I was paging through my copy of Sharp's "Complete Guide to Handloading" and noticed something I'm unfamiliar with, seating depth listed in thousandths of an inch. Example; 45 Colt, 255 Lyman bullet with a seating depth of .425". I assume that is the distance from the case mouth to the base of the bullet, but not sure. The load data is from the mid thirties and this stat for seating is used on all calibers and bullets listed in the text. So, any of the old timers have an explanation, that know for sure how this method is used, or what the dimension means?
Bullet seating depth
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Toprudder
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I wish that info was a standard part of load data today. How deep the bullet seats in the case has a direct effect on pressure.
I was just thinking the exact same thing. It would be an added data point to use when comparing different bullets of the same weight but different profile.
-Jeff
Toprudder
Member
Here is a link you might find helpful in that regard.
http://www.jbmballistics.com/ballistics/lengths/lengths.shtml
243winxb
Member
May have been listed in the 1970's Remington Dupont IMR powder guide??
I remember seeing it some where years ago. Its where i got the idea from.
Seat the shank of all bullets at the same depth from the case mouth. Full diameter of the bullet, not the boatail. Then record the OAL.
243 Winchester cartridge.
Jim Watson
Member
Yup. Seating depth, inversely related to powder space, has a strong effect on chamber pressure, especially in straight handgun cases.
But it requires three measurements and ARITHMETIC! Can't have that.
But it requires three measurements and ARITHMETIC! Can't have that.
That chart and a little math should get me what I'm looking for. It would be nice to see a recommended seating depth though. I may have to do some calculations on some of my loads with same weights and different profiles (manufacturers) to see how close they are on seating depths. Interesting stuff....
-Jeff
Does anybody know for sure? I wonder about the method too. I understand less case capacity = higher pressure and bullet lengths vary and while I have no trouble with OAL, a bunch of knowledgeable reloaders used this dimension in their every day reloading. Sometimes a feller just wants to know...
Zendude
Member
I calculate seating depth when using load data for bullets of the same weight and similar type as the ones I have. It gives me a rough idea of the minimum OAL that I should use (for pistol cartridges).
Seating depth from the case mouth is relatively useless if the case length varies and we know it does significantly.
Let me be clear, I don't know the answer to the OP's question.
What is more interesting to me is the volume of space for the powder, and that is better estimated by the distance from the base of the shell to the base of the bullet. There are still variances in the web of the shell and the thickness of the case walls, but the variance in volume is greater with case length than those other variables given brass of the same manufacturer/headstamp.
While COAL seems to be the preferred method in modern times, by itself it doesn't give a good idea of the case volume. We must also know the bullet length. Where load data specifies a specific bullet that is easy enough, but just "158 gr. LSWC" is not specific enough without the bullet length. More obviously, a 158 gr. jacketed hollow-point is going to be significantly longer than a 158 gr flat nose lead bullet.
Let me be clear, I don't know the answer to the OP's question.
What is more interesting to me is the volume of space for the powder, and that is better estimated by the distance from the base of the shell to the base of the bullet. There are still variances in the web of the shell and the thickness of the case walls, but the variance in volume is greater with case length than those other variables given brass of the same manufacturer/headstamp.
While COAL seems to be the preferred method in modern times, by itself it doesn't give a good idea of the case volume. We must also know the bullet length. Where load data specifies a specific bullet that is easy enough, but just "158 gr. LSWC" is not specific enough without the bullet length. More obviously, a 158 gr. jacketed hollow-point is going to be significantly longer than a 158 gr flat nose lead bullet.
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243winxb
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if the shank is in the neck the same depth for different bullets , the OAL would be different for each bullet.
There are just to many variations in bullets of the same weight.
Different core hardness, different jacket thickness, bearing surface lengths, diameter variations, ogive differences causing one to have more or less jump to the lands. lots of ways to get pressure differences between two brands of bullets of similar weight.
Sierra uses 4 different BHN hardness lead alloys for there bullet cores.
I don't know the sensitivity. A vintage Gun Digest had an article about bullet seating depth, the 38 Special, and pressures. Yes pressures went up as the internal volume went down.
Here is the thing, as long as you are loading that 45 Colt within "standard", and standard for the 45 Colt is pretty low pressure, you don't need to worry about slight variances in seating depth. There will be pressures differences due to seating depth but if you seat to the crimping groove and no deeper, you are not going to have problems. Now if you seat the bullet beyond the crimping groove, like below the front band, then by golly, you are going to experience higher pressures. But, given the low pressure nature of the 45 Colt, you are unlikely to damage anything. This is why I like low pressure cartridges. If you can do the job at low pressure than life is so much better. Remember the old driver educational film that emphasized "Speed Kills!!" The concept is similiar. Little changes don't result in big pressure problems if your cartridges are operating at low pressures. However, lets say you want to run pressures at 454 Casull levels, which is around 60,000 psia. Little changes in anything and everything are going to make huge differences in the pressure curve, usually increasing the pressure. Pressure is not your friend, it does not like you, it will bite you in the butt if you give it an opportunity. Keeping pressures low will keep its fangs out of your posterior.
