Is bullet jump a mechanical or pressure phenomenum?

[Andrew Leigh]

You can look back in my posts and see that I recently debated whether to chase lands or not. So glad I did and I absolutely owe credit to gamestalker for the method of doing a powder work up close to the lands. Previously at 2.8" COAL my best was 0.8" with 40.9gr and 1.1" (@100yrds) with 43.6gr imr4895 in .308 under 168gr SMK's. After loading 0.010" off my lands and redoing a powder workup I got this:

0.375" center to center 4 shot group in the upper right with 41.2gr. As to mechanical vs pressure: I have no clue but awesome question and good read on the replies.

So effectively you are using the OCW method from Dan Newberry, very solid and effective method. I used to use that too but found it rather wasteful of ammo.

You appear to have another node at 43.2gr. You may want to load up and down by 0.2gr and explore that node as well.

Have you been able to replicate that 41.2gr. group?

I would love to test your loads against QL and see if we cannot get some consensus here. Did you chrony the loads? If so would you be willing to provide some data, if so PM me you e-mail address and I will send you a form to complete. I will then input the data into QL and see if the QL output corresponds with your actual data.

 

[Centurian22]

Yes, I have read and enjoy using the OCW method. I agree it uses ALOT of components. The first time i used his method I followed it to a 'T' and skipped bigger steps low in the range, also I was running it at 2.8". This time I used 0.4gr steps the whole way through and ran 0.010" off the lands. I just shot that yesterday so have not re-tested it for repeatability yet. Unfortunately I do not yet have a chrony or know someone with one I can borrow. I would LOVE to have the load checked in QL (and was actually asking W.E.G. In another thread if he would mind running it. With out chrony data but with other info can anything be gained from QL? I would be happy to provide any and all info I have or could obtain.

As for 43.2, I might explore it later on but its at max (or slightly above in some sources) and while proven safe in my rifle I want to get the best brass life I can. Being this is only a 'match' load and not hunting I'm not worried about chasing velocity. When I switch to 165 Sierra Gamekings I might look for the 'upper' node for better terminal performance.

 

[Andrew Leigh]

I would LOVE to have the load checked in QL.

Yip without speed the QL exercise becomes rather fruitless.

I developed am Excel Spreadsheet that automatically works out the loads as per OCW if you don't already have anything similar.

 

[918v]

Here we differ a little. I work out two loads in QL at the two fastest and safest OBT nodes. I then download 3 rounds with a mild load as foulers and 5ea of the anticipated nodes. I head off to the range and pop them over a chrony. I then take the average speeds recorded and add back the loss in velocity from muzzle to chrony and arrive at my "real" MV.

I don't go for the fastest node either because it is usually too hot. The reason I make 11 batches .1grs apart is I that the smallest ES never occurs at the predicted OBT. There are just too many variables to perfectly sync QL to the particular powder lot. If I find myself with a load that shoots well throughout this range, I simply pick the charge giving me the smallest ES. Often, the charge giving the smallest ES is not the most accurate. So then I have to balance ES with accuracy. Do I want a .3" group with 30 FPS ES or a .5" group with 15 FPS ES?

 

[Andrew Leigh]

Mmm perhaps may have a go at your method.

One of my biggest problems actually is that I think I load better than I shoot. This means that sometimes I do not get to prove my loads correctly.

I recently took the decision to spend more time practicing than messing about on the range doing load development. Every time I go to the range it is tons of paraphernalia, setup and distractions. I have vowed to take 50 rounds, targets and a rifle to the range next time ...... and that is it.

 

[Sunray]

"...have agonised over jump..." Quit doing that. It's not that important. And a .375 isn't ever going to be a target rifle.
"...want my groups to dramatically close up by simply seating differently..." Never happen. Far too many variables.

 

[spitballer]

Sometimes I wonder if seating away from the lands isn't more useful to control pressure than it is to increase accuracy. I would think ball ammo under high pressure would be safer to load away from the lands.

 

[moxie]

Safer how? In what way?

 

[Centurian22]

Sunray: if a ragged hole .375 inch group "isn't ever going to be a target rifle" then what is in your mind and how much would it cost? I'm not going to hijack this thread but suffice it to say I strongly disagree with your opinion. Again I'm not expecting bench rest same hole groups out of a $300 gun but 3/8" is plenty good for me. Have a nice day.

 

[Gtscotty]

I think he was saying that a .375 H&H is probably not going to be a dedicated target rifle...

 

[Oldman1151]

Has anyone done any bullet jump testing with pistols?

