Cartridge OAL and why

[2 Wild Dueces]

I dropped out from this forum for quite some time. The principals of accuracy reloading have not changed a bunch. Had some thoughts on making the cartridge fit the chamber.....and thought I might share. Kinda wish I had this anology when I owned Stoney Point Products. Here goes:

Most bottleneck cartridges are tapered.....and the tapered cartridges fit chambers which are tapered to closely accept them. Kinda like putting a funnel (the cartridge) into a slightly larger funnel (the chamber). The better the fit of those funnels....the more centered the tips of the funnels (which represent the bullet and the bore).

This is why a concentric cartridge.....which properly fits a chamber will shoot straighter than one with a cartridge laying on "the bottom" of a chamber. Get those dimensions to closely mirror one another.....and you can make magic happen.

Then too.....Bullets seated close to the rifling do allot to center the bullet (AND the CARTRIDGE) with the bore.....which serves to promote accuracy: Big Time!

Immagine if the cartridge "lays" in the chamber loosely.....when the trigger is pulled.....the firing pin strikes the primer.....driving it forward at a slight (and most likely variable) angle. This can be substantially different from one cartridge to another. How can this promote accuracy?

Instead, remove the loose tolerances.....get the bullet closely aligned with the bore of the rifle.....and bingo!......accuracy results.

I originated the Stoney Point OAL Guage (now made by my friends at Hornady) and the various measuring tools by Stoney Point. (comparators, rimfire, and headspace gauges). I can assure you they are very good tools, and quite precise, when used in the way they were designed.....and STILL will provide the needed results when used correctly. Tho....you still need to be able to read instructions, and take some time to learn how to measure correctly and to use the data supplied.

I always strived to keep the integrity of these tools to promote accuracy for the average reloader and accuracy buff....and it sometimes pains me to see folks that cannot measure the thickness of a dime twice in a row....yet think they have all the right data when it comes to measuring other things.

Practice a bit with these precise tools folks. Use your calipers to measure some hair or the thickness of a piece of paper....and coins...and such. Keep in mind....that you are measuring a tapered bullet (ogive) and try to get a straight-line measurement (which the tools do afford). It will take a bit of practice, proper set-up, and some "jiggling" to remove the slack from your measurements.

Not everyone comes up with the same dimensions of a dime. If you cannot get the same dimensions when measuring such objects.....then you may not be ready for the OAL Guage and comparators. Work on it a bit. Practice makes perfect.... Just say'in.

Tom aka: 2 Wild Dueces.

 

[dirtman]

Good post wild.....

 

[Wildfire]

I must agree. Yet not all dimes are the same.
I was always told that any bullet actually touching the rifling was too far in and could create high psi. They need some jump rather than being in full contact with metal.
That jump does not need to be much. .010" is used in my Rem. 700s {.223} and they are one hole rifles. My Remington PSS in .308 had a long throat of .250" and there was no way to reach that with any round that would feed from the box. I used 168 nat. match bullets and that rifle too was one hole at 100 yards. I do agree matching brass to the chamber is the correct way to get the best accuracy. Fire formed cases should match the chambers very close. New brass was never as accurate as brass that had been fired in that rifle. The 168 grain bullets in the long .250" throats had to jump .180" before hitting metal. They always did very well. While I could not see that well others fired 1.5" groups at 500 meters with my rifle. Ya I know, perfect days and all with very little wind. I do use RCBS precision mics and they too work very well. I have found premium factory ammo with head spacing at as much at .015" difference in the same box. I call that unsafe. But they sell it and never look back. Then too a $100.00 dial caliper will do better than a $30.00 one. I also like nice tools.

 

[Bart B.]

a concentric cartridge.....which properly fits a chamber will shoot straighter than one with a cartridge laying on "the bottom" of a chamber. Get those dimensions to closely mirror one another.....and you can make magic happen.

I agree, but here's the reality of what happens.

