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Old January 28, 2014, 08:54 PM   #1
std7mag
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COL rifle...

I used an empty casing, with a bullet to get a measurement of COL.

For the Sierra 160gr. HPBT I got a COL of 3.251". So I was planning on loading to 3.241". Just .009" under what the loading data tells me.

However when I used the same method to for another bullet, I got some way out there readings.

Nosler 140gr. Ballistic Tip.
I got a reading in the gun of 3.371", which I would normally load .010 under that for 3.361". This is WAY over what all the load data's are telling me for COL.
It does fit the chamber, and also the magazine.

Should I go with a shorter COL, which would give me a ton of free length before the lands, or load to the gun???

How is this going to effect the pressures/velocities???

Thanks!!!

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Old January 29, 2014, 10:09 AM   #2
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I might be confused on this but are you trying to measure the distance between the bullet ogive and the barrel lands? If so, you'll need a comparator tool. Sinclair and Hornady both make them, I use the Sinclair. Once you find the maximum distance from the cartridge base to the ogive (CBOT) for your rifle, you can then know exactly how far off the lands the bullet is sitting.
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Old January 29, 2014, 10:24 AM   #3
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If you're going to reload from the rifle's magazine, seat bullets so the over all length (head to tip of bullet) of the round's about 1/16th inch shorter than magazine length. Then make a dummy round with one (no powder or primer) to see if it chambers easily and extracts easily. That's good enough for reliability in use. The actual distance bullets jump to the lands will only have a spread of a few thousandths inch; not a concern.

Use that cartridge over all length for working up loads. Pressure and muzzle velocity will change, but typically in a small enough range not to be of any concern if you know what high pressure signs are to look out for.
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Old January 29, 2014, 10:35 AM   #4
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Different bullets have different profiles and shapes and will have different OAL to the lands.

If you are going to use different bullets and load them a specific distance off the lands you must check each type and weight of bullet separately.

If you are going to measure Max OAL, then reduce .xxx off the lands you Ignore the manuals OAL. It has nothing to do with you,your rifle and your OAL technique.

From Western Powders.

SPECIAL NOTE ON CARTRIDGE OVERALL LENGTH “COL”
It is important to note that the SAAMI “COL” values are for the firearms and ammunition manufacturers industry and must be seen as a
guideline only.
The individual reloader is free to adjust this dimension to suit their particular firearm-component-weapon combination.
This parameter is determined by various dimensions such as 1) magazine length (space), 2) freebore-lead dimensions of the barrel, 3)
ogive or profile of the projectile and 4) position of cannelure or crimp groove.


Quote:
How is this going to effect the pressures/velocities???
The closer you are to the lands the higher the pressure. Reducing OAL will Reduce pressure.
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Old January 29, 2014, 11:56 AM   #5
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Quote:
The closer you are to the lands the higher the pressure. Reducing OAL will Reduce pressure.
I don't believe this to be completely accurate...

With a given bullet, seated to a given depth, in a given case, you have a given amount of space for powder. (lots of givens, I know)

Seat the bullet deeper, and you are reducing the amount of powder space, which generally increases the pressure.

Seating the bullet further out (longer COL, closer to the lands) increases (slightly) the amount of powder space, and generally decreases the pressure (again, only slightly)

Seating the bullet against the lands DOES increase the pressure, because the resistance to the rifling means the pressure spikes a bit, which does not happen when the bullet gets a "running start" when it hits the rifling.

From the bullet wts you gave, and your screen name, I am inferring you are talking about the 7mm Magnum? Couldn't see where you named the cartridge anywhere....
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Old January 29, 2014, 12:32 PM   #6
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Quote:
Originally Posted by 44 AMP
I don't believe this to be completely accurate.
I should have clarified this pertains to Bottle necked rifle rounds and not pistol rounds.

Unlike a straight walled pistol round, In a bottle necked round the case capacity occupied by seating the bullet deeper is a very small percent of the total capacity and has little affect.

There are a couple theories as to why decreasing OAL in a rifle round decreases pressure. Unclenick has a few, but I'll let him post that if he wishes.

I will however post a couple of Unclenick's graphs on OAL vs Pressure.





This is a theory from John Barsness of Handloader/Rifle Magazines. He basically says that peak/max pressure is not achieved until the bullet has left the case, so seating depth has little affect.

It decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.

Handgun powders are much faster-burning, and even regressive, meaning pressure peaks when they're first ignited. Thus seating them deeper allows more time for initial pressure to build.


One can also go to Hornady's web site a read their Theory/Explanation.

Found here. Scroll down to the bottom for "bullet jump vs pressure.

http://www.hornady.com/ballistics-resource/internal

Last edited by steve4102; January 29, 2014 at 12:57 PM.
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Old January 29, 2014, 04:30 PM   #7
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I find it interesting that Hornady uses bad information regarding "headspace" in that link above. As a contributing member of SAAMI, they don't want to use standard terms and definitions for headspace that SAAMI has established. They'll confuse lots of folks calling that space between the bolt face and case head "headspace" while using it to also mean the distance between the bolt face and shoulder. Head clearance is the proper term as stated in SAAMI's glossary for that gap between the case head and bolt face.

