Bullet Length in Relation to OAL

[Unclenick]

MarkCO and Metal god,

This illustrates what Bart and I are saying:

Note that if the case were shorter because of greater shoulder setback, but the neck was the same length (trimmed in a shoulder-registering trimmer), and the bullet seating depth into the mouth was the same, though there would be a larger head clearance between the breech and head, the bullet jump would be unchanged. This is what Bart and I are talking about. If you keep that Shoulder-to-Ogive throat contact diameter distance the same, bullet jump stays the same. Neither Bart nor I are addressing how you load ammunition to hold that distance constant. We are just stating what the principle involved is.

I have generally found that because seating dies contact the bullet ogive further up from the part of it that contacts the throat, where the OP was finding ogive variation, my gauge can find a couple or three-thousandths of variation in shoulder-to-ogive length even when case head-to-shoulder distance is perfectly consistent. I use my gauge to sort my loaded rounds on the assumption matching that distance helps match the bullet profiles. I've never done any rigorous study of the practical effect on accuracy at long range, but my expectation is that will provide a closer BC match between bullets.


Metal God,

As I said, I can't recall my source. I'll have to run the experiment for myself. I have the feeling it was new brass, so there may have been a softer shoulder than reloaded brass and it may have had a bigger runup into the chamber shoulder, letting it gain a little more inertia. It may have been done with magnum primers, too.

 

[RC20]

ONLY if the should is bumped back precisely the exact same amount each time, which it is not. You can not use a floating datum against a hard datum and get consistency. Sounds good, but it is wrong.

Your shoulder distance is a few thousands at worst.

Bullet tips are worse variation so yes you use a device that measure on the bullet shoulder.

Equally if you are measuring to the shoulder of the bullet the case shoulder distance is not relevant, you are going form the base of the bullet to the shoulder.

The shoulder will have affect on trimming, but that too is not relevant for COAL.

The biggest variable is the bullet itself and how consistent the mfg process is.

None of them are super exact, you just take an average and set the COAL where the particular gun and bullet combo shoots the best.

Some designs like to be right at the lands (you have to be careful on loads and excess pressure) and some like some jump and some like a LOT of jump.

 

[Metal god]

Thanks Unclenick , I guess the question is .... How much will that .002 variance effect internal ballistics resulting in external ballistics variances ? Did someone say above something like 1 or 2fps difference ? If so does it matter that you/we have that .002 variance in the first place ? How many times have we heard something like ... that small of a variance will be washed out by the noise of all the other variable ?

As for new brass vs work hardened brass do to a couple firings is an interesting thought as it relates to the firing pin setting back the shoulder . I can see how that may effect the results . I don't remember the number of firings my cases had but I'd bet they were fired twice and reloaded once from new . So that would have to include a second reloaded sizing cycle to do the test . Although not reloaded 10 times I can see how my cases would have been slightly more work hardened then new cases . This makes me want to go pull some Fed GMM and rerun the test haha . I also could just anneal some cases and try the test again but I feel annealing may result in softer shoulders then some factory new cases . Holly cow I just remembered I have a bunch of Lapua new cases I can test , I'm getting that little tingly feeling .

The test was done several years ago on another forum in a thread that was not specific to this issue so finding it would be much harder then I'm willing to tolerate . I still have the pics I took of the cases in a comparator before and after but not context other then my memory of the test and writing about it .

 

[Unclenick]

The possibilities always seem to be infinite, don't they? I don't think 0.002" of seating depth will have much effect, though if someone is trying to seat 0.005" off the lands, it could affect total gas bypass enough to increase velocity SD some, I suppose. As I said, I think the main value in making the measurements is to sort bullets off toolset A from bullets off toolset B, which may have a slightly different radius. The resulting BC difference may affect vertical stringing at 1000 yards measurably, but unless I try it, I can't say for sure. But that was my reasoning in making a tool to check the number. Since Redding's Instant Indicator makes the same measurement, a call or email to them might garner at least some anecdotal evidence that it makes a difference and under what circumstances. If it helps any, they won't be shy about sharing the data.

 

[Bart B.]

