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  #1  
Old 11-07-2009, 02:53 PM
Skip Sackett Skip Sackett is offline
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Exclamation Seating depth versus OAL

There are times when I think folks get confused over these two issues.

The difference between OAL and seating depth.

Some will ask: "Aren't the two the same thing?" To which I reply: "No."

When asked to explain I usually use this photograph:



While both of these bullets are of the same caliber and weight one can readily tell that they both have different lengths. When loading these bullets you can attain the exact same OAL but which one is going to have the deeper seating depth?

It may not matter in the low pressure 45ACP round. One thing for sure though, it will in the 40S&W that is being fired in a "Tactical Tupperware" or anything else with an unsupported chamber. Now just to dispel any rumors that you might take from that statement, I love my G22 RTF and handload for it for every occasion.

So OAL does matter but more importantly bullet seating depth. It plays a much more important role on pressures than does anything else. That information is taken from an older Speer manual where they did actual tests on the subject.

That brings us to another situation that crops up. "What do I do when I can't find a recipe for the EXACT bullet I have?"

Like this:


One can see that the same OAL is attainable with either bullet but which one will have more of it in the case? Which one will have the lower pressure using the same amount and type of powder? Well, unless the bullet is jammed against the rifling, the one with less seating depth.

By the way, this is the whole principle of Elmer Keith's bullets.

Just a thought.

Last edited by Skip Sackett; 11-08-2009 at 08:36 AM. Reason: made an old timer mistake!
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Old 11-07-2009, 03:06 PM
Jellybean Jellybean is offline
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Which one will have the higher pressure using the same amount and type of powder? Well,..., the one with less seating depth.
Now I'm really confused! I though increasing the seating depth increased the pressure.
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Old 11-07-2009, 03:46 PM
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Default Do it right?

The loaded cartridge has to fit into the chamber and not jam in the clip, magazaine, cylinder -- whatever. It's usually safer to load 5 test rounds than 1,000 that are too long, too hot, or too weak.

At the end of the discourse, it is good to have loaded ammunition the works correctly in the selected weapon.
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Old 11-07-2009, 06:49 PM
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Now I'm really confused! I though increasing the seating depth increased the pressure.

It does generally, but bullet composition and bearing length play just as big a roll in pressure as anything else does. A bullet with a longer bearing surface will almost always turn up higher pressure than a bullet with a shorter bearing surface when both weigh the same, and are seated to exactly the same length, or a harder makeup like a jacketed bullet can and almost always does turn up higher pressure than a cast bullet of the same weight with the same depth of seating. But......... a cast bullet that is too soft and fired in a high pressure round like the .454 Casull can turn up extremely high pressure also.

A cast bullet that is seated deeper than a jacketed bullet, or a cast bullet that has a shorter bearing surface than another of equal weight, but is seated deeper, may not necessarily turn up higher pressure, and odds are it won't.

Last edited by Gun 4 Fun; 11-07-2009 at 07:01 PM.
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Old 11-07-2009, 09:26 PM
Jellybean Jellybean is offline
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Gun 4 Fun wrote:
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It does generally, but bullet composition and bearing length play just as big a roll in pressure as anything else does.
Absolutely, but this thread wasn't about bullet composition or bearing length.

If it were, you would have to know the chamber and bore diameter to calculate just how much of the ogive on the LRN bullet would play a part. In the case of the two bullets pictured above the LRN appears, to me anyway, to have less seating depth and less bearing surface. But close up photography can be very decieving.

Last edited by Jellybean; 11-07-2009 at 09:31 PM.
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Old 11-07-2009, 11:01 PM
Gun 4 Fun Gun 4 Fun is offline
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This thread was about what causes pressure to rise, sometimes excessively. Seating depth is certainly one, but bearing surface as well as composition of the bullet are too, and have everything to do with pressure issues.

Little if any of the ogive comes in contact with the rifling lands, let alone the grooves, so the only way to know for sure how much really needs to be checked on a fired bullet when determining bearing surface.

