Question on cartridge length consistency

[Unclenick]

What 44 AMP said is true. My reason for posting both all the bullet dimensional variation and the fact shooters are measuring the wrong thing to insure consistent jump, was to point out that controlling actual jump to better than around 0.005" is mostly wishful thinking. It can be done, of course. I made a gauge to measure it years ago, and you can sort your rounds with it and then go back and, using the micrometer adjustment on a seating die, bring the longer bullets down to match the shorter ones. But that begs the question whether or not such control gets you anything. I've not see evidence on paper that it does. I think the reason is that even if you get perfect matching jumps, the variation in ogive length from the bullet base means the annular opening between the bullet and the throat that is responsible for allowing gas bypass will still differ from bullet to bullet.

Dr. Lloyd Brownell suggested back in 1965 that the bypass was what was responsible for throat proximity affecting pressure and he had the plots to prove it (read his study for this and lots of other information). If you look at super slow motion movies of bullets leaving muzzles, you see the bypass gas and some powder particles clear the muzzle first.

I have been left to conclude apparent improvements achieve by tiny incremental changes in bullet seating depth are most often actually random group size changes and not real. Berger seems to have come to the same conclusion empirically.

Now then after the firing pin strike on the primer it has been theorized that there is enough pressure build up to unseat the bullet and shove it into the lands of the barrel before the powder actually ignites to the point of building it's pressure.

Please feel free to correct me if i'm wrong.

You are wrong, mostly. Not sure who "theorized" the idea, but HP White labs did a bunch of work decades ago that showed a high power rifle bullet doesn't even begin to move until pressure reaches 10,000-12,000 psi, which is more than a primer can provide. When a primer unseats a bullet, it happens much more slowly. The bullet has too much inertia for the primer pressure to overcome within normal firing event and barrel times. The exceptions are in very small volume cases where the primer pressure actually can get high enough to beat the powder in moving the bullet. The .22 Hornet is famous for having this happen. The result is erratic velocity due to uneven powder ignition. Many pistol cartridges with short powder spaces appear to do the same thing. .45 Auto, for example.

Incidentally, when a high power rifle case fires, the brass starts to expand. The neck does this from the lowest point of contact with the bullet rolling forward. If your chambers aren't too loose in the neck area, you will have noticed that you can't drop a bullet into a fired case. This is because the mouth of the case starts leaking gas before it expands completely. This creates a pressure drop along the narrow space between the bullet and the neck, so there isn't enough pressure to finish expanding the mouth. The result is the mouth curls in very slightly (see sectioned .308 W case image below). The gas leaking through there before the bullet moves forward significantly is the gas bypass I mentioned above.

 

[F. Guffey]

You are wrong, mostly. Not sure who "theorized" the idea, but HP White labs did a bunch of work decades ago that showed a high power rifle bullet doesn't even begin to move until pressure reaches 10,000-12,000 psi, which is more than a primer can provide.

On different forums there are members that worked at arsenals, I have no problem disagreeing with them, one of them reported cold welding that caused the bullet to hang onto the neck and required 500 pounds of effort to separate the bullet from the inside of the neck. Cold welding just gave reloaders something to worry about.

I felt there was something wrong with the story, he left out a factor, in my opinion the factor he left out was time. If I chamber a round, pull the trigger and then eject the case with no neck I know the neck left with the bullet. And then there is the occasional neck that separates but does not make it down the barrel.

Anyhow; if the pressure does not push the bullet out pressure is shoved back between the neck of the case and chamber, again if the neck does not seal the chamber between the chamber and the case the case will collapse.

I remove a case after firing, I examine the neck; if the neck does not have carbon on the outside of the neck the neck of the case expanded and sealed the chamber. If I do not have the timing worked out pressure escaped back and between the chamber and case.

And then there was the one that claimed gas does not pass the bullet, and I said I have a picture of visible moisture , a smoke ring then the bullet and hot high pressure metal cutting gas eating away the muzzle.

I am beginning to believe time and distance is a little more than a reloader can keep up with. all of this stuff happens in miliseconds.

F. Guffey

 

[F. Guffey]

If your chambers aren't too loose in the neck area, you will have noticed that you can't drop a bullet into a fired case. This is because the mouth of the case starts leaking gas before it expands completely.

I go to the range, get set up, load a round and then fire it. I take the next round and place the bullet into the neck of the fired case. If the bullet touches the neck of the fired case I put the rifle away and will not fire it until I have clearance for the neck to expand, if the end of my case neck is reduce in diameter I shorten the length of the neck to prevent it from getting into the throat of the chamber.

I have never had a case bevel inward when firing.

F. Guffey

 

[RC20]

It not that 44AMP was not right, its how you bring someone along on the learning path.

For the very experienced reloader, yea, no big deal. When you are first getting started and you see all sorts of erratic results that make no sense and are a violation of specs you want an answer.

The perfectly accurate tech answer is not always the one that helps.

I think the answers should be directed to that end.

 

[toxo]

Thanks guys.
I came on here with a few questions and this thread has answered most of em.
I'm new to this malarky. I only reload the 17HH and I've got the basics and have reloaded a few hundred rounds but for one reason or another I've got a lot of cases of different lengths.