ONLY if the should is bumped back precisely the exact same amount each time, which it is not. You can not use a floating datum against a hard datum and get consistency. Sounds good, but it is wrong.

That does not happen either. And that one does not even sound good.

Do you think new belted cases don't stop against the chamber belt ridge used for headspace?

Regarding both types, what do you think they stop against upon firing the first time? One popular misconception is that the rim stops the case against the extractor claw. Another is the case lays on the chamber bottom when fired.

 

[Bart B.]

Forget OAL, you want ogive to land distance held constant. Where the tip of the bullet is in relation to the start of that lands, that never sees the gas pressure nor touches the rifling, does not matter. And, you don't want 0 clearance from the lands to the ogive. It depends on the rifle and the bullet. A jump of 0 means you are literally, chasing the lands, which is a fools errand. Back if off where it works well for the given parameters and adjust at each cleaning of the bore, which for most of my rifles is about 800 rounds.

Soft seating bullets so they set back a few to several thousandths when chambered has been popular with long range competitors for decades. The US Palma Team has used custom chamber reamers to ensure the issued ammo has bullets seated into the throat when chambered.

Yes, they're chasing the lands. I've done that for all my long range handloads for bolt action rifles.

 

[Bart B.]

I did that shoulder setback test some years ago.

Federal nickel plated 308 Winchester case shoulders were set back almost .010 inch.

Regular Federal 308 cases, about .004 inch.

Thin wall WCC58 match 308 cases weighing only 150 grains, .006 inch setback.

 

[scatterbrain]

Long ago, and far away, I had a first year production Remington 22-250 that was used a lot. Ground Hogs were over running the blue grass and farmers would put us up for the weekend to hunt.
I would start low with a powder load and work up to max, bullet length was determined by what would stay in the case until loaded in the chamber, if the round was removed without firing I used a cleaning rod to remove the bullet from the barrel. My velocity would be some where above 3800 fps. Cases lasted about 5 reloads before necks, and bodies would split, or primers would fall out. Shots ranged from 200 yards to 500 yards and we did our part to help out, one farmer said he had never seen ground hogs out at night before.
Chasing the rifling reduced my group size by about 50%, SD meant nothing to me.
Coyotes and a career ended the great times spent in the fields and now I hunt the SD.
For whatever the reason, bullet profile is just another variable and I will go with the 2 fps difference it could make. Now, what's next. Have a great day.

 

[Bart B.]

Unclenick,

Regarding the picture in post 21, I thought "freebore" was the cylindrical section from chamber mouth to where the angled throat tapers down to the bore diameter. It's a couple thousandths or more than bullet diameter. Some SAAMI spec chambers don't have any; 30-06 and 300 Win Mag for example.

A SAAMI representative explained to me it's that part of the "bore" that's "free" of rifling.

 

[Bart B.]

I actually tested the ability to set the shoulder back with just a firing pin strike several years ago . It was in a thread on another forum which Bart B was actively participating in . I tried it with 2 different rifles and manufactures . 1) was a 308 Ruger American , 2) was a Savage 308 model 10 . Neither one was able to set the shoulder back using an empty case striking a live primer . All cases had the same number of firings and shoulders were bumped .002-ish .

If I recall correctly based on Bart's input , we concluded that modern or commercial firearms likely lack the power in the firing pin spring to do so . It's believed that the firing pin strike setting back the shoulder of a case is more likely to happen with older military rifles with there heavy firing pin springs creating significantly more force to the primer then commercial rifles built today.

I didn't agree to that conclusion. Older military spring force was about 20 pounds or less. Modern day ones are typically 25 or more.

https://www.gunsprings.com/Rifles & Shotguns/cID2

 

[hounddawg]

I actually tested the ability to set the shoulder back with just a firing pin strike several years ago . It was in a thread on another forum which Bart B was actively participating in . I tried it with 2 different rifles and manufactures . 1) was a 308 Ruger American , 2) was a Savage 308 model 10 . Neither one was able to set the shoulder back using an empty case striking a live primer . All cases had the same number of firings and shoulders were bumped .002-ish .

