Bullet slump

[Brad]

When I talk of slump I mean changes in bullet shape from what is loaded in the rifle and what exerts the muzzle.
A couple questions. When does the major change occur? In the chamber, throat, or bore? Does the shape change in a shorter or longer period of time? I realize those are very relative terms.

How do YOU use or combat these changes? Go with the flow or fight back?

I am speaking of fixed chamber guns only. A revolver is another beast entirely with a unique set of needs.

 

[Josh]

I am a go with the flow kind of guy, I look at the target and allow it to talk. Now I do believe that slump happens at the throat, where pressure and velocity both jump instantly. This would somewhat explain why you can shift gears in powder burn and recover accuracy.

A fast powder like 4198 pushing a bullet will produce more G's on the alloy in the inital inch or so, where something like 4350 will produce less G's due to a "longer" push instead of the harder "punch" of a faster powder.

I try to stay away from any bullet that has minimal nose support when pushing for speed.

 

[Ian]

"Slump", or what I prefer to call "bump" (meaning the force of change comes from pressure behind, causing plastic flow of the bullet per the laws of dynamics, rather than the nose setting back due to static inertia, which I'm not too sure really happens as often as is talked about), occurs when and where you make it occur.

Like Josh said, powder burn rate (rate of pressure rise after ignition) affects when bump or slump, if any, occurs.

Two other MAJOR factors are alloy and throat/bullet shapes. Amount of initial bullet movement to contact the throat, and whether the bullet shape matches for quick, full contact or whether the tapers are mis-matched and there is a little bit of wiggle room and gradual increase from zero to full contact as the bullet engraves.

Bullet shape and how it interacts with the throat during the engraving/swaging/alignment process creates varying amounts of resistance to the powder burn, and varying resistance curves against which the powder performs work.

Alloy is moved into the plastic stage in part (just squeezing metal out of the way of the lands) or in full (as in some or most of the bullet becomes plastic and changes shape within the confines of the case neck, throat, and barrel), and alloy constitution and temper are variables which determine just how, where, and how much metal is moved during the firing event.

So, the three factors of bullet shape (broad-stroking the factors of static fit, seating depth, bullet body size relationship to throat and others here), bullet alloy, and powder burn rate are all inseparably intertwined and each affects the other. I call the whole thing Dynamic Fit. Difficult to predict outcomes as the variables are too many....but this is the principle secret to high-velocity accuracy (starting the bullet straight) with cast bullets and why there is no short-and-sweet explanation of how dynamic fit at launch is achieved.

 

[Rick]

Excellent description Ian. Since brad posted the question I've been sitting here trying to figure out how to go about explaining all that . . . Now I don't have to.

 

[Josh]

I am not as fluent in the keyboard as most here, and Ian conveyed the point much better than I could.

 

[Brad]

Well spoken Ian. Dynamic fit is the real issue. Making a mould that fits statically is pretty easy with the custom mould makers we have today. Pound cast sent to Veral or Tom and it is done. What happens after ignition can, and will, change the bullet shape.

Now, how do we know if we have a bullet design issue or a pressure curve/alloy issue?

 

[Ian]

[typing with seven fingers and two thumbs currently]. You can fuss over static fit 'til the cows come home and never make a rifle shoot past 1800 fps because all that work might just go to pieces when you light the fire. The majority of cast bullet shooters seem to have learned from Lyman and "conventional wisdom" that casting a bullet from acww and sizing to groove diameter and making sure the parallel bore-riding nose gets some marks on it from the lands when chambered is good enough, not realizing that it doesn't just go from loaded to fully engraved in one magic shot unless you're really, really lucky.

Wanna shoot half-inch groups instead of 2" groups at 1800 fps? Wanna shoot an inch at 2400 fps out of a factory rifle? You're going to have to learn exactly what's going on in the first half-inch of bullet travel and make what's going wrong go right with emphasis on a straight start and geting consistent bullet deformation shot-to-shot. I think a whole book could be written on that first half inch of bullet travel if enough experiences were shared, and I'm not talking rambling theoretical discussions, but things like "I did this and this happened, then I changed this and this happened, so I changed this other thing like so and this happened".

Maybe we could start that book right here?