If a bullet is too soft, it can over obturate, which acts more like a bore obstruction, and pressures spike quickly. In early Freedom Arms guns chambered in .454, too soft bullets were a big problem, since the only bullets available that could withstand the high pressures generated by that round were those from F/A. A lot of handloaders tried using standard .45 Colt bullets with terrible results, and some guns were even damaged by their use.

If you use two identical bullets with one being cast, and the other being jacketed, fired from the same barrel, and using the exact same load data and seating depth, the cast bullet will show less pressure everytime. Again, composition is in play. Same goes with bearing surface. The one with the longer bearing surface will give higher pressure, all else being equal, everytime.

Again, a bullet that has a longer bearing surface than another, while both way the same, will show higher pressure, if they have the exact same seating depth.

Bearing surface, and composition can affect pressure every bit as much as seating depth.

Last edited by Gun 4 Fun; 11-07-2009 at 11:04 PM.
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Old 11-07-2009, 11:28 PM
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This thread is completely ignorant of varying brass lengths. OAL is more crucial when dealing with varying brass than seating depth IMHO. I do not know about everyone else, but I tend to load up rounds that are not only the same make and model, but even from the same lot. The same however can not be said of the brass that I am loading in. I have yet to meet a person who loads several different type of bullets with the same load data, expecting the same result.

That being said I see OAL as much more crucial than seating depth, as it determines case volume. Constant seating depths not only would be a pain in the ***** with varying length cases, but it would result in varying case volumes, thus varying pressures.

If I am misinformed please educate me
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Old 11-08-2009, 12:00 AM
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Seating depth is more important than OAL as far as pressures go.

For case length to be an issue, you would have to let them get way too long before they would affect pressure as much as seating depth or bearing surface, and that would be because they can't expand at the mouth properly to release the bullet when they are fired, which will jack up pressure.

How much space the bullet takes up in the case, as well as how much of the bullet is in contact with the bore play far greater roles in pressure than cases that are over length by a couple thousandths.
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Old 11-08-2009, 12:10 AM
ajpelz ajpelz is offline
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I respect what you have to say, and you may very well know more than me, but to your post seems to somewhat contradict itself. You say OAL is less important, but space taking up in the case is more important. Doesn't OAL determine case volume? Not seating depth? Seating depth determines bearing surface not case volume when dealing with varying case lengths, right?
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Old 11-08-2009, 12:19 AM
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Bearing surface is determined by how much of the bullets sides are parallel and will bear against the bore.

Case volumn is affected by how deeply the bullets base is seated into the case. If you have two identical bullets, but one has the crimp groove closer to the base than the other, the bullet with the crimp groove closer to the base will not be seated as deeply if both are crimped in the crimp groove, but will have a greater overall length since more of the bullet will be sticking out of the case. There fore it takes up less space in the case, and volumn is increased over the bullet that has the crimp grove closer to the bullets nose, which will necessarily have more bullet seated into the case, which creates a shorter overall length.
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Old 11-08-2009, 12:27 AM
ajpelz ajpelz is offline
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But if one case is longer than the other, won't that bullet have to be seated deeper to maintain the same case volume? And OAL would be the same so bearing surface would not be changed?

P.S. thanks for dealing with me... just a curious guy trying to learn.

Last edited by ajpelz; 11-08-2009 at 12:30 AM.
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Old 11-08-2009, 12:44 AM
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I am glad to try to help you understand this.

Technically, yes if a case is longer than another, and bullets are seated using the same crimp groove, the case that is longer will have more internal space if you use the same load data. If one case is longer, then you would need to seat deeper in the longer one to maintain case volumn, and OAL, that is correct. But the crimp plays a roll in how you seat, so you have to take al things into consideration.

But.....as I said earlier, IF you have let cases get that much too long, you have other problems to start worrying about. Also, you would need to implement better reloading habits, as no case should gain so much length that it can effectively increase case volumn enough to affect pressure that much. Unless you are using a round for an auto, you should be using the crimp groove provided, unless you have enough experience to know how to crimp in different locations, like over the front driving band on some bullets like Lyman 358429, which is too long for the N frame .357's when crimped in the crimping groove. There again, case volumn comes into play when you change the crimp location from what is shown in the manuals.

If you are using the same bullet in both cases, then bearing surfaces will be the same, and not play a role in pressure.

