I know I'm just a junior college grad, but help a dummy out here please. Why are the numbers I'm getting using my comparator and seating guage so different to what Sierra manual calls for.
Cobt vs. Sierra coal's
- Thread starter Dan wilson
- Start date
RegionRat
Gold $$ Contributor
What are the specifics?
Many times, the length dimensions of seating depths given in loading manuals are a good starting point but not a law of physics. Other times, depending on details, they can be rigid and moving them can cause issues.
Sometimes, it is caused by a lack of standardization in what tools we use and where they make contact, for example using a comparison versus an absolute method.
Describing the details of the cartridge and intended use would help answer your question.
Many times, the length dimensions of seating depths given in loading manuals are a good starting point but not a law of physics. Other times, depending on details, they can be rigid and moving them can cause issues.
Sometimes, it is caused by a lack of standardization in what tools we use and where they make contact, for example using a comparison versus an absolute method.
Describing the details of the cartridge and intended use would help answer your question.
I'm testing sierra#2145 165gr SBT they call for 3.34 coal. My guage tells me the cartridge is 3.483 into the 300 win mag. When I follow the recommended procedure my comparator tells me my base to olive is 2.9. Subtract off .020 and the desired seating depth is 2.88 . To me it just seems to short.
shoot4fun
Gold $$ Contributor
Why do you have two threads with exact same questions?
I would say, reading the manner in which you've explained, that you are in a state of confusion.
Coal= cartridge over all length
Cbto= cartridge base to ogive
2 different measurements.
Cbto= cartridge base to ogive
2 different measurements.
RegionRat
Gold $$ Contributor
Dan, I assume you mean that your bullet touches lands when your at 2.9 with the comparator, but that should be the comparator value called Cartridge Base to Ogive as @1badgoat points out, not the 3.340 COAL.
It is okay to work within less than 0.020” of touching lands but let’s assume you like the 0.020” jump value. What COAL do you get with a bullet set to 0.020” jump?
ETA: some sporter guns can benefit from tuning that seating depth, others do not. Let’s assume your experience isn’t high and you start with something like 0.020” jump just to make sure the magazine function is good and that you don’t have any issues removing a live round. Nothing wrong with that type of starting point. If your COAL is as 3.483 with a 0.020” jump, test your magazine feed to make sure it works and you are good to go. You will not have anything dangerous with that type of seating depth difference using that bullet and cartridge.
ETA2: I forgot to add, 3.340 was a historical COAL max used for old 30 caliber specs and it is often a default for things like 30-06, 300 Mag, etc. It is not a law of physics so some rigs like things shorter or longer than 3.340 and that is okay.
It is okay to work within less than 0.020” of touching lands but let’s assume you like the 0.020” jump value. What COAL do you get with a bullet set to 0.020” jump?
ETA: some sporter guns can benefit from tuning that seating depth, others do not. Let’s assume your experience isn’t high and you start with something like 0.020” jump just to make sure the magazine function is good and that you don’t have any issues removing a live round. Nothing wrong with that type of starting point. If your COAL is as 3.483 with a 0.020” jump, test your magazine feed to make sure it works and you are good to go. You will not have anything dangerous with that type of seating depth difference using that bullet and cartridge.
ETA2: I forgot to add, 3.340 was a historical COAL max used for old 30 caliber specs and it is often a default for things like 30-06, 300 Mag, etc. It is not a law of physics so some rigs like things shorter or longer than 3.340 and that is okay.
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Thanx goat, after comparing I dropped over .5" over the coal. After I take off .020 from the compared 2.9" seems short to me. Sierra publishes 3.34" for that bullet. I don't think I'm doing anything wrong.
I'm all good with that Rat, let's say I scoot a bit closer, say 7.5/1000. That's a depth of 2.8925. Is this normal cartridge length for a 300win mag? I know this is a relative thing. BUT am I being to picky about this?
Take comparitor off of calipers and measure your OVERALL LENGTH.
2.8925 is only 9 hundredths longer than a factory SA cartridge length.
OK as suggested by another poster I may be guilty of being confused. Please bare with me. So my OAL GUAGE was 3.483 ... and if all the planets align on the magazine front and if I decide to back off the 3.483 by .020 , why did I buy the comparator?
RegionRat
Gold $$ Contributor
No, it says you are paying attention and until you gain experience that will allow you to learn faster and keep your eyes and fingers where they still work too....
All three of the popular 30 cals have some potential for bullet seating depth tuning with the Sierra 165 GK. It is very similar to the 168 MK. It works well in 308, 30-06, and 300 mags. (And for the money, does a very good job on accuracy and terminal ballistics too. So it makes a good baseline hunting bullet choice.)
