at "touching" is over mag length. Is that correct? If so, I wouldn't worry too much about it. As long as the bullet you're using will shoot jumped, it shouldn't matter too much. A little extra room in the cartridge does mean more powder to hit a given pressure/velocity, but as long as your target pressure/velocity isn't over MAX, by definition you'll have the room to add more powder. It's also possible the load will tune in under MAX pressure anyhow, so I wouldn't worry too much about a little extra case volume. It's usually too little case volume that is a problem. As long as your case fill ratio is somewhere in the neighborhood of about 93 to 103%, it will be fine.

As far as seating depth, if you determine the MAX COAL that will feed reliably from the mag, you can always carry out a uni-directional seating depth test. That is to say, you start at MAX COAL, then keep moving the bullet incrementally away from the lands; i.e. shorter and shorter COAL or increasing jump; hoping you find an optimal seating depth window before you run out of sufficient case volume.

With regard to using the OAL gauge, I start with the bullet/set screw such that the bullet is moswtly down in the case with just the nose visible. I then push the holder/case/bullet into the chamber with my left hand, holding it tight into the chamber. I loosen the set screw with my right hand and gently push the plastic rod until I feel the bullet just ever so slightly touch the lands. I then keep using the left hand and fingers to keep the case firmly into the chamber while holding the pushrod in place with just a little tension while I gently tighten the set screw. I use a cleaning rod inserted from the muzzle to gently push the entire apparatus out of the chamber. As an "internal control", if you have jammed the bullet into the lands any appreciable distance, you will feel resistance when trying to push it out with the cleaning rod. Is so, I start the measurement over from scratch. When I first started using the tool, I practiced over and over with the same bullet until the reading obtained was reproducible every time. It took a bit, but with a little practice using the same bullet repeatedly, you'll find out what works for you.

 

[RegionRat]

All of the above advice is very good, plus a thought.

If you are just starting to learn this measurement, when you have your value it may be a useful exercise to build a dummy round and try the stripped bolt technique to see when you get a tight bolt closure. Doing both methods helps you get a feel for the measurement method differences.

Some folks get to where the two methods converge within 0.005” or so, and other folks find a bigger offset between the two. At the very least, you will get a feel for how you push with the plastic rod, versus the force when you close the bolt, and the effects on those numbers.

Also, it is a little forgiving in that you only use the value as a reference starting point in a search for a seating depth. The results of the CBTO once you find your best seating depth are what matters more. Don’t panic if the sample measurements aren’t super tight.

 

[Ernest T]

If I understand you correctly, you'll be loading to mag length, but COAL with bullets seated at "touching" is over mag length. Is that correct? If so, I wouldn't worry too much about it. As long as the bullet you're using will shoot jumped, it shouldn't matter too much. A little extra room in the cartridge does mean more powder to hit a given pressure/velocity, but as long as your target pressure/velocity isn't over MAX, by definition you'll have the room to add more powder. It's also possible the load will tune in under MAX pressure anyhow, so I wouldn't worry too much about a little extra case volume. It's usually too little case volume that is a problem. As long as your case fill ratio is somewhere in the neighborhood of about 93 to 103%, it will be fine.

As far as seating depth, if you determine the MAX COAL that will feed reliably from the mag, you can always carry out a uni-directional seating depth test. That is to say, you start at MAX COAL, then keep moving the bullet incrementally away from the lands; i.e. shorter and shorter COAL or increasing jump; hoping you find an optimal seating depth window before you run out of sufficient case volume.

With regard to using the OAL gauge, I start with the bullet/set screw such that the bullet is moswtly down in the case with just the nose visible. I then push the holder/case/bullet into the chamber with my left hand, holding it tight into the chamber. I loosen the set screw with my right hand and gently push the plastic rod until I feel the bullet just ever so slightly touch the lands. I then keep using the left hand and fingers to keep the case firmly into the chamber while holding the pushrod in place with just a little tension while I gently tighten the set screw. I use a cleaning rod inserted from the muzzle to gently push the entire apparatus out of the chamber. As an "internal control", if you have jammed the bullet into the lands any appreciable distance, you will feel resistance when trying to push it out with the cleaning rod. Is so, I start the measurement over from scratch. When I first started using the tool, I practiced over and over with the same bullet until the reading obtained was reproducible every time. It took a bit, but with a little practice using the same bullet repeatedly, you'll find out what works for you.

