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gschwertley
veteran member
Posts: 251
(1/16/05 11:25 pm)
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Some Thoughts on Cartridge Over-all Length
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Most of you already know this, but it may come in handy for someone. Over the years, I had to learn a good deal about reloading through my own devices, and a few times the hard way. Finding out information is easier now with the internet.
We are accustomed to having the use of a reloading manual to give us all the required data we need in loading a given cartridge, including the cartridge maximum over-all length. In what I have heard called the land of "Milsurpia", however, sometimes (!!??) we do not always have a reference book at hand to give us the necessary data for loading various components in the military calibers.
There is no such thing as "one" maximum o/a length for any given caliber. The max. o/a length will be determined by the profile or shape of the bullet. This profile is governed by what is called the ogive (pronounced OH-jive), that is the amount of curvature between the shank and the tip of the bullet. Some bullets come to a sharp point; others bell out in the sides on their way to coming to the point.
For the sake of this discussion, I will compare two that seem different to me. They are the .30-06 and 7.62 NATO service cartridges. To my eyes, the .30-06 comes to sharper point than the 7.62, which seems to balloon out a bit before it comes into a point.
Now we have to mention the leade of the rifling. This is the area in the rifle barrel where the rifling starts, just ahead of the mouth of the chamber. The rifling has a ramped, gradual beginning. This is because the bullet needs a smooth introduction to the rifling to maintain proper chamber pressure. If the bullet hit the sharp edges of un-ramped rifling going, say 2,500 feet per second coming out of freebore (that unrifled area just ahead of the chamber and before any rifling starts), the engagement would be too abrupt and chamber pressures would spike.
Getting back to comparing the .30-06 and the 7.62 NATO, I have never checked to see if the leade in these two calibers is different or the same. Let us say just for the moment that they are. If they were, the two different bullet shapes would require different seating depths because of the ogive. If you were to seat them to the same depth, the 7.62 NATO bullet with its different ogive might be jammed into the rifling when chambered. If so, when fired, dangerous chamber pressures could result.
The seating depth will determine the cartridge max. o/a length. It is said that optimum cartridge o/a length is obtained by seating the bullet at that point just short of leade engagement. Said another way, with minimum amount of freebore. This may not always be the case for a given rifle, but it is commonly suggested. Some reloaders don't like to play it this close and leave a little margin for safety.
There are different ways to take this measurement in a given rifle. One method you can use is with taking measurements with a cleaning rod or dowel; I have forgotten how to describe that one. You can look that one up on google if you are curious. The method I use is to make up a dummy cartridge (no primer or powder), and experiment with seating depths using practical knowledge about the cartridge or by comparing it to factory-loaded rounds. If the bullet is seated out far enough where the rifling contacts the sides of the bullet, it will leave burnishing marks. You can use black felt-tip marker on the side of the bullet for repeat attempts. Just keep experimenting until you get the seating depth to where the rifling will no longer burnish the bullet. If you want to play it safe, seat it a little deeper.
Then there is the world of shooting lead bullets in these rifles. This takes more care and usually more experimentation to be successful in accuracy. I have said before that casting and loading lead rifle bullets is to shooting as flyfishing is to fishing. It takes more knowledge, care and practice. Casting your own lead rifle bullets is a time-waster, too, just like tying flies, but for the patient person, it is very rewarding.
I mention lead rifle bullets here due to their unusual shape compared to military "ball" (actually pointed) bullets. Many of these have long, rounded noses. It makes you wonder about seating depth on these. Relax. The part of the bullet that is ahead of the driving bands, if properly selected for your rifle, is exactly the width of the rifling. When inserted into the chamber, the long sides of that part of the bullet are in the barrel, just resting on the lands of the rifling. The part of the bullet that engages the rifling doesn't start until it gets to the driving bands. You can seat the un-banded part of the bullet out as far as you want. If the rifle has a lot of free-bore before the leade, many lead bullet shooters seat part of the bands outside of the case to get them as close as practical to the leade.
Many of our milsurp rifles have generous leade in the rifling and therefore bullet seating depth hasn't been much of an issue. If you use bullets with a cannelure (that little knurled groove around the shank of the bullet), this just about tells the reloader where a "proper" seating depth is. There are exceptions to this practice, however.
That brings me to the one milsurp that in my experience requires caution in bullet seating depths and cartridge max. o/a length. This would be the Swiss rifles, the Gewehr 1911 and the Karabiner 31. I don't know about the earlier ones. These rifles in my experience have a very short freebore before the leade; be careful with overall cartridge length when loading for this round. If you were to use with the Swiss, say, milsurp pulled 7.62 NATO bullets with their generous ogive, then for sure you would not seat them in the cannelure. These should be seated below their original cannelure to get a safe max. o/a length in the Swiss.
yockey5
Owner
Posts: 3635
(1/16/05 11:35 pm)
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Re: Some Thoughts on Cartridge Over-all Length
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Believe it or not GS, I understand everything you have said here. Thanks
Don
Yesterdays Weapons forums
http:pub151.ezboard.com/bsteyrmannlicher
Why did I leave that m95 sporter in Nashville?
mikmarjon
Administrator
Posts: 2652
(1/17/05 1:08 am)
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Re: Some Thoughts on Cartridge Over-all Length
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Me too.I use the pulled .308 bullets in 147 grain and I heard that they need to be seated a little more on a loading board,for that very reason.I seat about .010 under and they seem to work great.Still very accurate also.
mike
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txpete
contributing member
Posts: 26
(1/17/05 10:39 am)
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Re: Some Thoughts on Cartridge Over-all Length
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well written and very good information.
when loading for milsurp rifles this really comes in handy.I smoke the bullets and make a dummy round for setting OAL.
it really helps out with the finn mosin nagants as some have "D" chambers and throats and some don't so OAL is very important.
I cast my own bullets and it is a little more work but alot of fun when you get to that special load that works.right now I have the 185 gr lrn gc and a .314 sizer on the bench just waiting for the brass to come in from graf's.
grafs does have 7.7 jap brass in stock for 27.99 a 100 which I think is a good deal and good reports on the quality of the brass here on the internet.
my fav cast bullet is for my 45/70.I just like hearing that bullet hit home .I gave some to a good friend and he dropped his buck with one this year.
pete
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