Last edited by Gun 4 Fun; 11-08-2009 at 12:49 AM.
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Old 11-08-2009, 12:53 AM
ajpelz ajpelz is offline
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Thanks. That post is somewhat reassuring my instincts.

Like I said, I always use bullets that are the same length. I understand we are talking only thousandths of an inch, but that seems to be what this thread is all about

I mostly load autos, so I am using a TC. Either way, I always thought OAL and case volume dictated pressure more than seating depth. You learn something everyday, right?

I mostly use speer gold dots and Rem golden sabres for reloading, so I do not work with cannelures often. I set my seating die to my desired OAL, which determines case volume, and go at it.
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Old 11-08-2009, 12:58 AM
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Like I said in another thread the other day, in an auto, there is only so much room to play for OAL when dealing with max lengths. I never try to deep seat a bullet, but I will try for a longer OAL than listed if I can do it and still have my ammo function through my magazines. It lowers pressure, and will usually (but not always) improve accuracy. I use a TC for my auto's too, as do most people, especially if there is no crimp groove.
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Old 11-08-2009, 08:35 AM
Skip Sackett Skip Sackett is offline
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Now I'm really confused! I though increasing the seating depth increased the pressure.
Jelly,
Sorry for the mis-statement. You of course are right. I will fix the original post too.
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Old 11-08-2009, 08:50 AM
Skip Sackett Skip Sackett is offline
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I hate to disagree with folks about the intent of my original post but, bullet seating depth has a much greater impact on pressure in handgun cartridges over all of the other factors mentioned in this thread. The other things are issues, and to that I agree but bearing surface has no factor in KBs in unsupported chambers.

The damage is done long before the bullet enters the barrel.

The article is a must read. And as soon as I find it again, I'll post it!

It has pictures and everything!
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Old 11-08-2009, 11:42 AM
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For you folks that are trying to wrap your head around the ramifications of this seating depth thing, you might want to do a little research on Boyle's Law of Gases. I also strongly recommend that you seek out load manuals from the manufacturer of the bullet you are working up a load for. You also would be well advised to compare that data with that of the powder manufacturers. I also strongly recommend getting a chrono to verify the results in your individual firearm. For you people that shoot jacketed bullets from manufacturers that also sell loaded ammo with your bullet of choice.........buy a box of factory ammo and examine it for COL. It never hurts to go back and re-read your load manuals to make sure you are not developing any BAD HABITS!
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Old 11-08-2009, 12:10 PM
dennis40x dennis40x is offline
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I’m not going to get hyperventilated over the topic of OAL (AKA- Over All Length) versus seating depth.

First I’m not a Handloader but a Reloader there is a difference. 99.9% of what I reload is dedicated to punching holes in paper. That said there are various OAL which are usually dependent on configuration for bullets of the same weight. The 200Gr SWC employed with the 45ACP is a good example of this. Unless I’ve over looked what’s been written the expansion ratio hasn’t been touched in depth. It was either in a Speer or Hornady manual of years past an example was made of the 9mm-Luger with a change of (X-Dimension 3/32 in?) the load went from acceptable PSI standard to catastrophic.

I’m not into making the 38Spl a quasi 357Mag or the 44Spl a 44Mag. For across the course shooting highpower rifle the AMU had the “V8” load for the M-16 rifle in match shooting. Let’s just say it was pushing the envelope.

I’d say we’d all be better off just to apply standard loads with components specified in manuals starting at the low end then working up as opposed to starting at the maximum going up from there.
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Old 11-08-2009, 03:34 PM
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Skip, you don't owe me an apology. With your vast knowledge of reloading I was pretty sure that it was just a case of "think one thing and type the opposite" -itis.

But I do owe you an apology for causing the thread to get hijacked. Sorry about that.
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Old 11-08-2009, 05:14 PM
Skip Sackett Skip Sackett is offline
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Jelly,
I don't think you caused the thread to get hijacked either. The caution was to understand the need to overlook OAL sometimes and consider seating depth.

I just got a book from NKJ_Nut as a prize. It is Phil Sharpe's book on Handloading (reloaders need not read! ).