A cartridge with a 30 degree shoulder pushed into its chamber and before the bolt's closed will lay in the chamber bottom until some force moves it. Here's the forces it has to contend with before it's fired:

* Chambering in a Mauser type action, Win 70 for example. The case rim's slid up between the bolt face and extractor lip/claw and stays there until the bolt's closed. There's a few thousandths clearance between bolt face and case head as well as case shoulder and chamber shoulder. How much depends on the difference between chamber headspace and case headspace. The case body is often pushed against the chamber wall at its back end by the extractor forcing it there. Meanwhile, the case can rest on the chamber bottom with a few thousandths clearance to the chamber wall at the body right behind the shoulder as well as the space mentioned between case head and shoulder to the chamber at those places. The case neck and bullet are a couple thousandths below center to the bore when the rifle's held horizontally.

* Chambering in a push feed action, Rem 700 for example: As the bolt's closed, the bolt head's extractor pushes the round full forward until it stops against the chamber shoulder. The bolt's in line ejector has beep pushing the round forward against the chamber shoulder for the last near 1/10th inch of the bolt moving forward. The ejector alone keeps the round firmly against the shoulder. Therefore, the shoulder of the case is well centered in the chamber shoulder. If the case neck (and therefore the bullet) is well centered on the case shoulder, therefore so will be its neck and the bullet be well centered. And the case body doesn't touch the chamber wall anyplace except its back end at the pressure ring; it's diameters are smaller than that of the chamber in that area.

Along comes the firing pin to fire the round. Its tip would normally be about .060" past the bolt face when it's full forward on an empty chamber. When it starts moving towards these cartridges in these rifles, here's what happens:

* In the Winchester rifle: the firing pin tip is a few thousandths past the bolt face when it first touches the round in that chamber. Its force drives the case forward until its shoulder stops against the chamber shoulder. Remember it's 30 degree shoulder angle? Good. Remembering one standard in high school trig classes, the sine of a 30 degree angle being .5 is crucial to the rest of this event. For every thousandths inch the round goes forward after its shoulder touches the chamber shoulder, the case will rise .5 X .001 which equals .0005 inch taking the case neck and bullet with it. When the case shoulder is completely into the chamber shoulder, the case shoulder will be in full contact with the chamber shoulder. The funnel syndrome (which I like to call the Dixie Cup syndrome) has worked. Two identical shaped cones center on each other when they're touching together. Meanwhile, the case front stays perfectly centered in the chamber shoulder until the firing pin has dented the primer at least .025" at which point it fires the primer that burns the powder whose gas pushes the bullet out of the case into the rifling.

* In the Remington rifle: the firing pin does exactly the same thing. Only difference is the case is already full forward in its chamber so the firing pin has a bit more movement in front of the bolt face before it hits the primer. After hitting the primer, all else is the same except the round stays hard into and well centered in the chamber.

In both rifles, the force of the firing pin can set the case shoulder back a bit. More with small shoulder angles, less with steeper shoulders. Keeping the case shoulder well centered in the chamber shoulder until the pin stops moving, the bullet leaves and pressure drops so the case can shrink back a tiny bit from the chamber walls.

Use your calculator to see how much your case shoulder angle moves the case neck up or down for each thousandth inch of movement forward by the bolt or firing pin.

All of this is why brand new cases, well dimensioned, will chamber in and center bullets just as perfectly as any resized case will. If you're partially full length sizing your bottleneck cases so the bolt binds a tiny bit when closed on chambering them, doesn't matter what type of action you have; case shoulder's pushed full into and well centered there by its tight fit in the chamber. Best example was when new .308 Winchester cases were unprepped then loaded with 155-grain bullets with up to .003 runout and IMR4895 powder with 3//10ths grain spread in weight then shot in a few dozen match rifles from around the world. They all shot that ammo in to about 1/2 MOA at 600 yards for accuracy. All sorts of chamber and bore and twists were in those barrels. Go figure.

All of this is why most of the benchrest folks finally decided to quit neck sizing their fired cases. Proper full length sized cases have their necks and bullets better centered on the case shoulder than neck sizing. Neck sizing dies let the case body wiggle off center from the neck axis while sizing. Full length ones keep the case body well centered on the neck while sizing. It's also why Sierra Bullets proved back in the 50's full length sizing made bullets shoot straighter than neck sizing and they've used that to date testing their bullets for accuracy. Their first ballistic tech's favorite saying was "The case has to fit the chamber like a terd in a violin case."

Take two Dixie cups horizontally pushing one into the other then watch the inner one move up then into the top one until they mate just like the case shoulder centering on the chamber shoulder. Their centers are then very well aligned.