That particular bullet is very forgiving in most chambers and tolerates everything from hard jam to big jumps. That is not to say that there is nothing to be gained from learning to tune with seating depth, but that takes some experience as well.
A quick anecdote to explain what a good starting point would be if I knew nothing about the chamber... Suppose I was going to ask a gunsmith to make a custom hunting rig. He would attempt to chamber for a light touch to 0.005” jump for a dummy round that I would hand him. So, where would I then place that particular hunting bullet for a starting point?
I would observe the COAL when the junction of the bullet bearing diameter hits the boat tail, and line that up slightly (~0.010) above the neck to shoulder junction of the case to avoid seating into a potential donut later on. Of course, this just gets me a sturdy bullet seat. I also already know enough about that particular bullet at that kind of length to know it will feed from a magazine.
Could I have selected a longer length, yes. Could I seat deeper, yes but then I risk a donut issue and rob myself of case volume too. Going a little farther out allows me some room if I find the tuning like a little more jump so I can then seat a little deeper without hitting that donut again.
I’m guessing you get the picture by now. Here is some nightstand reading on depth tuning.
https://precisionrifleblog.com/2020/03/21/bullet-jump-and-seating-depth-reloading-best-practices/
That is one article that leads to several. Reading those is food for thought and comes for free, so I would soak it up while you practice cause eventually you will want to learn to run a sweep for powder charge and one for seating depth.
RegionRat
Gold $$ Contributor
Dan, as long as folks are civilized, there are no worries answering questions or teaching terminology. We don’t all start out with an expert mentor or parent that reloads, so it is okay to come onto a forum and ask for help.
Remember that on this forum, we might be answering at a third grade level one minute, and post doc level the next. It is okay to say where you are on that scale up front and nobody is going to flame you for being a rookie as long as you are civilized.
It is always best to bring some background and homework to the party to show you have at least some skin in the game. I have been a student and also an instructor my whole life. I can take orders, or give them. Today I am stuck waiting on the city inspectors so ask away and I will type for free... LOL
Now were gettin somewhere your COAL is fine even if you subtract. 020.
Your comparator is for sorting bullets, and measuring shoulder set back on brass during sizing.
BBTO= Bullet base to ogive, this aids in consistent seating depths and is also used for your....
CBTO= Cartridge base to ogive, which is the smaller number you were referencing.
These are the 2 most important measurements for keeping a consistent distance to the lands to maintain accuracy once seating test has been preformed.
COAL just let's you know if cartridge will fit the magazine.
Bullets vary the most from ogive to meplat in forming the shape of point as its being drawn down, this is why sorting bbto is critical to uniformity.
shoot4fun
Gold $$ Contributor
Dan,
I went down to the shop and took some measurements of my 300WM ammo. Here is what I got and what I saw:
My 300WM loaded with 190 SMK, which is well off the lands and still fits easily into a AICS 300WM magazine, measured 3.465" COAL. Sierra's latest manual, Edition VI I believe, shows the same COAL for the 190 SMK and the 165 SBT at 3.3340".
My eyes were saying that my ammo looked short, IE the bullet appeared to be seated very deeply.
Maybe that is what has confused you?
The bottom line on COAL is, at least for me, that it is not a key number to dwell on. I only care that my ammo (no matter the cartridge) will fit the magazine box and feed 100% reliably. My main concern is CBTO, as that is telling me just how far off or in the lands those bullets are seating.
I do try to follow a self-imposed rule that the bullet be seated at or close to a full bullet diameter into the neck but there are times when I don't even follow that "rule" if the rifle likes it out farther.
thank you all for your time and your hospitality. i have for some time did the coal test on my bolt guns, ie. loading in a cartridge into the breech, 10 times getting a different reading each time, tossing the high and lo, then avg the rest , and taking .010 off of that and it seemed to work out very well. being older im tired of that technique. hornadys guage does it all quicker and easier. all i care about is accuracy for hunting. form what im getting is the fact just the guage is enough to get that job done. my comparator is just a tool for the future, if i follow you correctly. so my 3.483 guage measurement -10 then maybe -20/1000 is getting me the groups i want is awesome.....
my question is how does one interpolate the reading from the guage along with the reading from the comparator to get the precision you have set forth.
ok Bc, im ok now,,,,i did exactly what you said to do. i am good coal is 2.47 .01 off the lands. what was driving me nuts was the 3.34 in the manual. i still have to check the magazine. i see what you mean about the seating depth, plenty of room for that,,,,and THANK YOU.....
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