Hornady's section on neck only versus full length sizing is atrociously incorrect regarding how a bullet aligns with the bore when the round's fired, but that's another issue. I don't think they've ever taken a real close look at what happens when the round's chambered compared to when its fired. Same assumptions that so many have made over the years put in print again.

And sudden increase of resistance to the bullet going down the bore will increase pressure a bit at that time. It'll go back down once past that point. One point's where the bullet starts to engage the rifling after it's moved a tenth of an inch or so out of the case.
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Last edited by Bart B.; January 29, 2014 at 05:00 PM.
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Old January 29, 2014, 07:45 PM   #8
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Ok, so instead of buying a comparator, I used an empty casing, and one of the bullets that I plan to reload. Insert bullet into neck, ensuring that it moves, but not freely. Inserted into chamber. Closed bolt. Removed casing with bullet carefully so as not to drop, or bang it against anything. Used digital calipers to measure COL for that bullet to where it touches the lands.
Was told to take 0.010" off of that measurement for a reloaded COL.

As I mentioned before, with the Sierra I got a measurement of 3.251". Thus I had planned on reloading to a COL of 3.241".

With the Nosler however I got 3.371". This is considerably longer than what Nosler lists in their reloading.
Note, I did this 5 times just to be sure I wasn't screwing it up!!

Too much travel before the bullet reaches the lands is bad. (so I'm told)


What I'm wondering is if you would load to the 3.361", giving me 0.010" to the lands, or load to the manual???

I don't know if I'm worried about total pressure, as opposed to a spike in the pressure curve...
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Old January 29, 2014, 08:06 PM   #9
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Quote:
Too much travel before the bullet reaches the lands is bad. (so I'm told)
Not really, OAL and distance to the lands is different for every rifle. Mine may like it up close and personal and yours may not. Only experimenting will get you the answer you seek.

A few years ago Barnes did a study on OAL vs Accuracy vs Pressure. What they found was there is often times more than one OAL sweet spot. One up close and another much further away.

So , The less the jump, the more accurate the load is not really a Myth, but it is not totally correct either.

Start .010 off, work up some loads looking for accuracy and pressure. If you find that magic load, great. If not reduce your OAL and start the load work up all over again, repeat until you are happy with your accuracy, velocity and pressure.
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Old January 29, 2014, 08:36 PM   #10
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Will do!!!

Thanks all!!!
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Old January 29, 2014, 09:37 PM   #11
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Std7mag, that won't work perfectly every time, being that a bullet will somewhat stick in the rifling and when you open bolt it is pulled out somewhat from the case. I know because I tried that Like a million times and got different answers most times.
The purchase of the Hornady Lock-N-Load Comparator and the COAL length gage has worked flawless for me on many different rifles with many different bullets, I measure using the comparator with the bullet touching lands and record measurement then I usually subtract .010 and use that until powder charge is found then I tweak length from there..

I'm loading 150 grain Partitions right now, using IMR 4350, and the length is only .003 off the riflings, and it still might need to be pushed back some but thats where I'm starting this data..
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Old January 29, 2014, 11:00 PM   #12
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Quote:
Too much travel before the bullet reaches the lands is bad. (so I'm told)


What I'm wondering is if you would load to the 3.361", giving me 0.010" to the lands, or load to the manual???
Too much travel is bad, but how much is too much? Note that for many years Weatherby rifles had approx. 1/2" of "freebore. This gave the bullet a good running start before hitting the rifling, and while Weatherby rifles were never noted for match winning accuracy, they were plenty accurate enough for hunting game.

As to "loading to the manual", the manual does not know where your rifling begins. The COL is a rule of thumb number that should work in all guns, but may not. Most guns feed better if OAL is less than listed maximums. IF you want to set your bullet 0.010 off the lands, the manual can tell you where it ought to be, but cannot tell you where that actually is...
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Old January 30, 2014, 12:01 PM   #13
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44Amp, Weatherby's ammo made for their rifles didn't have their bullets seated so deep as to compress the powder charge. Seating bullets so deep they compress the powder charges is what often raises peak pressure as the powder's more compacted close to the primer. That chart that Unclenick's often put up showing how that happens is a good way to graphically explain it.

If Weatherby's ammo was fired in a more conventional chamber with minimum freebore (throat length), it would have higher pressure that the norm in their own barrel's chambers. Such is common when folks chamber barrels with a short-throat chamber for the Weatherby ammo; often high pressure signs happen.
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Old January 30, 2014, 12:43 PM   #14
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44AMP, This makes for some good reading

http://www.bergerbullets.com/effects...e-cbto-part-1/

My 270Wby doesn't have the Wby freebore and you are right Wby freebore will effect accuracy and in a custom you get to play with that.
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Old January 30, 2014, 03:18 PM   #15
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This is an excellent thread. Thanks for all the information.
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