Immagine if the cartridge "lays" in the chamber loosely.....when the trigger is pulled.....the firing pin strikes the primer.....driving it forward at a slight (and most likely variable) angle centering the case shoulder in the chamber shoulder. This will be very repeatable and precise from one cartridge to another. This promotes accuracy. The principals of accuracy reloading have not changed a bunch; that's for sure.

Want to actually see it happen? Get your Wilson case gauge, slip an empty resized or new case into it, look straight into the front end. With the tip of your finger the horizontally held gauge and moving it back and forth a tiny bit, watch the case mouth move relative to the front of the gauge. The case mouth will be perfectly centered in the gauge mouth only when the case if fully into the chamber. Backed off any amount and the case mouth goes off center in the gauge mouth. Or, remove the barrel from your rifle, cut it off at the chamber mouth then use that chamber section to check you own cases in it; thing's will behave the same way.

Meanwhile, back to the bullet jump issue.

Cartridge overall length alone does not determine how far the bullet jumps to the rifling. It's how far the bullet contact point at rifling diameter is to that same diameter in the throat (throat is the angled part of the rifling at the front of chamber constant diameter free bore section to the bore diameter, some cartridge chambers have no free bore at all) when the round's fired. As most cases have a few thousandths spread from head to shoulder reference, or case headspace, that spread will be transferred to the bullet's position in the chamber. And a .308 Win chamber erodes away at that point about .001" for every 35 to 40 rounds shot, so bullet jump increases that much if the seater die's set to the same place for all rounds fired. Some way overbore cartridges erode the barrel .001" for every 10 rounds fired. Are you going to "chase the lands" as the old saying asks?

 

[Nature Boy]

That was great Bart. Thanks!

Regarding our ability to get accurate measurements of our process. In the manufacturing industry it's called "Gage Repeatability and Reliability" . It's a way of analyzing the variation of the measurement tools used and the human that uses them.

Google "gage R&R" if you want to learn more. I won't bore folks with the details here.

 

[2 Wild Dueces]

Just some food for thought.....

Folks that play the bench rest game have all sorts of practices the average hunter cannot or shouldn't do. Many folks shooting bench rest guns will seat the bullets hard into the rifling or on the rifling.....and have very small dimintional tolerances between the chamber and the cartridge brass. I used to have .0002" (two ten-thoussandths of an inch) between my (turned) case necks and the chamber.....and the bullets were some .005 into the lands.....kinda a crush fit. That is not allot of clearance to release the bullet and is NOT recommended for hunting rounds or for those that cannot measure such things. How you gonna unload that gun? - without a cleaning rod and / or potentially spilling powder into the action?

Still, reducing all the jump.....so that the bullet is seated against the lands and the cartridge case against the bolt face will serve to center the bullet within the bore.....and if the cartridge is concentric (especially the bullet with the case head) then the result is often improved accuracy. .....this assuming the plunger or firing pin does not impact the alignment aforesaid. Assuming the case necks are reasonably of similar dimension all around......the rest of the brass case won't do much to center such a round within the bore.....as the tapered bullet is already centering the round in the bore.

^ Such arrangements may prove useful for long-range, target shooting or varminting......but for hunting rounds these loads are to be avoided IMO. Too hard to unload the gun without firing it. Instead form brass that closely fits the chamber and keep a minimal bullet jump to the bore will afford acceptable (very good) accuracy for hunting - in my guns.

 

[2 Wild Dueces]

I must agree. Yet not all dimes are the same.
I was always told that any bullet actually touching the rifling was too far in and could create high psi. They need some jump rather than being in full contact with metal.
That jump does not need to be much. .010" is used in my Rem. 700s {.223} and they are one hole rifles. My Remington PSS in .308 had a long throat of .250" and there was no way to reach that with any round that would feed from the box. I used 168 nat. match bullets and that rifle too was one hole at 100 yards. I do agree matching brass to the chamber is the correct way to get the best accuracy. Fire formed cases should match the chambers very close. New brass was never as accurate as brass that had been fired in that rifle. The 168 grain bullets in the long .250" throats had to jump .180" before hitting metal. They always did very well. While I could not see that well others fired 1.5" groups at 500 meters with my rifle. Ya I know, perfect days and all with very little wind. I do use RCBS precision mics and they too work very well. I have found premium factory ammo with head spacing at as much at .015" difference in the same box. I call that unsafe. But they sell it and never look back. Then too a $100.00 dial caliper will do better than a $30.00 one. I also like nice tools.

