Rimless Bottleneck Cases; How They Fit Chambers

Discussion in 'General Rifle' started by Bart B., Aug 31, 2010.

  1. Bart B.

    Bart B. G&G Addict

    From another thread: .308 v 7.62 NATO

    Close, but the objective is to prevent the back end of the case breaking off from head separation. Tight headspace is the cure.

    Again, remember that the front of the case (shoulder) is well centered in the chamber shoulder when it fires. This is the key to bullet alignment with the bore. If you don't understand how this happens both before and when the round's fired, forget anything about where the case is behind the shoulder; it doesn't matter.

    On a rimless bottleneck round, the back end at the pressure ring will always be pressed against the chamber wall opposite the extractor. Any off center amount will be equal to half the diameter difference between case and chamber at that point. If a .308 Win. case diameter at the pressure ring is .4703" (SAAMI spec.) and the chamber's diameter at that point is .4714" to .4734' (SAAMI spec.), the case head center will be between .00055 and .00155" off center; always in the same direction. Which means while the case shoulder's still well centered in the chamber shoulder, the bullet tip is about half that much off center. More importantly is the fact that each and every round's set this way and they all are very repeatable in this regard. In 7.62 NATO chambers, the difference will be a thousandth or so more due to their slightly larger diameters.

    Bullets are pushed out of the case neck long before the neck expands. Any neck expansion will always be behind the bullet as it exits the neck anyway. It only takes a few dozen pounds of pressure to push the bullet out of the neck. By then, the bullet's already aligned with the bore.

    Regarding where the case head is as it's held by the extractor, note the extractor lip has enough clearance to the bolt face to easily allow the rim to fit into it when loaded from the box magazine in actions with Mauser style claw extractors as well as the push-feed post '64 Model 70 style bolts with the sliding extractor in the bolt face. There's a few thousandths clearance between the rim and the bolt face behind it and the rear of the extractor lip in front of it. Plunger style ejectors push the rimless bottleneck case forward and they stop with their shoulder against the chamber shoulder before the rim stops against the extractor's lip. Same for external claw style extractors.
     
    Last edited: Aug 31, 2010
  2. samuel

    samuel G&G Newbie

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    Thanks Bart for taking time and effort to explain the happenings in a chamber at the milisecond of ignition when firing a centerfire cartridge.I hope everyone gets a lot out of this,as I have.Again,thanks for sharing your expertise and spending the time. ,,,sam.
     

  3. Bart B.

    Bart B. G&G Addict

    Best fit of rimless bottleneck case to chamber for accuracy.....

    Then properly full length size your fired cases.
     
  4. samuel

    samuel G&G Newbie

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    Haha!I bet you knew that would bring a reply.I still believe neck only resizing is more accurate and easier on cases if done properlySo do most bench resters and gun guru,s,which by the way,are usually engineers or scientists,or both.To be fair to you and the other members,I will first post what I believe,and then post a view that sides with you.(And I think I can find what Bart Bobbit said on this) ,,,Full Length, Partial & Neck Resizing ,,,http://yarchive.net/gun/ammo/precision_resizing.html ,,,sam
     
    Last edited: Aug 31, 2010
  5. samuel

    samuel G&G Newbie

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    I cant seem to get what I want on here.The point I wanted to make is Bart calls his system FLR and in my opinion it is partial resizing.He doesn't bump it back all the way.That is what I have to do after about 4/5 shots.I have plenty of time to go for accuracy and use the "bumped back" cartridges for plinking and such.The problem with Barts system is short case life.But this is my opinion.Read both articles and make up your own mind.All I want is for you to have the straightest flying bullet possible.The straightest ones for me are neck resized only.(this is a very,very old,old argument.) ,,,sam.
     
  6. Bart B.

    Bart B. G&G Addict

    30+ reloads full length sizing rimless bottleneck cases ain't short case life. I've got as many as 47. Friend of mine did a test with his match rifle shooting one case 56 times. The group at 100 yards was a bit under .3" center to center.

    Virtually all of the high power match rifle records have been set with full length sized cases. Look 'em up on NRA's web site then contact the person who set 'em.

    Go figure.........
     
  7. samuel

    samuel G&G Newbie

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    I have known competitors in about all fields of firearms and done a little shooting myself.(but not nearly as much as I would have liked to) One thing I always chuckle at is when people want to talk about shooting,especially long range centerfire,they always use BENCHREST shooters as a benchmark.While it is a fact that BR,ers are the finest shots out there using all the gimics and gadgets,they (in my opinion,only my personal opinion) are also BS,ers.I qualify that statement by stating that one week all the rage is one thing like cut riflings overwell,you know what I mean.One time the greatest is new cases,the next once fired.They just go on and on.Once i was at a range and synthetic stocks were the only way.The next tome,only laminated.One time it's neck,next it's partial,next it's neck. And then along come the analists.By the time the average shooter gets anything they have converted back to something else.For such a simple act of loading a cartridge,placing it in a firearm,sighting and sending it down range,there are about a zillion ways that possibly make it do better.My finding is what does great for me sometimes doesn't do too good for someone else.Bart,I have tried your FLR and my NOR works best for me. (as it does for a lot of other competitive shooters.)When you read an article,if they are for something like neck resizing,they always add that it is the way most bench rest shooters do it.If they are for something else like FLR they never forget to say most BR shooters do it this way.It is my opinion that most competitors don't tell all of their secrets and even if they did I might not be able to get it to work for me. (but you can bet your sweet bippy I will try if it even hints at a straighter bullet flight.)Competition shooters are always trying something new,and then they try something they already tried all over again. ,,,sam.
     