First question; How do I determine what length to trim to?

Second question; Are cases that fall below saami specs useless? If not how far below can I go?

Third question; How important is the amount of "hold" on the bullet in the case neck?

I've noticed that some reloaders crimp and some don't. I'm loading two types of bullet, one is flat based and one is boat tail. My logic tells me that the longer cases will have a better hold for the boat tail and the flat based will be better off with the shorter cases?

 

[AxlMyk]

Then I happened to put a charged and primed case on the bench rather than in the die until I reached for a bullet to seat and noticed the case rocked a bit. I reseated the primer a bit more and that completely eliminated the discrepancy I have been seeing. Check your primers.

This was my first thought. The primers.

 

[Arbrn Rngr]

44 amp, If you are measuring your coal and pulling back .03 or less, you will not be able to eyeball that and must use a caliper. Jamming that bullet into the lands could increase pressures that would make one unhappy.

 

[Arbrn Rngr]

Ammo.crafter, I use the exact same technique. Then I check it with a 2 foot 1/4" threaded rod that screws into my fire formed brass, drilled and tapped for my 1/4" threaded rod. I check it 5-6 times both ways for consistency. I use a single stage Lee cast Iron press and the Lee seating dies give me spot on accuracy for my C.O.A.L ( minus .02). Hornady Steel manual dial calipers lock down and each finished round is a perfect match.

 

[Wrought]

I always assumed that its safe to shoot shorter necked cases just not long ones. I set my bullet seat down onto a test round, once i load a test round to my desired depth with no primer or powder.. I make sure most cases are the same length but theres always a couple shorter ones but my dies are set to load them to the same col regardless of case length. I also never crimp for a bolt action, only pistols and pumps/levers. My old pump .35 rem can shoot pointed bullets, i crimp them real good and the tips usually dont get to nicked up

 

[cw308]

You can use the short cases for foul shots until the grow , I trim every firing , find your trim range an trim in the mid range . the only way to accurately measure your rounds is from base to ogive .OAL is important when feeding from a magazine . Fire your new brass then concentrate on accurate case measurements.

 

[cw308]

Unclenick
Your diagrams are the best .

 

[RC20]

Primers should be at least .003 below back of the case.

 

[Unclenick]

I go to the range, get set up, load a round and then fire it. I take the next round and place the bullet into the neck of the fired case. If the bullet touches the neck of the fired case I put the rifle away and will not fire it until I have clearance for the neck to expand, if the end of my case neck is reduce in diameter I shorten the length of the neck to prevent it from getting into the throat of the chamber.

I have never had a case bevel inward when firing.

This is very interesting to me, because I have seen it frequently, just as the sectioned case shows. The case mouth may have sprung back enough that it won't allow a bullet to slip in without friction, but the neck behind the mouth is large enough to, so I've never considered it a sign of a problem. How hot the loads are seems to affect how much of it there is, too, with milder loads leaving the brass less completely formed to the neck area of the chamber.

 

[RC20]

Question:

If you know where the bullet touches the lands, and then regardless of where, have reference on the OGIVE of the bullet, does it matter where that tool you use and its line is?

 

[cw308]

Jamesf553
Is having a problem getting his bullets the same length , if the bullets were exact they would , but the bullets aren't exact , that's why for the seating distance to the lands is measured from the ogive . if his seating die is set properly , his OAL wouldn't be consistent but his ogive setting would be exact . By changing the die adjustments to make the rounds the same length his ogive settings would be all different . Ogive setting is most important .

 

[Dufus]

I have found the only way to have consistent exact ogive to lands is to use CNC made bullets.

Otherwise, manufacturing tolerances will prevent most any other bullet to be the exact measurement as the one before or the one after.

 

[hounddawg]

I have found the only way to have consistent exact ogive to lands is to use CNC made bullets.

Otherwise, manufacturing tolerances will prevent most any other bullet to be the exact measurement as the one before or the one after.

That is exactly right.



here are 15 Sierra 69 gr .223's.BTM is overall length or base to metplat, 2nd number is base to ogive. If your seating die is seating off the metplat the ogive to landing (jump) numbers will vary and if seating off the ogive the COL will vary.

btm bto	
0.902	0.509	
0.902	0.512	
0.9	0.509	
0.901	0.509	
0.903	0.51	
0.899	0.508	
0.899	0.51	
0.899	0.512	
0.899	0.509	
0.902	0.511	
0.9	0.509	
0.903	0.509	
0.898	0.509	
0.899	0.512	
0.901	0.512

 

[44 AMP]

ok, so a couple folks agreed I'm right, but thought my answer could have been better framed for a beginner, so I'll try again, and try to explain some basics.

The first question was about the difference in length in the first time fired cases.

The answer is that this is perfectly normal. Generally speaking the first time brass is fired, it stretches the most. And, each individual case, even from the same maker and lot#, fired in the same chamber with the same load, has the potential to stretch slightly differently than the other cases. so some will be slightly longer or shorter than the average length of the other fired cases.