Hardly anyone bothers with measuring the depth in the case because it is a bother and as long as you seat to reloading manuals OAL's, and stay within published charge limits, the probability of problems goes way down.
HEY, Wait a minute. I know about case volume vs. presssures, variations in bullet length for the same weight, different case volumes, and all the other things some are "lecturing" about. I read a "classic" text from the 1930s and asked if anyone knew how the seating dimension was arrived at and why. Some of the replies have little to do with the subject. "If the shank is in the neck the same depth for different bullets , the OAL would be different for each bullet". Yes, but as I stated all the calibers and all the bullets in the text use a dimension in thousandths of an inch to determine seating depth. Not all use the same dimension. If this method is "useless" then why is a "famous" gun author/shooter/reloader/expert (Sharpe) publishing this info and the text was reprinted 3 (or 4) times? Does anyone else own a copy of "The Complete Guide to Reloading"?
Personally, I seat bullets to the crimp groove when loading for my revolvers and for all intents and purposes, disregard book OAL. I have been reloading for over 30 years and this works quite well for me. I was merely asking if anyone knew about this method, and appearently no one does...
Personally, I seat bullets to the crimp groove when loading for my revolvers and for all intents and purposes, disregard book OAL. I have been reloading for over 30 years and this works quite well for me. I was merely asking if anyone knew about this method, and appearently no one does...
Jim Watson
Member
I do have the book and thought "seating depth" was a useful datum. I did wonder about showing two different values for the same bullet, since I, like you, consider a revolver bullet's crimp groove to be the control on seating. Elmer Keith didn't always, and Skeeter Skelton dealt with one of Elmer's problems by using a bullet with two crimp grooves.
I thought the term was self explanatory and didn't get into "know for sure" stuff.
I thought the term was self explanatory and didn't get into "know for sure" stuff.
Toprudder
Member
Quite often the COL is simply a dimension that will allow the loaded round to chamber in the intended gun. Nothing more. Plunk test will tell you that. Some 45acp bullets can be seated to the max of 1.275" and still plunk, others must be shorter than 1.200" or they will jam into the rifling and not allow the gun to go into battery.
Generally speaking, flat nosed or hollow point bullets must be seated to a shorter COL than round nose bullets, for that very reason.
Does that help?
243winxb
Member
Anyone have an old one laying around?
Nope. I have "plunk tested" thousands of rounds in my semi-autos (even my Garand) and I have "chased the lands" in my 308, and again, I understand "modern" bullet seating methods. I understand that OAL is basically bullet shape vs chamber depth. Jes wanna know...
Toprudder
Member
I ran a few Quickload data sets and imported them to Excel to see the effect of muzzle velocity and peak pressure versus seating depth on a very sensitive cartridge 9mm. I did this in about an hour on a Saturday morning when I had nothing better to do.
Here's one of them:
This is not to be taken as real data, just a demonstration of what we're talking about in this thread
You can see the MV is fairlly linear as you shorten the COL and increase the seating dept, but the peak pressure goes up exponentially. Now the placement of the curves are arbitrary and I placed them and the scale so that you can see one is linear and the other is exponential, but most of us would only have a chrony and not a pressure transducer to measure the actual numbers.
Yes, a varying case height makes seating depth not reliable, but if you know the true bullet length and set the COL, the case height differences will not matter because the volume in the case will be the same if you set the COL. Of course 9mm has wildly varying case volumes too so its best to keep a good margin of safety.
Here's one of them:
This is not to be taken as real data, just a demonstration of what we're talking about in this thread
You can see the MV is fairlly linear as you shorten the COL and increase the seating dept, but the peak pressure goes up exponentially. Now the placement of the curves are arbitrary and I placed them and the scale so that you can see one is linear and the other is exponential, but most of us would only have a chrony and not a pressure transducer to measure the actual numbers.
Yes, a varying case height makes seating depth not reliable, but if you know the true bullet length and set the COL, the case height differences will not matter because the volume in the case will be the same if you set the COL. Of course 9mm has wildly varying case volumes too so its best to keep a good margin of safety.
243winxb
Member
A bullet with a seating depth of .452 in the case is going to be one funny looking bullet 255 grain bullet. Looking at some bullet designs on NOE's website shows a 285 grain SWC should have .390 of the bullet inside the case. a 255 grain SWC should be have .320 inside the case. You have to get up to a 325 grain bullet before you're in the that range. Unless the bullet in question was full wadcutter style bullet that measurement doesn't make sense, and if the bullet is a wadcutter they should have made it more clear.
I routinely load 38 Special bullets in 9x21 cases to be shot in a S&W 929. I adjust the load depending on the amount of bullet in the case. Different styles of bullets have wide variances for the amount of bullet in the case. I regularly use a Lee 140 SWC which has a longer shank than just about any 158 grain RN or SWC bullet out there. The nose of the 140 grain bullet is very small and the front driving band is medium sized. There are some 158 grain bullets that have close to a tenth an inch of difference in the amount of bullet seated in a case when seated into the crimp groove. As rsrocket1's graph shows a tenth an inch can make a big difference.
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