Bullet jump on 5.56x45 when slammed into battery

W

Western Living

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Minuteman
Sep 27, 2020
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#1
I posted this in the 5.56 thread, but no one seemed interested in discussing it there. This is therefore a re-post.

Summary: bullets jump when slammed into battery in a semi-automatic action. I tried increasing "neck tension" and crimping. Increased neck tension did not stop the jump. Crimping did not stop the jump but seemed to limit how far it continued. We should expect some jump because a hard stop into battery works just like an inertial bullet puller. Is a crimp into a crimp groove necessary to limit this jump, or is neck tension on a bullet without a crimp groove sufficient? Crimp grooves can be detrimental to accuracy if they're formed by smashing the bullet (maybe not if cut on a Swiss lathe, but we like less costly bullets).

My die is setup with a bushing to size LC brass necks to .243" and then pull a carbide expander ball through them to leave an OD of .244". When I seat a bullet, the OD is .0246". This would be commonly referred to as two thou of "neck tension." The amount of tension also depends on how much of the deformation the brass undergoes when the bullet is inserted is plastic and how much is elastic. My brass was brand-new Winchester brand from Lake City, fired and resized only once by me. I haven't messed it up with improper annealing nor have I work-hardened the brass neck by resizing it many times. I'll refer to this two thou delta between sized and seated as two thou of neck tension with the understanding that there's a little more involved there.

Bullet Jump
With two thou of neck tension and no crimp holding the bullet, I measured the overall length and then let the action on the rifle strip it from the magazine and slam it into battery with the muzzle pointed down. I measured it again and saw the bullet had jumped 1 thou. The cartridges I'm using for these tests have no powder or primer. Because the rifle has a floating firing pin, I did not want to be trying to induce a slam fire. I'm not surprised the bullet jumped because when a semi-automatic action accelerates the cartridge toward the chamber and then abruptly stops when the breech closes, it works just like an inertial bullet pulling hammer hit against a hard surface. Newton's first law explains why the bullet, once accelerated, will continue in motion even if the brass is stopped. Friction in the brass case neck is the only thing stopping the bullet from continuing until it hits the lands, unless it is crimped. The friction was enough to allow the bullet to jump only one thou. However, I repeated chambering the same cartridge several times and the bullet continued to jump 1 or 1.5 thou each time.

Crimp Test
Next, I used a Redding micro-adjustable taper crimp die to crimp the case of a different cartridge in the bullet's crimp groove. I repeated the process of measuring the overall length, chambering the round with the full force of the action, and measuring how far the bullet jumped. With the crimp, the bullet still jumped a thou. However, when I repeatedly chambered the same cartridge, the bullet jumped a total of three thou and then stopped moving farther with subsequent chamberings. I suspect the edge of the crimp groove slid forward against the crimp, and that it would take more force to push the crimp open.

More Neck Tension
To get more neck tension, I first removed the carbide expander ball from the die. Now the brass would be .243" before seating the bullet and .246" after -- three thou of neck tension. I also selected a different brand brass, a Speer nickel-plated case, and after sizing it with the .243" bushing without expanding it and then seating a bullet, it measured .247". The Speer case's brass is thicker in the neck -- four thou of neck tension. I repeated the tests on uncrimped cartridges and found the bullets would still slide forward about a thou every time they were slammed into battery. The bullet in the Speer case continued to jump as far forward as eight thou with repeating chamberings.

Crimp + More Neck Tension
I resized cases and seated bullets with 3 and 4 thou neck tension, and this time I also crimped them. I repeated the tests. I found the bullets would jump a thou, and after repeated chamberings they had jumped about three thou. After that, they would not jump farther.

Conclusion
My results suggest to me that increased neck tension will not stop bullet jump. I don't have more bushings to test 5 or 6 thou or more of neck tension, but at some point seating the bullet is going to plastically deform the neck so that it is not actually holding the bullet under any additional spring tension than that which is achieved with a larger bushing.

My results suggest a crimp will not stop all bullet jump, but that a crimp into a crimp groove can limit bullet jump to some smaller distance within the groove provided the inertial force on the bullet does not overcome the crimp. Certainly, my hammer-style bullet puller will overcome the crimp.