  8. Bart B.

    Bart B. G&G Addict

    Sam, benchrest rifles are not necessarily the most accurate rifles. They may well shoot the smallest groups in competition compared to other disciplines, but that has no bearing on what's the most accurate. Knowledgeable shooters know why.

    Do you know how many long range benchrest records have been set shooting full length sized cases....or even new, unfired ones?
     
  9. texnmidwest

    texnmidwest Sir Loin of Beef Forum Contributor

    :popcorn:

    Great discussion guys!!! Great info!
     
  10. samuel

    samuel G&G Newbie

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    That wasn't the point I was trying to make. (altho I certainly don't understand your statement that BR rifles aren't the most accurate but may well shoot the smallest groups.If they shoot the smallest groups,what rifle is more accurate?) The first point I was making is that everyone, (some,not even competition shooters make their point and try to verify their statement by adding "most bench rest shooters use this method,or do it this way".If the statement the person is making is accurate and works to benefit accuracy,why do they need to verify it with bench resters? another point I was making is bench resters use all of the gadgets and systems ever heard of but they also try things and abandon them only to go right back and try them again,such as stocks.They are all one thing one time and then it is something else.A gun editor (or others) will see/hear of them doing one thing or using a technique and think that is what everyone should be doing.So,(in whatever way,gun rags,forums,range talk) they truy to spread the good news and enlighten others to the miracle technique the Bench resters are using,and by the time they educate everyone to this "BENCHREST" secret technique/system,the bench rester has decided it wasn't so great,quit doing it and is using something all togather different. (altho,the BR may very well go back and try it again) From my experience,the only way for shooters to go is like I say NRO and you say FLR.The thing I hope all intrested parties will do is try your system against yours and see what works for them,(also in their rifle as it may not be as good in another rifle) As I stated,I tried FLR and it wasn't as accurate "FOR ME" as NRO.That doesn't mean it isn't or is the best or worst and if I were wanting top accuracy I would try it and even if it didn't seem as good,bug you until I was sure I had the same technique you use.It sounds so simple to set your dies,insert a case,and pull a lever.But there is much more to it.Possibly something I did or didn't do caused my FLR to not be as accurate as NRO.Or,on the other side,possibly something I was or wasn't doing NRO was making them more accurate than FLR.Personally,I hope everyone listens to you and tries your system and I hope they make it work.The one "BIG" weakness with NRO is within 3to5 rds (depending on how big of a fire you build in the chamber) the shoulder "MUST" be bumped back.This happens even using collet and mandrell.When the shoulder is bumped back you have a variable,even if you fire one round to re-form the case.I don't call your system a full FLR,just a partial,but it is the same every time.My experience,you are still pushing the body in so you are pushing the shoulder up.This,first,is working the case causing a possible short life.But beyond that,the precision to push the body/walls in and then bump the shoulder back the necessary .001" to .002" would be precision almost beyond comprehension to the average reloader. (and possibly the reason I couldn't get the accuracy you do.) As they say,the devil is in the details. I know some are getting a lot out of your posts and this thread so keep them enlightened. ,,,sam.
     
  11. Bart B.

    Bart B. G&G Addict

    You first have to learn how each shooting discipline's competitors have the rifle held while aiming at the target with the least amount of movement and which one has the most. Then you have to understand why each one is what it is.

    In long range matches (600 to 1000 yards) a benchrest rifle resting on supports barely touched by humans held on target such that its point of aim stays well inside a 1/20th MOA circle on the target then fired in virtual free recoil will easily shoot 5- or 10-shot 0.5 MOA groups (often in less than 5 minutes).

    In a high power long range match with that match rifle slung up in a prone position and held against a humans shoulder with both hands and their beating heart causes the rifle's point of aim (for the best shots) to be bouncing around between 0.8 and 1 MOA on the target (at least 16 times bigger than the stool shooter) and their grip and position ain't perfectly repeatable for each of 20 to 25 shots fired in 15 minutes (making several wind corrections on their sights) and managing a much heavier trigger pull then they end up with a 1.5 MOA group shooting the winning score. The high power match rifle's got to shoot all those shots in under 0.5 MOA at that range to do that well. Oh, I forgot; the high power match rifle's got aperture sights.