I would have to agree with you MG. Thinking in terms of the physics involved most if not all of the energy from the firing pin will be used to deform the primer cup, very little will be transferred to the shoulder. It takes a good amount of force to move brass which is why you will never see a inline full length sizing die for use in a arbor press

edit - article by David Tubb on firing pin pressures, notice he uses inch ounces for measuring the force applied, not inch pounds

https://www.davidtubb.com/index.php?route=account/download/free&download_id=28

 

[Bart B.]

I would have to agree with you MG. Thinking in terms of the physics involved most if not all of the energy from the firing pin will be used to deform the primer cup, very little will be transferred to the shoulder. It takes a good amount of force to move brass which is why you will never see a inline full length sizing die for use in a arbor press.

Why did Holland & Holland put a belt on the rimless bottleneck 30 and 37 caliber cases over a century ago?

How much force does a 2.5 ounce firing pin moving about 20 fps pushed by a 25 pound spring put on the cartridge?

 

[hounddawg]

How much force does a 2.5 ounce firing pin moving about 20 fps pushed by a 25 pound spring put on the cartridge?

read the link I provided above and you will know how much energy it takes. Not only that but can you tell me how much of the energy transferred from the firing pin to the primer is used to deform the primer cup and crush the pellet?

 

[Bart B.]

read the link I provided above and you will know how much energy it takes. Not only that but can you tell me how much of the energy transferred from the firing pin to the primer is used to deform the primer cup and crush the pellet?

I'm asking you for the answer.

What about post 32?

 

[hounddawg]

I gave you a link. I read it. If you want to learn something then I suggest you read the entire article and if not

 

[Metal god]

Not only that but can you tell me how much of the energy transferred from the firing pin to the primer is used to deform the primer cup and crush the pellet?

IMHO that is something that can not be over looked . The deformation of the primer cup has to reduce significantly the energy transferred to the case shoulder contacting the chambers shoulder . The firing pin pushing the case forward can't be looked at as if the firing pin is fixed to the case and 100% of the spring energy is transferred throughout the cartridge . The prime must be absorbing a significant amount of that energy .

Primer cups are actually quite soft in this context IMO . A light AR floating firing pin dents the primer cup when closing the BCG . This to me shows how much potential forgiveness the primer cup has when the firing pin is driven into it .

I'm now thinking shoulder set back is less and less likely by a firing pin strike . Can it happen ? I think so but the question might be better asked , is it likely to happen ?

Maybe I'll run some test today with new cases and CCI and Win primers in 4 different 308's

EDIT , that all said I did run some test several years ago to see if letting the BCG fly home on a loaded round in the AR-15 platform will set back the shoulder . The answer there is not only yes it can but it is actually likely to do so . My test showed the shoulder was set back anywhere from .0005 to .0015 when the BCG fly's home stripping a round from the mag . Not only that , if I repeated the test with the same cartridge the shoulder was set back again . I don't remember the number in total thousandths but at some point the shoulder did/does stop being set back . I want to say somewhere between .002 and .003 was the total set back I was able to induce . Note this was not the firing pin contacting the primer but rather the BCG slamming the case into the chamber that caused the shoulder set back .

Interesting fact here as far as my test and how I think of it as fact ( I mean CNN did not fact check it so take it for what it's worth ) The fact I was getting the shoulder to set back made me try sizing the case to a case head space .002 longer then the fire formed length of a case from that firearm . This was to see if the long case from head to datum point would chamber or would it lock up the bolt . To confirm the case head space was long I road the charging handle home the tapped on the forward assist to be sure the case/ cartridge was to long to chamber freely . It was and I then ran the test of letting the BCG fly home stripping the cartridge from the mag . The bolt went into full battery and I was able to extract the round with minimal effort ( Sorry my memory is not 100% ) on the extraction part . I know extracting was not an issue but don't recall if the bolt was slightly stiffer to open or not .

 

[MarkCO]

It is actually easy to test. Get some plasti-gauge, put it on the shoulder so that it does deform enough to take a measure. Then fire that case with a primer only and re-measure.
That was how I determined how much to bump the shoulder back before I had case gauges. It works. Not a single time I have tried it was there any change at all in the measurement.

I used the same method when I would have a "stuck case" that had case gauged properly in my match AR15 and found that yes, the crimped bullet was pulling the shoulder into contact at the shoulder, so I bumped back the shoulders a little further to fix the problem.