 

[pseudonymity]

Now of the above there is only one factor that to my mind that is purely mechanical and that is concentricity, the rest ALL result in a pressure change. The pressure change in turn will affect speed and barrel time.

So I have asked if I am not fooling myself. If I am on an OBT node I am in tune with the harmonics of the barrel and therefore any change in jump would require a change in load.

I am not so sure that the jump to the lands would have a significant effect on barrel time necessarily. Have you seen any experimental data that shows that? The jump seems to effect both peak pressure and the pressure curve, but I suspect that the bullet acceleration rate is a function of pressure, and that overall the total energy expended before muzzle exit may be close to the same for small differences in jump.

My personal guess is that in situations where minimal jump makes a difference, it is compensating for a lack of concentricity somewhere else - a poorly manufactured die, uneven case necks or a chamber reamed slightly off the bore axis.

That being said, it is nothing more than just a guess.

 

[lauderdale]

Old Man 1151 start a new thread? I would like to see anything one might no for sure! If their is reason to or is it a can of worms? sorry for High Jack. Iam interested!

 

[Dthunter]

Controling variables is what its all about. Seating depth in relation to the rifling lead holds "Some " merit. Many different things/techniques improve precision and accuracy. Its the sum of all the small things, so to speak.

One thing to concider, I have seen two very experienced benchrest shooters shoot each others rifles on the same day, and niether of them could shoot each others rifles as well as they could thier own. Same stock, and both 6mm PPC. Interesting eh?

Searching for accuracy and precision is fun as heck, try what you want. If it works for you, add it to your tool box of techniques. If it doesnt, move on. In the end, "our own personal limitations will govern the accuracy potential of a well tuned firearm, not just seating depth".

Doing load evaluation at longer range opens up the average group size due to environmental conditions for sure. But if the shooter pays attention to them, you will be rewarded with a valuable visual aid in the results.

You will see patterns emerge in the bullet holes for each group. Verticle/horizontal, nice and round shapes to groups,etc.

I have been shooting long range for a number of years, and it has taught me allot in my shooting and reloading. Give it a try

 

[918v]

Has anyone done any bullet jump testing with pistols?

I have.

I have reduced 100+ FPS extreme spread to 15 by reducing OAL from 1.150" to 1.125" in some 9mm loads. I have reduced group size by 50% by reducing the OAL in my .45 ACP from 1.275" to 1.255" with some 230 FMJ RN. I have gotten the same results by increasing the OAL from 1.135" to 1.150" in my Glock .40 S&W with 165gr bullets.

OAL is a viable load tuning tool in handguns for sure. It has more of an impact in handguns because the cases are small in capacity and the change in case volume is greater.

 

[spitballer]

Safer how? In what way?

Safer because it's less likely to upset the internal mechanisms of autoloaders with unnecessary pressure? Isn't that what the longer leade is about?

 

[moxie]

That makes no sense.

Look, the little bit of extra pressure generated by seating the bullet a teeny bit closer to, or farther from, the lands is not harmful. What's harmful is a double charge. That will upset the internal mechanisms in a big way. It's called a kaboom.

Are you aware of a kaboom or other catastrophic event involving an auto as a result of minor tinkering with OAL?

Yes, if you seat a bullet very deep in a case you can raise pressure dramatically. But when does it actually become unsafe? I've not heard of any instances.

The minor tweaks discussed in this thread are simply for the purpose of hunting ever better accuracy.

Really, don't overthink this. Just get a good reloading manual, read the front matter, and follow the rules. You'll have safe, and accurate, ammo.

 

[gamestalker]

Hey there Moxie.

Regarding bottle neck cartridges, I totally agree with you, never heard of, or ever experienced a catastrophic event due to variations in seating depth, even when jammed. However, in the Speer #10 they make reference to pressures more than doubling when the bullet was seated .030" deeper with a pressure tested 28,000 cup 9mm load, pressures jumped to 62,000 cup. I don't know if that qualifies as catastrophic, but I would venture to guess it may be on the brink? It's a claim by a trusted publisher, but then again?

GS

 

[spitballer]

I'm a simple man and that's probably why I stay with a simple bolt action. I don't trust myself with anything more complicated.

Nevertheless a thread worth reading.

 

[moxie]

GS,
Won't argue with Speer. 9mm is something you have to be careful with. But, again, I've not heard of any instances of damage or kabooms attributable to overpressure resulting from minor OAL tweaks, moving the bullets in or out of the case a teeny bit.

For perspective, think about how many 9mm rounds with serious bullet setback (often unbeknownst to the shooter) get chambered and fired all the time, apparently without ill effect.