Yes, mag length is shorter than the COAL length with my measured CBTO.

 

[Ernest T]

Thank you all for the tips. I assumed this was a “foolproof” process, but I’m coming to realize there is some skill and experience involved in achieving accurate, repeatable results.

 

[Coyotefurharvester]

A clean,dry chamber(properly sized brass) and a stripped bolt start with the bullet way out. Use seating die to seat in 0.005 incrementally until bolt closes. Repeat with a second bullet 0.010-0.012 long and seat in 0.0025 increments. More than likely your caliper is +,- 0.0010 and bullets vary, especially lot to lot and type of bullet. But you will have an actual measurement, which will be very helpful when you want to check or compare different bullets quickly with the Hornady tool.

 

[Mark W]

Thank you all for the tips. I assumed this was a “foolproof” process, but I’m coming to realize there is some skill and experience involved in achieving accurate, repeatable results.

IMO, you should simply strive for a repeatable technique. All you really need is a known repeatable point from which to start your seating depth testing. You will also have to do this for every bullet that you intend to test. They will all be different. It’s also a good idea to recheck when changing lots of the same bullet.

 

[Ernest T]

Thanks for your help, I think I can make accurate, repeatable measurements now. I just have to figure out what to do with them! I have some time since I have new brass I'm going to load to SAAMI specs and fire before I have to decide how to use those measurements to reload.

 

[Bronsin]

Look on youTube for Winning In The Wind video called How to find rifling touch point. I found it accurate and easy to do. YouTube won't let me re-post it.

 

[KMart]

Call me crazy. I use the same tool to find touch. I will test a few measurements and unless I get some weird measurements, I call it good and write that # down. If there is .002-.003 difference, that is ok. I do it this way because in load development, I am going to do a seating depth test anyway. I just need a starting point. I can usually find something that will shoot well between .015 jump and .015 jam. That is the important number, where it shoots best.
If you are checking touch in a newly chambered barrel, remember that after about 20 shots, your original touch number will increase a few thousands as the sharp edges on the throat are worn down.

 

[Ernest T]

I just realized I used the term CBTO when what I'm actually measuring is the distance from the cartridge base to the ogive where it intersects the lands. After many tries, that final measurement is 2.264 with these bullets.

I need to load and fire the new brass, before I worry about reloading once fired brass. With that in mind, my magazine opening is 2.870 so I decided to go with a COAL of 2.820 to leave plenty of space in the mag, but get closer to the lands. A bullet seated to that length resulted in a CBTO of 2.192 with the 140 grain Hornady ELD Match bullets. I worked up four loads starting at the minimum listed in the Hornady manual. That's still .072 off the lands and already .020 longer than the max length specified for the bullet.

For comparison, I have several boxes of Hornady 140 ELD Match factory ammo and the CBTO measurement of that ammo is 2.184 and COAL is 2.810. Both measurements + or minus a couple thousandths. These rounds I'm making to fire the brass are closer to the lands than the match ammo.

 

[Ned Ludd]

I just realized I used the term CBTO when what I'm actually measuring is the distance from the cartridge base to the ogive where it intersects the lands. After many tries, that final measurement is 2.264 with these bullets.

I need to load and fire the new brass, before I worry about reloading once fired brass. With that in mind, my magazine opening is 2.870 so I decided to go with a COAL of 2.820 to leave plenty of space in the mag, but get closer to the lands. A bullet seated to that length resulted in a CBTO of 2.192 with the 140 grain Hornady ELD Match bullets. I worked up four loads starting at the minimum listed in the Hornady manual. That's still .072 off the lands and already .020 longer than the max length specified for the bullet.