^ I suppose I have owned well over 30 various calipers over the years. Some top-shelf units....and some low cost models. I always took low cost calipers to the SHOT Show as I would have them set all around.....and didnt worry about losing them. They seem to last a long time and give good service and results. Today, a reasonably accurate caliper can be bought for $30....and it will suffice for many reloaders. You may be able to "rack" the measurements with them a bit more than the expensive ones......but with a little practice that really does not become a big issue. Just don't rack em or force the jaws. Keep in mind that a caliper is not the "end all" of measuring instruments. They are good for basic needs.....but when you start splitting thousandths of an inch....these are not what's needed.

Still.....I think the average reloader does not spend enough time evaluating the capability of these tools, nor practices how to use them reliably, and often times does not set things up to get proper, and repeatable readings with these interments. < THAT was originally the purpose of my post. The instruments, tools, scales, and measurers are often better than the operators.

 

[Nature Boy]

The instruments, tools, scales, and measurers are often better than the operators.

That's almost a truism

 

[Reloadron]

Folks that play the bench rest game have all sorts of practices the average hunter cannot or shouldn't do. Many folks shooting bench rest guns will seat the bullets hard into the rifling or on the rifling.....and have very small dimintional tolerances between the chamber and the cartridge brass.

The outdoor range I have shot at for well over 20 years is Kelbly's Range which in the bench rest community is home to the annual super shoot. While I have played around a little with cartridges like the 6mm PPC I am not a bench rest type guy. However, surrounded by the bench rest community I watch these guys and follow what they do. Interesting to see what works and what doesn't and it is hard to argue with a guy placing bullets through the same hole at 200 yards.

Most load right there on the range. They show up with about 5 pieces of brass and use those five pieces all day. Most will just start a bullet into a case and then literally chamber the round, using the bolt to seat the bullet. They use a constant but gentle force on the bolt, the same each time round to round. They want their bullets touching the lands but not jammed into the lands. Over and over again they repeat the process with perfect uniformity. They are not very big on scales either. Matter of fact, most can't tell you what their charges weigh but they will tell you exactly where that micrometer is set on a Harrell's Powder Throw. Again, just like their bullet seating process they apply the same pull time and time again for repeatability. They develop a sort of knack for repeating the same motion with each drop identical to the previous drop.

Still.....I think the average reloader does not spend enough time evaluating the capability of these tools, nor practices how to use them reliably, and often times does not set things up to get proper, and repeatable readings with these interments. < THAT was originally the purpose of my post. The instruments, tools, scales, and measurers are often better than the operators.

With that I can agree. There is no shortage of cool tools out there available to the hand loader or reloader. Being able to separate those which are useful from those which are not is a science unto itself and once we know which tools are useful mastering them is another step unto itself. The very best tools are useless if we can't master their use.

Thank You Bart for your usual contributions and knowledge....

Thanks For Your Post Wild Deuces....
Ron

 

[Bart B.]

Observed at Kelby's range shooting one hole groups.....

Matter of fact, most can't tell you what their charges weigh but they will tell you exactly where that micrometer is set on a Harrell's Powder Throw

The best of those measures meter stick powder into cases with a 2/10ths grain spread. Exact weight ain't important; exact volume is.

 

[Reloadron]

Observed at Kelby's range shooting one hole groups.....The best of those measures meter stick powder into cases with a 2/10ths grain spread. Exact weight ain't important; exact volume is.

Yep. Thinking maybe more like 1/10th grain but either way, you will never see a scale out there.