 

[Brad]

I really wish my berms were more conducive to recovering bullets intact and less deformed. I think the spent bullets would tell a whole lot about happened to them upon firing.

 

[Josh]

I would also like to say that any rifle, before loading cast for extreme accuracy, should have a jacket bullet accuracy load worked up with the rifle in the configuration it will be shooting cast. I say this because there are some rifles that will only shoot 1" at 100 yds or only 1.5" at 100. You will constantly chase your tail with cast thinking it is the projectile/powder/alloy/etc. When in all reality the rifle will never do as goos as you wish. I have a wonderful winchester 30-06 that is a perfect example of this.

 

[Ian]

Sure, there are plenty of problems that are rifle-related, and getting a feel for how the bedding and barrel are and some other things with jacketed loads is a good idea. That said, there is a lot more that can be done with cast to make a contrary rifle shoot straight than there is with jacketed...for example with cast bullets we can compensate for abnormally large groove diameters or very worn throats.

 

[Brad]

Great point Josh. If you can get equal, or close to it, results with cast that you got with jacketed then you are onto something.

 

[Josh]

Agreed Ian, there is plenty extra we as casters can do to put a bandaid (so to speak) in our rifles. My point was more of a "jumping off point" for accuracy, as we can compare what we are doing vs what the jacketed load can do.

A jacketed load accuracy test should not be the end all for accuracy, but more of a bookmark or rally point.

 

[Ian]

Very true. Shoot some factory loads, maybe handload a few j-words, see how the barrel does when it warms up, get a feel for the trigger, check the mechanicals out, confirm the sights or scope and mounts are good, then make a pound cast, measure some fired case necks, check headspace, clean the bore really, REALLY well, and start studying what bullet mould and bullet/case sizers will be needed.

 

[9.3X62AL]

GREAT THREAD. THIS is why I keep coming here, discussion content and quality. Looking on and listening closely here.

 

[freebullet]

You can recover your boolits intact and un- expanded in my boolit catcher bucket. With handgun rounds it slows them to a gentle stop. Most come out as fired.

The rubber mulch/sand I used in that bucket for catching handgun bullets could be used to make a bigger catcher for rifle boolits. We could use a tote or something longer to recover them intact. I reloaded some 45acp that were already fired. Just tumble lubed & loaded. They shot fine the second time too. The bucket weighs 20lb something capable or stopping cast rifle would be around 40lb.

 

[Brad]

Rubber mulch and sand? Interesting. The mulch is cheap enough. I may just give it a try this spring. How much sand?

 

[Ian]

I have made mulch traps and have plenty of it on hand. It works ok by itself, never tried it mixed with sand. My recycler traps are all filled with used #4 blasting sand and it rakes off the bullet surface pretty badly. Maybe rounded "silt" sand would be better than the sharp granite abrasive. Sand mixed with mulch, hadn't considered that but it sounds worth a go.

My plan has always been to make a tube trap with oiled sawdust, but it would be very messy and a lot of work since I'd be using used engine oil from work and sawdust that included a lot of actual wood "dust". Probably a wheel barrow to mix it up and then shovel it down a large PVC pipe. Be a real pain to get the bullet back without a metal detector.

 

[Brad]

This is something I will pursue.
How tight should I pack the mulch? Loose or cram it in?
Anything that helps recover bullets in better condition can only help.

 

[freebullet]

I posted the exact fill in the stickied bullet trap thread over yonder. I posted pics of the whole unit, the folding stand, & recovered boolits. Brad your welcome to borrow it as I use it rarely.

Didn't mix the sand. I put an inch sand in bottom of bucket. A thin sheet metal plate, 3"of sand. A layer or 2 of carpet to keep sand & mulch seperate & then fill with mulch & carpet over the face under & over the lid so the stuff don't fall out when the lid gets shot up a bit. I fired over 1k 45acp into it in 1 day. 357mag won't reach the plate near the bottom. Was guna shoot some x39 at it before I switch to a new bucket.
A slightly longer unit could easily catch cast rifle boolits.

 

[Ian]

I remember that now. Not sure a rifle bullet won't get damaged going through a bucket lid at 2500 fps, but there's only one way to find out and a different sort of lid or perhaps a "small window" cut in the lid and covered with soft rubber sheet or recycled cardboard could be used to contain the mulch.