Not all bullets have a crimp groove. Many more accurate bullets do not because pressed crimp grooves deform the bullet and result in poor mass concentricity and wobbly bullets. Perhaps crimp grooves cut into a solid bullet on a lathe do not create this problem. Certainly, bullets into which a crimp groove is smashed by a Factory Crimp Die do. For bullets without a crimp groove, it seems best practice not the crimp them and to be aware they can jump some when slammed into battery by a semi-automatic action. If they jump one thou, that doesn't seem to be a big problem to me.

If a bullet has a crimp groove, a taper crimp of the case mouth into the groove can help limit the distance of bullet jump from the cartridge being slammed into battery. However, unless the cartridge is expected to be repeatedly slammed into battery many times, it's not clear that any benefit is derived from the crimp since the crimp is not likely to prevent the 1 or 1.5 thou the bullet is likely to jump upon being chambered the first time, and any ability of the crimp to limit further jump upon additional chamberings won't matter after the cartridge is fired. It could matter more if the recoil spring rate was greater. The M249 uses a piston operated gas system with a greater reciprocating mass, fires from an open-bolt, and has a higher rate of fire that is expected to be sustained for longer. It is highly likely therefore that it has a higher recoil spring rate, and that the cartridge is accelerated to a higher velocity before it is stopped in battery.

Questions
  • Do bullets in 5.56x45 cartridges jump when chambered in your semi-automatic rifle? If so, how much?
  • How much delta is there between your sized and seated necks?
  • Do you crimp your cases when the bullet has a crimp groove? If so, taper or roll?
  • If you crimp your cases, why do you do so? Have you done any tests to verify that it is effective?
 
#2
Western Living said:
Questions
  • Do bullets in 5.56x45 cartridges jump when chambered in your semi-automatic rifle? If so, how much?
  • How much delta is there between your sized and seated necks?
  • Do you crimp your cases when the bullet has a crimp groove? If so, taper or roll?
  • If you crimp your cases, why do you do so? Have you done any tests to verify that it is effective?
Click to expand...
My bullets do move about 0.001" to 0.003".
My neck tension interference is roughly 0.0025" or whatever it takes to get a seating force greater than 35 pounds.
I do not run cannelure bullets, so I do not crimp.

If I am discussing prairie dog ammo along the lines of 40 VMax, or XTC ammo along the lines of 77 SMK and 80 SMK, I don't run a cannelure.

Note that MIL ammo uses a neck tension but also uses a sealant, as well as a crimp. The crimp is intended to fight set-back more so than extension, however the total of those features means the bullet remains in place from being slammed into battery since the acceleration forces do not exceed the grip of those combined features, notably the sealant.

If you have a force-pack or ability to measure seating force, try and push one of your loads in, and then give a legitimate sealed M-193 load a try for comparison.

One further comment. It is good to think about these factors for semi or full auto ammo for your rifles, pistols, and shotguns. The workmanship is your responsibility. Semi and full auto ammo does warrant a moment to consider inertial movement as well as set-back. These actions are known to be violent on the ammo compared to human operated actions.

That said, once you come to grips with the idea that unless you use sealant your AR-15 rounds will be changing from being slammed into battery, you also come to grips with the idea that is part of the territory and you start to look at seating depth in a different light. We tend to favor tangent ogives due to their forgiveness of seating depth in these chambers, and we test with the assumption the bullets will move a little.

When we play the XTC games and run mag length ammo at the 200/300 yard lines, or when we run 80 grain ammo in single feed fashion at the 600 yard line, the final judge of our workmanship is the score. The performance of XTC loads against the standard target is a matter of record. The varmint ammo performance is yet another. Since it would be exceedingly rare to hear of a hand loader that uses sealant, it is apparent that bullet movement and seating depth in an AR isn't something to get too worked up about. That is not to say to forget about it, but it is to say that you must assume your bullets will move and plan accordingly. YMMV
 
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#3
Have you considered a LEE Factory Crimp Die?? They are not expensive: Lee 90817 Factory Crimp Die 223 Remington 5.56x45 New In Box #90817

I use them for all 5.56x45 loads, because I'm looking for consistency and safety from my AR's and not chasing the smallest possible MOA. I use this die regardless of whether the bullet has a cannelure or not. A mild crimp with this die is usually enough to keep a bullet from moving at all. It's crimping action is similar to that used on most factory 5.56mm ammo.
 