Ron

 

[2 Wild Dueces]

I've shot a few rounds down range a Kelby's and participated in a couple of super shoots....and it is among the best ranges and venues that exist for bench rest shooters. The bench rest shooting game is a specialty.....much like any other discipline. It's a game for those that like to live life in the fast lane of rille acuracy .....and have a good degree of know-how to rely upon.....and hopefully the budget to stay abreast. (grin)

Then too....many BR shooters just follow the top shooters and copy their equipment and try to emulate their ways. Don't see many of those in the winners column. But, the best of the best are the real innovators out there. The guys that set the pace and win on a regular basis. Those guys don't follow the same old stategy.....they set the pace. I appreciate those innovators our there that set new standards for accuracy. Still....the game doesnt appeal to everyone.....or me. The guns and the tech have improved so much over the past few decades.....that the game now is about how to read wind flags and trigger control and a bit of good luck. Most anyone can and will shoot screamer groups on occasion....and most all the guns have good pedigree and can shoot.

Alas....I'm getting a bit dated.....old school.....and feel like an old guy spouting-off on what is well-known to many others. My rant is over. (grin)

 

[Wildfire]

Some very good points in here. Many of which some of us just do and have never really talked about. Kind of fun to see some of this come out.
I do see many hand loaders today using I guess what I would call cheaper loaders and tools. I am fine with that and also know that even the cheaper ones work and can produce good results. I understand all about loosing the good tools if they are taken out of the shop. Been there and done that way to many times. I have used some cheaper stuff and it usually works for the intended purpose. Jim Carmichel once said he bought a rifle that had the first several inches of the bore and rifling completely shot out. Wasted. Yet he said that rifle was still very accurate. My comment about head space on some factory ammo was about how it was found to be very inconsistent. Knowing that it would not align with much of what has been said above. When we can see .010" difference between a box of 20 rounds in head space that can change everything. Bolt face fit and shoulder fit. The Remington 700 PSS rifles used to come with very long throats {free bores}. 1/4" or .250". Pretty hard to set oal with that going on yet they fired extremely tight groups from mine. I have always understood longer bullets to sort of self center as they traveled out or down the tube vs a short bullet which could get a bad start and maybe keep off track some. but then I have been wrong before and will again.
And yes I have watched bench shooters load at the shooting bench. Crazy it seems some do not even use a scale. Just put powder in until full and seat the bullet chamber it and fire that same round many times that day. They seem to do rather well with what some of us labor at our loading benches for hours attempting to get close to. Oh well it's all fun anyway.

 

[Wildfire]

Yep, It sucks to get old and get run over by the new crowd. I went back to old west cowboy rifles and revolvers to just have fun and hunt some. I must say, I think I am having more fun now.

 

[Bart B.]

It took about 50 years for the modern benchrest folks getting best results to switch from neck to full length sizing their bottleneck cases. Other disciplines learned that long before the modern benchrest matches started after WW2.

 

[Doublehelix]

It took about 50 years for the modern benchrest folks getting best results to switch from neck to full length sizing their bottleneck cases. Other disciplines learned that long before the modern benchrest matches started after WW2.

I had always heard that the BR crowd *always* neck sized only. I know nothing here other than hearsay, but thought I would confirm that you did not have a typo above.

 

[Reloadron]

I had always heard that the BR crowd *always* neck sized only. I know nothing here other than hearsay, but thought I would confirm that you did not have a typo above.

The BR guys did neck size only for quite some time. They were doing it 25 years ago at the BR range I previously mentioned. Then they returned to full length resizing where they seem to be today. The BR crowd seems to stay on the edge trying anything new and if it does not work out return to their previous methods. All in pretty short order. Maybe 20 years ago the BR guys were running with the moly coated bullets where I shoot and that seemed to come and fade away in a few years time. Not being a BR type it is merely what I observe watching the BR crowd where I happen to shoot and have a membership. Before I forget, the BR crowd I refer to are the 100 & 200 yard shooters, not the long range BR crowd.

Ron

 

[Bart B.]

Before I forget, the BR crowd I refer to are the 100 & 200 yard shooters, not the long range BR crowd

The high power match shooters holding rifles to their shoulders were getting test groups for their long range rifles with new and full length sized cases in the '60's and '70's the same size as long range benchrest rifles shoot today. Both rimless and belted bottleneck cases. Proper full length sizing is nothing new. Brand new unfired bottleneck cases are made that way. New belted case rounds can center nicely in the chamber when loaded then fired; nothing forward of the belt touches the chamber or bore.

Keeping all test shots fired at 1000 inside 6 inches has been about as good as one gets for decades. Once in a while, a tiny few-shot group sets a record. Rarely does its holder tell what the size of his biggest groups are.