    Go back to my posts on how I full length size fired cases. Read 'em carefully; several times. Hopefully, you'll realize that I not only size the case body and neck diameters down a couple thousandths or so but also set the shoulder back the same amount. Full length sizing means doing all three to some degree...at least according to manufacturers of sizing dies. Partial sizing typically does not set the shoulder back nor does it size all of the neck. Instead, it usually sets the shoulder forward causing the case between the shoulder and bolt face for each shot preloading the bolt differently for each shot. If the rifle's bolt face ain't square with the chamber axis, really good accuracy is impossible to attain and any sizing method may well show better results than another. Squared bolt faces guarantee consistant headspacing when the difference (clearance?) between chamber and case headspace is only a few thousandths.

    Yes, there are details that some folks don't observe, understand, and adjust for. Most folks I've explained my process to have been able to do it well. If one doesn't have the desire, tools, skills and knowledge to properly measure cases at various stages in the reloading process, I can't help do that unless I'm right beside them doing show-and-tell then guiding them through the process and finally letting them do it right themselves so they've mastered it and are elated at their new-found abilities.
     
  12. samuel

    samuel G&G Newbie

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    You hint that I might be slow to comprehend or understand.I have no problem comprehending even your definition of what you are doing.I only pointed out that to me what you call FLR is more like PR.Only to me.You or others can call it what you wish.A FLR (to me) is where you set the base of the die down to where it touches the shell holder.I understand your technique perfectly.I just stated I had tried it and got better accuracy from NRO.Others may find your technique more accurate and I hope they at least try. The one place/tool that most could check precision very simply but few ever do is chambering FLR cases in the rifle they are using.Even NRO I check to be sure the case can be chambered with little or no effort.If there is the least resistance the shoulder probably isn't set back far enough.Also,just using standard run of the mill resizing dies I have taken fired brass and set the die too high and resized and kept setting the die down just a smidgen and resizing until the case chambered with no resistance and then set the die there and rechecked to be sure the case chambered easily.Trying to run with as little changing/resizing as possible I find frequent checks are necessary. ,,,sam.
     
  13. stinkybriches

    stinkybriches G&G Enthusiast

    my local gunshop owner gave me a copy of directions to set up the sizeing dies that i believe were origanaly printed by redding. it basicaly said to take a piece of fired brass and adjust the die down on it until it would just chamber.there seems to be lots of good info in this thread, cant wait to try some of the stuff out.
     
  14. Bart B.

    Bart B. G&G Addict

    I've had the same problem trying to keep changing/resizing repeats down to a minimum. In measuring FL sized case headspace with a case headspace gage (works the same as the RCBS Precision Mic), I noted that there was as much as 3 or 4 thousandths spread across a batch of sized .308 Win. cases. A friend suggested I do something to more uniformly lube the cases.After doing several tests with different case lubes on a pad and rolling several dozen fired cases on it, I learned I wasn't repeatable enough to lube each one the same. More lube resulted in shorter sized case heaspace; less lube made more friction and sized case headspace was longer. I had to figure out a way to lube cases to keep sized case headspace to no more than a 2 thousandths spread. I did get the smallest spread with a 50/50 mix of STP engine treatment and Hoppe's No. 9 bore cleaner (suggested to me by someone I now forget who).

    I tried a coffee can lined with 1/4" thick soft foam and 10 drops of my Hoppe's/STP 50/50 mix and put in 30 cleaned fired cases. That was the solution. Sized case headspace was then kept well under 2 thousandths.Turning a sizing die one full turn changes its height .071 inch. 1/16th of a turn changes headspace about .0045". To change the die's postion for changing the case shoulder setback only .001", turn the die in its lock ring such that it moves only 1/10th of an inch about its circumference. That's about 1/71th of a turn. 1/8th of a turn on the die changes it's height almost .009"; way too big of a step between tries.

    If the die's set only a thousandths or two too high for perfect headspacing and you feel a slight bind closing the bolt on it, turning the die down 1/8th of a turn (common, popular amount to lower the die) will no doubt let that case chamber easily. But the full length sized case may well have excessive headspace. That'll put that case into the dangerous area of reloading and most surely cause poor accuracy.
     
  15. samuel

    samuel G&G Newbie

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    Don't know what happened to my reply to you Bart.Maybe the dog ate it.I know it was on here and now it's gone.Anyway I stated your mix was intresting but wondered if it ate the foam up.I have seen crisco eat foam and anything petrolium really does a # on it.Possibly some kind of foam made out of neoprene or such.Also I stated I was glad you mentioned lubricants as it is very important to clean them off thourly before checking anything,even and especially in the chamber of your rifle.As is well established,if there is resistance bolting a round in,after several rounds you are really damaging your locks/firearm.Hope this one stays put. ,,,sam.
     
  16. SwedeSteve

    SwedeSteve Freedom Zealot Forum Contributor

    Very interesting read. Thanks to all who participated !!