The next question was about the difference in length of loaded rounds. The answer here again, is this is normal, given the very small differences in length, production tolerances of the bullets, and your seating stem type and where it contacts the bullet.

Most rifle seating stems are made with a cone shaped hollow, to all clearance for the tip of the bullet (so a soft point won't get mashed). The stem contacts the bullet ogive (the sloped/curved tapering part of the bullet) and that contact happens on the bullet at the same point of bullet width (diameter).

That point on the bullet can be varying distances from the bullet tip, depending on the bullet design, and the tolerances involved with both the bullet and the seating stem "hole" size.

SO, it is the difference in individual bullet length, between where the seating stem touches them and the bullet tip that gives you the slight variance in loaded overall length.

Consider, the next time you get new brass and new bullets you measure them, to see just how uniform they are before you start.

First question; How do I determine what length to trim to?

Anything below listed max will work. How many firings do you want before you need to trim them again?? The trim to length is not a set in stone number, it is a convenience number, to give you a few loadings (usually) before the case has stretched back to max length. The usual trim to length for most calibers is 0.01 less than max case length. Some cartridges are listed with trim to lengths 0.005" less than case max length.

Second question; Are cases that fall below saami specs useless? If not how far below can I go?

Useless? No. Not useful for a specific purpose, possibly. As long as there is enough case neck left to securely hold the bullet, the case can be used, meaning it can be loaded and fired safely. Accuracy, on the other hand could be different, every rifle has its own quirks.

I once had a trimmer setting "accident" and cut some .223 cases way too short. Like, the necks were only half as long as they should have been, short.
And, of course the error wasn't caught before loading them. I fired them in a Mini 14. There was no detectable difference in function or accuracy in that gun. In a precision bolt gun? who knows??

Third question; How important is the amount of "hold" on the bullet in the case neck?

The amount of "grip" in units of measure, I don't know, all I know is that it has to be enough, and it has to be uniform for accuracy. The old rule is at least one caliber length of case neck in contact with the bullet as a minimum.
Some folks say they get fine results seating to only a half caliber "depth". If you aren't loading "long" there's usually well more than one caliber of case neck gripping the bullet.

44 amp, If you are measuring your coal and pulling back .03 or less, you will not be able to eyeball that and must use a caliper

I'm not sure where this comes from, as I didn't mention anything about how I measure or load anything. and I know I didn't say anything about "pulling back .03" or anything like that.

if it is a comment on my comment " if you, and your rifle can't tell the difference, except with a caliper,.." let me explain what I meant by that.

What I was referring to was all the "measuring to the ogive" and "bumping the shoulder back .xxx" using different gauges to measure to the lands, and the other "tricks" that have been mentioned.

Say for example, you do all the tricks, checks, etc., and with your best loads you get 3/4" groups. Now say you load the same loads, but without ensuring each case is exactly the same length (just all below max), and you seat to a bit below the max listed loaded length, and don't do anything else, and you still get 3/4" groups.

IF that is the case, then the extra work ensuring maximum uniformity of each round is essentially wasted. YOUR rifle may, or may not shoot the precision crafted ammo significantly better than "regular" ammo. The only way to know for sure is to try ammo loaded both ways (with your most accurate load), and see.

 

[jamaica]

Dufus: "Case length has nothing to do with COL.

Differences in bullet ogive is what is screwing you up."

Yes, this.

Here is what I do: Adjust the seating die to give you the bullet length I want with the first bullet. Then load and seat bullets. The calipers are put out of reach. Don't worry about a slight difference in OAL as long as they go in the magazine and load OK. Then go shoot and have fun!

About trimming cases, I shot a 30-30 for many years in a lever action rifle. These cases did stretch and I would have to trim them. These are the only cases I have ever needed to trim.

44 AMP gives good advice above.

 

[Unclenick]

You can minimize the variables in the above by measuring enough bullets to know their average length. Pick two that are as close to average as you have. Use the first one to adjust the seating die. Use the second to make a final adjustment. Then all the errors will be plus or minus, but center around the average.

Some bullets have bearing surfaces that are shorter than a caliber, so these obviously cannot be seated to provide a caliber of bullet grip (the old M1 Type military match bullet or the current Sierra .308 155 grain Palma bullet, for example). Just sure these shorties don't get loose in the case going up the feed ramp, and you are seated deeply enough and will have to depend on load tuning to make further improvements.

What you will find is that different tools "find" the ogive at different places. The Sinclair bullet comparator inserts and hex nut tool both seem to match chamber reamer throats well, so they catch the ogive pretty far down. A lot of seating dies catch it rather further up. This, IME, results in up to around a 0.002" discrepancy comparing a Redding Competition Seating Die to what the Sinclair insert finds on a 168 grain .308" SMK. A Lee Dead Length Seating Die catches them still closer to the nose. So you just have to accept there will be some degree of imprecision in bullet jump imposed by this. The best cure is to find a seating depth that isn't particularly sensitive to exact bullet location, but that still minimizes group size. This is why I am always suspicious of what shooters are getting with seating depths purportedly less than five thousandths off the lands. I know that in that range they could go from wider open to touching the lands just based on some bullet's tolerances.