 

[2 Wild Dueces]

Another story on barrel throats....

Shortly after developing the OAL Gauge.....I stopped at a local gunshop while searching for a big game gun. (in those times I was measuring all kinds of gun barrels just to learn about the wearing process in gun barrels) I used to stop at this store on a regular basis and became friends with some of the staff there. The store had two brand-spanking new rifles that were identical in every way.....and they were sitting side-by-side on the rack behind the counter.....and we were discussing those guns.

I knew the store operator, and asked if I could measure the throats in those new guns with my gauge.....and he watched as I did this. Using the same bullet and the OAL Gauge to measure........one of the guns had a reasonable throat with the bullet not needing to jump from the case mouth to the rifling. The dimensions were pretty much what was anticipated in a new gun. The other gun however required the bullet to travel (unsupported by the case neck) approximately 1/4" from the case mouth to the rifling.

I could only assume that the manufacturer had changed some tooling when cutting those chambers (only a few numbers different in serial numbers) or that they had no standards on freebore....or a careless machinist with a throating reamer, etc. Big differences in those guns......I never pursued to find the answer of "why"? with the maker of those guns.....and frankly I don't remember the brand anymore. I did not purchase either gun.

But....I asked the store operator which gun he would buy? He (and I) were quite astonished with the differences, but he said he knew which he would want too.....but he had to sell them both. I doubt very many buyers consider checking the barrel before buying.....but it may be a good idea on some factory-made guns if you have high accuracy expectations.

 

[Wildfire]

There is really nothing wrong about a long throat. Many with long throats are extremely accurate rifles. Mine was a one hole gun at 100 yards. Remington has been known to use long throats in the PSS and other tactical rifles. It reduces PSI, and allows for long range heavy bullets. I still have one PSS in .223. It has a shorter throat. It also has a 1-14 twist. It will toss amazing accuracy at very long ranges. I used this rifle in South Dakota P-dog shooting and have some very long kills. Many over 700 yards. This was with 55 grain Ballistic tips. The 40 grain Ballistic tips were one hole at 100 yards but did not handle any wind. .012" off the rifling. I can agree and disagree at the same time about bullet set back. But I never allow a bullet to touch the lands. That ups the pressure. Shooting in high heat like near 100 degrees already has a barrel rather warm. I have seen both ways work but still do not allow bullets to be into the lands.

 

[Big7]

Neck size ONLY. Seat .010" off the lands. Problem solved.

 

[chas442]

What a great thread.
It got me thinking.
If someone bumps the shoulder of a bottleneck case back as far as it will go rather than sizing it to the chamber, could that contribute to vertical stringing? Would it be possible that the case would not be properly aligned with the bore of the rifling and "laying" on the bottom of the chamber?

 

[Bart B.]

No is my answer to both questions Read post #4 in this thread.

Even when the back end of the case body is pressed against the left side of the chamber wall by an Mauser style extractor pushing it there, horizontal shot stringing doesn't happen.

 

[Wildfire]

Yes, I agree with Bart. Best reason I know for shot stringing is a barrel changing heat.
I had a very cool Remington 700 in .270 Win. Mountain light weight custom built rifle. It would put 3 shots in around a 1/2" at 100 yards. #4 would start up and 5 and 6 went higher yet. After 3 shots you would not dare touch that barrel. Let it cool to cold and 3 shots went back to zero.

 

[Bart B.]

Barrel heat alone doesn't cause shot stringing on target. Match rifles with all sorts of profiles from skinny to fat will easily put a few dozen shots fired 15 to 30 seconds apart into sub MOA groups through 1000 yards. I've put 10 shots from bolt guns into sub MOA at 300 in 60 seconds then again 3 to 4 minutes later. 24 from Garands in 50 seconds; sub MOA at 600 relative to point of aim. Go to a benchrest match and watch them put 5 shots grouped 1/4 MOA less in than 30 seconds during a lull in cross wind speeds.

That shot impact moving as barrels get hot happens when receiver faces are not squared up with their barrel tenon thread axis. Commercial rifles could be made that way for an extra $40 to $59 but vast majority are not. Your 'smith might be able to do that.

Or, shoot several junk loads to heat the barrel then shoot a 20 shot group to see how accurate your stuff really is.