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#4
I appreciate the replies. What got me into this was deviating from bullets with a crimp groove. I suspect crimp grooves a detrimental to accuracy. That would also be true of grooves dented onto the jacket with a FCD. A milder crimp with a FCD does not damage the bullet jacket. I also understand there are three different types of FCD -- the taper, collet, and carbide types. Sierra Bullets published an article stating that :

To counteract this tendency, the semi-auto shooter is left with basically two options: applying a crimp or increasing neck tension. The first option, crimping, brings up some other issues that can be troublesome. In general, crimping degrades accuracy. Most match bullets are not cannelured (which also seriously damages accuracy potential), a requirement for correct application of most crimps. Still, there are taper crimp dies available from most of the major manufacturers. Lee offers their “Factory Crimp” die as an alternative, which seems to be one of the better options for those bullets without a cannelure. That having been said, crimping is still, at best, an occasionally necessary evil. Avoid it if at all possible.

The other—and in our opinion, better—option is increased neck tension. This, in turn, leaves us with two more options depending on what type of equipment you’re using. The object of either is simply a tighter grip on the bullet. Using conventional sizing dies, (i.e., those utilizing an expander ball) this is accomplished by reducing the diameter of the ball itself. This can be done by chucking the expander/decapping rod into a drill and turning it down slightly with fine emery cloth or a stone. The goal here is to decrease the diameter two or three thousandths (0.002″ to 0.003″) under bullet diameter. This is a trial and error process, and must be done slowly. The end result is an expander ball that opens the case neck up somewhat less than the as-issued item. This, in turn, increases the grip of the case neck on the seated bullet.

Without a crimp groove, I needed to understand how much bullet jump would happen, what methods are effective to limit it, and how much it can be expected to be controlled.

I am skeptical that the sealant on LC cartridges is what fixes the bullet in place more securely. I'm not disputing the claim. I have no evidence to the contrary. What is it anyway? Cyanoacrylate?

Following Sierra's advice, my Redding Bushing die and expanding ball are as-issued and they result in 2 thou interference fit on the diameter. Instead of turning the ball down, I can simply omit it and get 3 thou interference fit because my neck bushing is precisely sized and does not excessively size the neck down. However, my experiments show that 3 thou is not better than 2 thou, neither is 4 or 5 thou.

What I'm understanding is that it may not be effective to increase the interference fit of the bullet and neck to 4 or 5 thou using a smaller bushing and expander ball or mandrel. An interference fit of 2 to 3 thou should be sufficient. I should just expect some movement of the bullet. I'm doubtful that a taper or roll crimp will grip the bullet any tighter unless it is pushing the case into a groove or denting a groove into the bullet. The taper crimp is useful for closing expanded mouths, but I don't expand 223. Otherwise, the crimp dies do seem to be effective at limiting bullet jump and setback within the length of the groove.

Without the groove, I doubt crimp dies do anything. The case neck is already pressed against the bullet jacket. The crimp die cannot move it to make it any tighter without deforming the bullet. It makes more sense to close the neck tighter before seating the bullet because then there is no incompressible bullet behind the neck that it is being attempted to push inward.
 
#5
More problems with bullet setting back during recoil than moving forward when chambering on heavy recoiling rounds ...and more of a serious problem with super high pressure if fired.

But that is not the problem here.

ARs with heavy bullets 500 gr sized .452" in 450 Bushmaster, add a light to moderate Lee factory crimp, will move forward.

Or the problematic 450 Bushmaster with .451 pistol bullets 185 gr would have such light bullet pull when sized in a regular Bushmaster FL die, that when the cartridge was fired all ya hear is the primer go off pushing the bullet a few inches into the bore along with all the powder....messy.
Not enough start pressure to ignite the powder.
Since ya can't crimp the 450 Bushmaster the problem was solved with the .451" pistol bullets but partially sizing the neck past where the bullets ends with a carbide 454 Casull die.
This left a bulge in the case neck easily seeing where the back of the bullet was located...but it solved the ignition problem.

But do they move out on chambering?...Absolutely...about .005" to .007" as loaded.

The 338 RCM 230 gr .0015", the 300 Blk 220 gr .0005" thought it would be more with heavy bullet. The 223 with 77 gr .005" no crimp.
Didn't test any light 40 gr or 50 gr but they should be less with less mass, depending on dia differences, of maybe if crimped.

So they all move out... the 450 Bushmaster can not be crimped hard, as is headspaces off the case mouth, with heavy bullets and one of the worst offenders of the ARs, as were my 77 gr 223s non crimped.

Since you have tried more neck tension and more crimp and still there is slight movement a hard mechanical lock in crimp into a bullet groove is probably the best way to stop movement of the bullet with these AR bullet pullers.
You'd have to test that for accuracy and S/ D variations to see if its a viable option.