It seems to me that something has been lost over the years in understanding in our fine hobby. I have had to rethink things before and have done so on this issue years and years ago.

Adjusting OAL for fit or function with disregard towards seating depth is an accident waiting to happen.

Who cares if it fits and functions if it is a time bomb just waiting to go off.

Remember, this isn't for those that get bullet x, find recipe y and use the exact components to build bullet z. I'm not saying there is anything wrong with that, don't get me wrong, and this isn't "aimed" at Dennis, many folks just want bullets that shoot and don't experiment one iota. I mean this, that's fine. They are re-loaders.

This was for those few of us out there that take bullet xx without any recipe and try to build a safe, functioning round for it.

Take for example the 45ACP. Say I want to put a 255gr LRNFP from a 45Colt in it and run it in my M625JM. Where do I start? Well, I can tell you, seating depth. I will compare the crimp groove on that bullet to one I have known data for near the same weight. I will seat that bullet in the case, maybe make up a dummy round with no powder and primer, I've done that, and see how it chambers. Maybe it does. If so I go on from there with the adapted recipe. Starting low and working up.

What if it doesn't fit and I need to seat it deeper? Then I will until I get a round that fits my chambers. I will carefully note how much deeper I had to seat it and adjust the powder charge accordingly, maybe reduce it a full grain to start out with, working up from there.

That seating depth makes all the difference in the world in pressure problems, I don't care what anyone says. If you don't believe that, find someone with a semi-auto who has had a KB due to bullet set back. OAL didn't cause the problem, seating depth did.

An experiment of mine focused on this issue. I took a load with known performance and seated the bullet only .030" deeper. All of the other factors were the same, powder, primer, lube, bearing surface, bore condition, yada yada. From that deeper seating depth I saw an increase of over 100fps from my firearm. Fortunately, it wasn't a maximum load. What if it was though and I had total disregard for this knowledge and seated the bullet deeper to get it to function or fit in my firearm? Maybe nothing, maybe something catastrophic and it would be due to my ignorance.

When folks want to argue and fuss over what they know it only shows how much they really don't!

Last edited by Skip Sackett; 11-08-2009 at 05:17 PM.
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Old 11-08-2009, 05:43 PM
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Quote:
Originally Posted by smith crazy View Post
Jelly,
That seating depth makes all the difference in the world in pressure problems, I don't care what anyone says. If you don't believe that, find someone with a semi-auto who has had a KB due to bullet set back. OAL didn't cause the problem, seating depth did.

An experiment of mine focused on this issue. I took a load with known performance and seated the bullet only .030" deeper. All of the other factors were the same, powder, primer, lube, bearing surface, bore condition, yada yada. From that deeper seating depth I saw an increase of over 100fps from my firearm. Fortunately, it wasn't a maximum load.
Didn't your OAL get reduced by the same .030"? I am mystified on how you can seat deeper and maintain OAL.
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Old 11-08-2009, 06:27 PM
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Quote:
Originally Posted by smith crazy
When folks want to argue and fuss over what they know it only shows how much they really don't!
If they don't agree with you, then they know nothing, right?

This is what I mean by being arrogant. You always come at these posts like you are the only one with any knowledge. A lot of us, have been at this a long time too. You can say what you want about it all, but now you are the one who is giving bad advice to newbies who may be reading this thread. Bearing surface length, and bullet composition have just as much to do with pressure as depth of seating. All three are equally important. Pressure doesn't peak until the bullet has engaged the rifling, plain and simple. Sure there is some presuure upon ignition, but it hasn't reach maximum until the bullet is being held back by the rifling.
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Old 11-08-2009, 09:24 PM
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Originally Posted by smith crazy View Post
Jelly,

I just got a book from NKJ_Nut as a prize. It is Phil Sharpe's book on Handloading (reloaders need not read! )


Remember, this isn't for those that get bullet x, find recipe y and use the exact components to build bullet z. I'm not saying there is anything wrong with that, don't get me wrong, and this isn't "aimed" at Dennis, many folks just want bullets that shoot and don't experiment one iota. I mean this, that's fine. They are re-loaders.
I read that book years ago. Sharpe’s jargon from the nineteen-thirties is nettlesome.