 

[Walkalong]

The difference in bullet setback in a bottle necked rifle caliber vs 9MM is quite large.

Think of it in terms of percentage of case volume taken up by the setback.

.060 in a .223 case or a .30-06 case vs .060 in a 9MM case is vastly different.

Yes, guns are designed to be able to handle the occasional over pressure load (Or we might all be in deep Kimsche), but they are not designed to handle them over and over. So sure, we get away with the occasional high pressure auto pistol caliber load due to bullet setback. It doesn't mean it is safe or cannot have dire consequences.

 

[GLOOB]

I think of it more of a pressure thing.

Take a break barrel pellet gun, for example. A Crosman pellet is a harder alloy with a bigger skirt than most every other pellet on the market. The skirt does not get into the chamber unless you forcefully press it in.

Some people use a pellet seater tool to seat the pellet deeper. Some leave the skirt sticking out the back of the chamber. What is more accurate depends on the particular gun in question. It can help or it can hurt.

As far as velocity, I think most break barrel guns will shoot slightly faster if you leave the skirt sticking on the edge of the chamber. Pressure will build higher, initially, before the pellet starts to move.

OTOH, CO2 guns usually get more velocity if you seat the pellet farther down the bore. They can inject more cold air +- liquid CO2 in the space behind the pellet in the short time that the valve is opened.

In a rifle, sticking the bullet against the lands is like having the skirt of a pellet on the rim of the chamber. It will probably lead to a greater initial pressure. This can make the entire charge burn more quickly, overall. This can increase velocity and peak pressure. Like in a pellet gun, this can be good or bad. In particular, if you are using a slow powder, this increase in pressure might result in higher consistency and accuracy. Or it might not. Depends on the gun, the bullets, and the load. Only the target will tell. But theoretically, it will reduce any deviation in burn caused by variation in neck tension.... because neck tension will be fairly moot compared to the force required to get the bullet into the rifling. So now, it's more a question of how consistent is the hardness, diameter, seating depth, and concentricity of the bullets, themselves.

With my bolt action cast rifle loads, I purposely load them into the lands in lieu of crimping. Lubed cast bullets don't get as much neck tension/friction as jacketed bullets, and I rather not crimp. I think this sorta makes up for the lack of tension, particularly when using slower powders. At any rate, peak pressure of these loads is well below factory loads, so increased pressure is not a safety concern.

 

[gamestalker]

Moxie, agreed, and IMO it would probably take the right mix of circumstances to cause a catastrophic even, max powder charge, extreme set back, and so on. And we all know that when we shorten oal, we should take the load back down and re-work from a sensible place in the charge table. But this is nothing new to a fairly well experienced reloader. But if a reloader were to start with a max published charge, and then reduce the oal .030" from the tested oal in the published data, it could possibly produce some excessive pressures with a small high pressure cartridge such as 9mm. And although I'm not willing to try and reproduce Speer's tested results, I would doubt it would cause a KB in a modern in good condition firearm, as well.

This is a really good and informative discussion we have going here. I like the perspectives and theories being presented on this topic.

GS

 

[spitballer]

That makes no sense.

Look, the little bit of extra pressure generated by seating the bullet a teeny bit closer to, or farther from, the lands is not harmful. What's harmful is a double charge. That will upset the internal mechanisms in a big way. It's called a kaboom.

Are you aware of a kaboom or other catastrophic event involving an auto as a result of minor tinkering with OAL?

Yes, if you seat a bullet very deep in a case you can raise pressure dramatically. But when does it actually become unsafe? I've not heard of any instances.

The minor tweaks discussed in this thread are simply for the purpose of hunting ever better accuracy.

Really, don't overthink this. Just get a good reloading manual, read the front matter, and follow the rules. You'll have safe, and accurate, ammo.

I respectfully disagree. I think it makes a lot of sense or they wouldn't have done it. Please explain the purpose of the longer leade with the 5.56 mm chamber, then.

 

[Andrew Leigh]

I all depends on where you are on the load curve. If you are at the mid point then there is sufficient headroom to compensate for these variations. It is when you are unknowingly at max pressure and you start playing around that it becomes a problem.

Before getting QL I was stupidly chasing a factory load speed on my .375, I got there with a local powder. When simulating in QL my pressures were running at 74 000psi vs. the 62 400 odd that is allowable, and I was more than .25" off the lands. Now imagine if I had jammed that load into the lands?

Also that the pressure curve is not linear as you approach the lands, the closer you get the more dramatic the rise.