I wouldn't worry about .001" or .002" jump if that's all there was every time.

The military uses a heavy crimp and some sticky asphalt goo on the bullets that's hard to remove. I use SS pins and wet tumbling for quite some time and twice to remove it on 50BMG pulled bullets that need to be resized because of the pulling.
 
#6
As stated above, a thou is nothing. Second, bushing dies are not going to size the neck as completely as a standard FL die, which is important when the neck is already very short. Without knowing how much seating pressure you’re seeing it’s hard to go deeper into it. So ditch the bushing die, play with the inside surface of the neck to increase friction, etc.
 
#7
I'm considering trying a Lee Collet Neck die, and also the same with the undersize 0.220" mandrel. I'm not talking about neck-only sizing. I have a FL bushing die that I can FL size with. If I remove the bushing and expanding ball, it won't touch the neck. Then I can size the neck with the Lee Collet. The collet won't pull on the neck the way the ball does. Alternatively, I could use a bushing with no ball and then push a mandrel into the neck, but as pointed out in the post #6 above, the bushing won't size as much length of the neck.
 
#9
The advice of @45-90 and @RegionRat is spot on. I will add that assuming only 0.002" tension is preferred is probably not the best approach for a semi automatic rifle. As they have noted setback and also having the bullet slip forward and and does occur. A primary safety issue is setback. As an RSO I have seen multiple issues with 223 and 556 loads forced back in the case. Usually these are caught and firing the rounds avoided but the potential for major issues is there.

I load a large amount of 223 for a bolt action using the LCD. However, I would never consider using only that die for sizing the neck of my 556 loads.
 
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#10
Thank you for the advice. I attempted 0.002" tension, but found the bullets slip forward. I also attempted 0.003, and 0.004, and the bullets still slipped forward. I agree that setback is a serious concern. I think it is somewhat mitigated when the bullet is compressing a powder charge, but it's also fair to say that powder is not the preferred means of holding the bullet in the case.

When you write that you would not consider using only a LCD for sizing the neck of 556, do you mean that you would not neck-only size without resizing the body? I can understand that. I just want to clarify if you see any problem with sizing the body with a FL bushing die and then using the LCD for the neck.

The reason I'm considering the LCD is because it should size a greater portion of the neck than the bushing die which only sizes the neck as far as the neck is inserted into the bushing. The SAC bushing I'm using also has a step in it. I think the Redding Type-S FL bushing die with SAC bushing is outstanding for bolt-actions or single-feeding, but I want to pursue a method that holds the bullet more tightly for an autoloader with a more violent action.
 
#11
Western Living said:
I just want to clarify if you see any problem with sizing the body with a FL bushing die and then using the LCD for the neck.
Click to expand...
If you pull the bushing out of the Redding FL Type S die completely, it comes very close to becoming the equivalent of their Body-only die and can be used to bump the shoulder and diameters without affecting the neck.

Now just humor me for a second. Are you trying to solve a performance problem with your 556 ammo for use in an AR?
Or are you just curious and thinking out loud? There is no harm in chewing the fat in the forum, so I am just wondering if you have an actual performance issue or are just curious?
 
Last edited:
#12
Western Living said:
Thank you for the advice. I attempted 0.002" tension, but found the bullets slip forward. I also attempted 0.003, and 0.004, and the bullets still slipped forward. I agree that setback is a serious concern. I think it is somewhat mitigated when the bullet is compressing a powder charge, but it's also fair to say that powder is not the preferred means of holding the bullet in the case.

When you write that you would not consider using only a LCD for sizing the neck of 556, do you mean that you would not neck-only size without resizing the body? I can understand that. I just want to clarify if you see any problem with sizing the body with a FL bushing die and then using the LCD for the neck.

The reason I'm considering the LCD is because it should size a greater portion of the neck than the bushing die which only sizes the neck as far as the neck is inserted into the bushing. The SAC bushing I'm using also has a step in it. I think the Redding Type-S FL bushing die with SAC bushing is outstanding for bolt-actions or single-feeding, but I want to pursue a method that holds the bullet more tightly for an autoloader with a more violent action.
Click to expand...

Consider a Forster FL die with a custom neck diameter.
 
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#13
RegionRat said:
...Now just humor me for a second. Are you trying to solve a performance problem with your 556 ammo for use in an AR?
Or are you just curious and thinking out loud? ....
Click to expand...