I know you’re not pecking at me. A shooter I am, I am and freely admit that Reloading allows me to shoot. It’s I’d rather be shooting than Handloading.

Still no one directly talks about the expansion ratio which is volume. The OAL is normally a do not exceed standard measurement but I’ve also seen minimum OAL addressed in loading data. The minimum OAL addresses issues in regards to pressure.

As for a rifle that is bolt guns and gas guns for match shooting the OAL is normally a limiting factor in gas guns. My bolt guns are throated to exceed the standard specified OAL but that’s a story for a different time and place.

Some times these group discussions’ be whizzing contests
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Old 11-08-2009, 10:00 PM
Skip Sackett Skip Sackett is offline
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Exclamation Duplication was what I was after.

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Didn't your OAL get reduced by the same .030"? I am mystified on how you can seat deeper and maintain OAL.
Yes, and it does seem like it is confusing. The point of my comment was this though, a seating depth difference of .030" increased the velocity by 100fps.

Let's say that the bullet SHAPE made the bullet sit that much more in the case. Seeing as I don't have a bullet stretcher I can't make both situations occur at the same time.

Now, if you happen to purchase bullets, as in the first picture I posted, from one manufacturer and some from another and they have a bullet stretcher ( ) you can end up with the situation you describe.

That is the point that I am speaking to. I know that there are other factors that play a part in pressure changes. I'm not speaking to those at this point. They are to be considered and I alluded to that in the original post.

Look at the picture of the two plated bullets. That is my point. It could be more egregious than those two bullets portray.

The old timers understood this. That is the reason that Elmer and Sharpe designed their bullets like they did. Same weight as nominal bullets for caliber but a whole lot less in the case. They filled that void with powder.

This thread isn't going to be a whizzing contest Dennis cause, I'm out of here!

Handejector, please delete my account!
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Old 11-09-2009, 11:00 AM
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I think Skips OP was aimed at the fact that just because data might be for a certain bullet weight, not all bullets of the same weight are of the same length. And yes that can have an effect on pressure.

As others have stated there other factors that will also effect pressures, such as bearing surface, bullet composition and hardness, temperature, chamber dimensions, barrel fouling and etc.

But let's look at the data given both in print and on the 'net. Most of them don't give a clue as to what any of these variables are. They list a lead bullet of a certain weight with no information of seating depth, composition, bearing surface or etc. And I know they are well aware that there are people out there that don't care to find out either, they will use it regardless starting at the max load. And it usually doesn't cause a problem. Usually, but occaisionally, in a worst case scenario, some poor fool will blow his gun up due to a lack of caution or attention. Always start at the starting loads if you are using any component other than the data was derived with. Inculding bullet, primer, firearm etc. It you read their disclaimers this is what they use to save their butts.

And a word of caution too. Just because you aren't getting the velocities a data source lists for a certain bullet, doesn't mean you aren't getting the pressures. This isn't a green light to add a little more powder until you get the same reading on your Chrony at the local range as they did in their lab.
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Old 11-09-2009, 12:32 PM
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re: "It's usually safer to load 5 test rounds than 1,000 that are too long, too hot, or too weak."

...oh sure.....NOW you tell me......
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Old 11-09-2009, 01:22 PM
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Quote:
Originally Posted by dennis40x View Post
Some times these group discussions’ be whizzing contests
...and sometimes these contests become personal and spread across several threads and that is unfortunate. Differing opinions are the lifeblood of any forum. If we all knew everything and agreed all the time this forum would be useless and deserted... FWIW

I often experiment with different bullets, powder, primer combinations. To be honest I do little else. Now I'm casting my own as well and this is just going to add one more variable to the mix. This thread served it's purpose. I'm going to be more attentive to ALL of the variables at the bullet end of the cartridge that can affect pressure. Thanks.
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Old 11-09-2009, 03:58 PM
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Differing opinions are the lifeblood of any forum. If we all knew everything and agreed all the time this forum would be useless and deserted... FWIW

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38spl, 45acp, cartridge, casull, colt, crimp, fouling, hornady, model 16, primer, tactical

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