CBTO LENGTH QUERIE

[mike243]

I have just started on reloading a new batch of ammo, I have measured the CBTO Length and it averages 2.2905 which is 0.107" longer than the standard length I have found on the interweb of 2.1835 for CBTO lengths, am I missing something or is it just that the throat has eroded far enough that is what I have to work with now, Ruger M77 mkll in .243win

thanks in advance guys, Mike

 

[Muir]

You know? Just measuring OAL is a lot simpler and ballistically more meaningful. ~Muir

 

[Smellydog]

You mean this stuff....

 

[PhilRS03]

I have just started on reloading a new batch of ammo, I have measured the CBTO Length and it averages 2.2905 which is 0.107" longer than the standard length I have found on the interweb of 2.1835 for CBTO lengths, am I missing something or is it just that the throat has eroded far enough that is what I have to work with now, Ruger M77 mkll in .243win

thanks in advance guys, Mike

Morning Mike,

It’s bullet dependant. The first point of engagement into the rifling is the ogive so that’s absolutely the correct way to measure. There’s no other way of measuring jump otherwise! If it’s the same make of bullet you’re using maybe start by sticking just that into a comparator and give it a measure. If you’re using a different grain or bullet design than normal, then that’s pretty Normal to see a shift in ogive position between bullets.

 

[Smellydog]

Ogives a sh......ok ok, I'm outta here.....

 

[ChesterP]

I have just started on reloading a new batch of ammo, I have measured the CBTO Length and it averages 2.2905 which is 0.107" longer than the standard length I have found on the interweb of 2.1835 for CBTO lengths, am I missing something or is it just that the throat has eroded far enough that is what I have to work with now, Ruger M77 mkll in .243win

thanks in advance guys, Mike

First rule of reloading: Forget what you find on the "interweb" unless the source is reliable.
There is no "standard length" base to ogive for different makes using the same chambering. It varies from rifle to rifle, hence I refer you to "1" above.
Chambers can be reamed which have different freebore lengths between rifle makes (distance to lands from chamber).

Whatever you have measured yours at, that is what it is, so is ALL that matters. Forget loading close to the lands. Just load to mag length and develop your load from there. There will always be several accuracy nodes for each OAL and these are related to a combination of internal pressures generated and barrel harmonics.

Find one that gives good accuracy which is far enough below max safe load (anything showing pressure signs should be avoided) and also accounts for ambient temperature. Load developing in summer when it's hot will generally show reduced MV's in winter/colder months but load develop in winter and you could be well overpressure come summer. I would recommend loading to a max of 1 full grain under max when developing in colder months. You should find a good accuracy node between half to 3/4 up from starting loads. Stick with one load for each bullet, and after than record OAL or use a comparator for BtoO and just make all future loads the same. Keep it simple. For stalking, you won't need to fret over sub half moa loads. Just find a consistent and accurate node and job done.

 

[Cottis]

It is not a generic measurement. The tool is a comparator. It compares. It would only be generic if every comparator was machined the same to the same tolerances by all manufacturers who offer these gizmos. Your chamber will be different, maybe, to the next one. Your batch of bullets might be different to another batch of the same.

If you must load a round with such accuracy, then really you should take all the bullets from your batch and find one that represents the average from the base of the bullet to a repeatable point on the ogive. Then using that bullet, find out what the max COAL length is for "your" rifle using that bullet. That is from the head of the case to a repeatable point on the ogive of the bullet that contacts your lands. Then your comparator can do its job and give a "measurement" to compare future rounds to.

It is not necessarily a bad thing to do but it mostly will illustrate quite how accurately chambers are cut relative to specs and my advice would just be to take that overall max cartridge length and reduce it by 30 thou and call it good. It will shoot fine. Or you can just ditch that and use what the powder and bullet makers say to use, get some calipers and measure from the case head to tip of the bullet and call it good. That works as well.

 

[mike243]

ChesterP, Cottis

thanks for the replys, most helpful, the length I found on the web I think is a factory ammo case dim, my concern was the length I was finding seemed long and from memory longer than when I last loaded any ammo for this rifle, it must be all of 10 years ago, so I assume that the deeper throat is just errosion from repeated shooting, I'm only starting afresh because I am almost out of my old ammo and cant get any H414, so I have had to change powder so thought I would start from the beginning again just so I know exactly where I am and there doesnt seem to be much more effort involved to load as good as I can over just throwing the components together any old how, just maybe a bit more thought and care, I have some reasonable kit, decent components and a reasonable-ish idea of what I'm doing, my office is in the garden so I sit out here and tinker with my reloading or tying hooks and making pole rigs for fishing rather than sitting in the house watching Eastenders of the real housewives of god knows where or some other equally mind numbing crap with she who lets me wears the trouser.

the bullets are from the same batch as previously loaded although I will need to buy another box shortly which I will measure a few of for interest more than comparison, but I cant imagine there will be such a difference to cause any problems

thanks again guys, much appreciated Mike