For comparison, I have several boxes of Hornady 140 ELD Match factory ammo and the CBTO measurement of that ammo is 2.184 and COAL is 2.810. Both measurements + or minus a couple thousandths. These rounds I'm making to fire the brass are closer to the lands than the match ammo.

Ernest - as noted previously, you can do a uni-directional seating depth test; i.e. moving the bullets incrementally farther away from the lands so loaded rounds never exceed mag length. Hopefully, that will allow you to find a seating depth that provides the accuracy/provision you're looking for. If for some reason it does not, one alternative might be to try a bullet that is more forgiving of seating depth - specifically, a tangent or hybrid ogive bullet. The Hornady bullets you're using are secant ogive, which can sometimes be finicky with respect to optimizing seating depth. Berger makes at least couple tangent/hybrid ogive bullets in the 130-140 gr weight range, and Sierra probably does as well. I understand you want to get things working with the components you've already chosen, as I also would. Just pointing out that you may still have additional options that might be made to work well if the 140 ELDMs don't want to tune in easily in your particular situation. Best of luck with it.

 

[Tylsmith33]

My question is why waste time getting an accurate jam measurement or even worrying about it when your aics max mag length is substantially shorter than what you need? I used to mess around with that stuff and ended up finding loads that shot good but didn’t feed worth a dam, or messed up bullet tips. Personally on guns like this I start at a length that will feed flawlessly from my mag and try to find a velocity seating depth combo that will shoot sufficiently. If you are planning to ditch the mag that’s a different story.

 

[Ernest T]

Ernest - as noted previously, you can do a uni-directional seating depth test; i.e. moving the bullets incrementally farther away from the lands so loaded rounds never exceed mag length. Hopefully, that will allow you to find a seating depth that provides the accuracy/provision you're looking for. If for some reason it does not, one alternative might be to try a bullet that is more forgiving of seating depth - specifically, a tangent or hybrid ogive bullet. The Hornady bullets you're using are secant ogive, which can sometimes be finicky with respect to optimizing seating depth. Berger makes at least couple tangent/hybrid ogive bullets in the 130-140 gr weight range, and Sierra probably does as well. I understand you want to get things working with the components you've already chosen, as I also would. Just pointing out that you may still have additional options that might be made to work well if the 140 ELDMs don't want to tune in easily in your particular situation. Best of luck with it.

Thanks for the suggestions, I do have some options to explore. I tried several loads today using RL-16 and the ELD 140 Match bullets and most didn't shoot worth a darn, with the exception below. I only made three rounds of each load so I didn't have enough to find out if this was just dumb luck or I was on to something. I'll make some more give it another try.

 

[Ernest T]

My question is why waste time getting an accurate jam measurement or even worrying about it when your aics max mag length is substantially shorter than what you need? I used to mess around with that stuff and ended up finding loads that shot good but didn’t feed worth a dam, or messed up bullet tips. Personally on guns like this I start at a length that will feed flawlessly from my mag and try to find a velocity seating depth combo that will shoot sufficiently. If you are planning to ditch the mag that’s a different story.

I didn't know that was the situation when I started. It's a little disappointing to discover the rife chamber is so large.

 

[RegionRat]

I didn't know that was the situation when I started. It's a little disappointing to discover the rife chamber is so large.

Don't loose heart!
You can learn a lot about shooting with what you have in games where the gun is body supported. You can build experience with position shooting and learning wind while you plan for the next barrel or rig.

Part of the learning curve, is learning what the best standards are for the goals of your shooting. You learn the better components, barrels, chamberings, scopes, triggers, stocks, etc., for your interests as you go. Till then, you run what ya brung the best ya can. No worries.

 

[Sandstorm]

Factory chambers are super endowed with free bore from my experience.
Your best bet was Ned's suggestion.. Start at Mag max and go in incements of .003 shorter till it shoots.
or switch bullets and try it again until you find one that works for you.
my 2c