 

[44 AMP]

Bullet Length in Relation to OAL

I don't see where what the shoulder is set back to, or "bumped" to has any direct relationship to bullet length and OAL.

Why did Holland & Holland put a belt on the rimless bottleneck 30 and 37 caliber cases over a century ago?

The people who designed those cases are long gone, so there's not going to be any answer from them. I would suggest two possible reasons for consideration;

First, marketing. Holland and Holland introduced the belt and the term "Magnum" with their .375 H&H in 1912. The belt was a unique identifier, something no other round (at the time) had, and gave people the (mistaken) impression that the case was thicker and stronger, to handle the "magnum" power.

Second, both the .375 and the later .300 have long tapered cases and fairly small, and shallow shoulders. H&H wanted something they could count on for positive headspace that didn't have the feeding complications of a regular rimmed case. The belt did that.

Note the era, and the commonly held beliefs of the times. JM Browning designed his .25, 32, & .38 pistol rounds with a "semi rim" for headspacing, because, at the time, he wasn't convinced a rimless round headspacing on just the case mouth would work acceptably well. Note that the round he designed after the rimless 9mm Luger proved the concept would work, the .45acp, is rimless.

I think the H&H engineers were thinking on similar lines. To ensure reliable headspace on their new round, they added a belt, and headspaced on that. Note that the Flanged (rimmed) versions of their rounds (intended for double rifles) did not have belts. They knew rimmed rounds would have no problems, but weren't so positive about their .375 case shoulder being enough, so adding a belt took care of that concern, as well as giving them something distinctive as a marketing feature. That's my theory anyway...

Now, back to bullet length in relation to OAL. When loading pointed bullets, regular seating stems do not contact the tip of the bullet. They bear on the bullet at some point on the ogive. Bullets of exactly the same measured length (base to tip) CAN vary in diameter at the point where the seating stem touches them, and variance here means that point is slightly further back or further ahead on the bullet, which results in a slightly different length of the loaded round. Now, the question is, how much, if any, does this matter?

For me, not so much. I've got a Win Model 70 varmint in .22-250 that I bought nearly 40 years ago. Ammo is full length sized, Seating die set using a Rem 55gr factory round for length. I don't know where the shoulder is "bumped back to" and I don't care. I don't know what the jump to the lands is, and I don't care. Ammo fits and feeds from the magazine and doesn't hit the rifling.

The rifle, in my hands, from a bench shoots 55gr slugs into 3/4" groups @100yds and shoots the 52/53g Sierra/Hornady match bullets into 1/2-3/4" groups and that works for me. I might possibly be able to do a lot more work and slightly improve that, but I can't shoot any better than that (and not quite as good in the field), so there's no point, for me.

 

[Bart B.]

44AMP,

Rimmed H&H cases didn't feed reliably from box magazines, belted ones did.

 

[Metal god]

I don't see where what the shoulder is set back to, or "bumped" to has any direct relationship to bullet length and OAL.

I believe Bart and Unclenick pointed all that out earlier . short answer is .... if you're trying to keep your bullet off the lands exactly .005 through out the ignition process . The length of your case head space , length from shoulder datum to bullet ogive closet to baring surface and the ability of the firing pin or loading of the cartridge to set the shoulder back can all effect where the ogive of the bullet is in relation to the lands upon ignition . I believe ... at least for me and the OP we concluded a couple thousandths was not going to effect internal ballistics enough to effect external ballistics . This was not to say there is no effect only that the effect is likely so minimal most other variables will likely cancel it out .

As to the firing pin setting back the shoulders . I just did a very small test with 3 different rifles and 2 different primers in 6 new Lapua cases .

FIREARM -----------------PRIMER-------------------PRIMER

AR-10 =__________CCI LR no effect_______Win LR no effect

Savage M-10______ CCI LR no effect_______Win LR no effect

Ruger American____CCI LR no effect_______Win LR -.002 setback of shoulder

Not sure what this means with such a limited test but it does seem to indicate it can happen but not every time .

On a side note the Win primers were louder and appeared to produce more residue/blast out the muzzle .