I am wanting to produce good quality 556 ammo for a semi-automatic rifle. I shoot factory LC cartridges and then reload them. The LC factory and my reloads are what I train with. I also have a few thousand once-fired cases from a law-enforcement range. I want to load those and store them. Whether I load cases to shoot or to store for shooting in the future, I want what I produce to be good. Especially, before I tool up to reload a few thousand, I want to have thought through the process and validated the result of what I'm going to produce.

I have not reloaded for a semi-automatic action much. I typically use the bolt-close method to size brass by bumping the shoulder just under chamber dimension, and a bushing die to achieve minimal neck tension. I don't necessarily crimp, and I believe bullets with a crimp groove are generally less consistent and accurate than bullets without. My experimental evidence suggests my methods are insufficient for a semi-automatic action, especially when using bullets without a crimp groove. I also read the kb article from Sierra bullets quoted in post #4 which also indicates the methods I've been using are insufficient.

Even so, I haven't actually had a performance problem yet. By looking, I can see my bullets are jumping in the neck when they're slammed into battery. If I didn't look, I could just shoot them unaware of what's happening, and it may never matter. Because I did look and I saw the bullets are jumping I can do one of three things:

  1. ignore it and load thousands of cartridges without any regard to it
  2. take more measurements to see if the jump is limited to only ever a thou or two, or if it can sometimes pose greater risk
  3. try to find a method that will hold the bullet in the neck better, esp. bullets without a crimp groove
 
#14
Western Living said:
My experimental evidence suggests my methods are insufficient for a semi-automatic action, especially when using bullets without a crimp groove. I also read the kb article from Sierra bullets quoted in post #4 which also indicates the methods I've been using are insufficient.
Click to expand...
Western Living said:
Even so, I haven't actually had a performance problem yet.
Click to expand...
Okay, this is why I am getting a little lost. There is a gap between "my methods are insufficient" and "haven't actually had a problem".

We need to shift the focus from the keyboards to the testing at the range. If we then discover we have an actual problem, we can work it out.

It is good to ask good questions, especially ones about your own safety. However, it is also the case here where you couldn't be in a more statistically beaten down topic that has been done for generations.

Bullets slammed into battery by AR actions will move statistically. It isn't a reason to stop provided you are not doing something unorthodox. If your loaded ammo will not survive being loaded, unloaded, and then loaded and fired, then we have a real problem.

At the same time, we aren't advising folks to run a light neck tension on a load for a semiauto, but we are also trying to persuade beginners not to make their first goals to load giant batches before they have their Dunning-Kruger Curve under their belts.

Give yourself some time to learn the ropes of loading for the AR before you mass load a bunch of ammo you will regret.

Run small batches more often, and give yourself a chance to get used to the idea that these rigs are hard on brass. By doing many smaller loading cycles, you will learn faster than doing fewer big ones and with less chance you load scrap.

You sound like you would benefit from more testing and less worrying. Start with the popular pet load recipes to clone your favorite ammo and give yourself a chance to learn the ropes. You will be fine. Good Luck and in for the range reports.
 
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#15
What spring is in the AR that you are using to chamber these rounds that are "jumping" setting depth?

Are you manually dropping the bolt, or firing allowing the gun to cycle, or both?

.003-.004 neck tension should be good to go but I have seen some require more for various reasons.

Make sure you are not running a bushing and then draggin the expander ball back through the neck.
 
#16
As far as as the LCD the issue is springback. it occurs in the direction the metal was bent/shaped from. So the
case neck sized with the LCD tend to open in diameter with both short and long term setback. With a regular FLS or bushing die the case neck is made smaller than the mandrel or expander so that the neck tends to reduce in diameter due to springbuck. The degree to which all this occurs depends on both time and case neck hardness. Does it matter to performance? I don't know but I want more than 2 or 3 thousandths for my semi-autos.
 
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#17
Have you checked to see what the actual bullet jump is from your bullet at load length to the touch point in your throat? In most AR chambers I've found that the jump to the lands is more generous than most think (depends on the bullet and profile). If you have a 50 thou jump to the lands, I wouldn't worry about the bullet moving a few thou when chambered .... unlikely that it would change accuracy that much.

If you can load at max mag length and get within 10-15 thou it might make a difference... but I haven't found too many chambers that have a short jump.

I would also pay attention to the number of firings on the brass and make sure you're not getting too much spring back (or the neck tension you expect)
 
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