Crooked launch: Dynamic Fit gone awry.

[Ian]

I found this bullet in my cull bucket. It was recovered from my sawdust trap, as I recall fired out of my 18" LR-308 at over 2400 fps. Alloy is approximately 6%Sb, 4%Sn. Less than .002" TIR when chambered, all the brass sized for .002" headspace, seated just off the lands, about .010".

Static fit was pretty good. The bullet wasn't jammed into the throat but the front band was sized to just scuff the rifle's long, parallel freebore, and the case and case neck were snug and centered in the chamber. In other words, the bullet was presented straight to the bore centerline before the trigger was pulled.

But after the trigger was pulled, the bullet went crooked in the throat and after squeezing through, the nose was no longer concentric with the bore.

Here's one side of the bullet, note how far up the nose the land engraves are:



Other side of the bullet. See the difference in land engrave length? Also note the seating punch ring at the base of the ogive is not square to the bullet's center of form:



The factor of crooked launches...and how to prevent them...is core to every dynamic fit discussion. Here is proof positive of a crooked start. The load that produced this result will still still group 2.5" at 100 yards with no particular flyers.

Let's talk about why this happened in my rifle with this bullet and what we can do to prevent it so we can get better long-range groups.

By the way, I got it to shoot ten into an inch at almost the same velocity by changing just one thing, but I'm not telling yet.

 

[waco]

Variation in case neck thickness?

 

[Ian]

Something worse than that. Case neck wall thickness variation is something I personally see affecting HV groups somewhere below 1.5" at the very worst, more like under an inch at 100. I suspect the relatively minor wall thickness variable is much more pronounced at longer ranges, like 200-500 yards.

This bullet got EXTREMELY crooked in the throat as the launch commenced, likely tilting sideways before moving forward.

Why?

 

[CZ93X62]

Wide throat/wide chamber specs? Chamber/throat/bore not all looking in the same direction?

 

[RicinYakima]

Crooked gas check? Alloy too hard, making immediate gas cutting? Damaged alignment from magazine to chamber?

 

[Brad]

Riveting?

 

[JWFilips]

More tension on the thicker side of the neck. Also A well fitted cartridge that has an irregular neck thickness is never concentric to the bore

 

[freebullet]

Neck tension or crimp.

 

[popper]

Sized too small. Gas cutting (slight) on the base bands. Too hard for the powder and nose slumped into the groove. No trailing edge on the most front band (there should be some slight drag there). First pic your finger seems to hide the crooked base, like something collapsed under the GC.

 

[Rick]

Sized in an in & out sizer and sized off concentric. Try a push through sizer nose first with a flat punch against a flat square bullet base. Never a poor fitting nose punch. Push the bullet into the die base first at any angle off center and that is exactly how it will be sized, loaded, enter the chamber and exit the muzzle.

 

[Ian]

Much of what looks like gas cutting on the one side is damage from the trap media. You can tell because the abrasion is on the front side of the bands going down into the lube grooves whereas gas cutting generally makes linear cuts without arcing over the bands and back down into a groove again.

The bullets had the gas checks seated in one operation, push-through sized in another, and lubed in a final step in a die the same size as the push-through. The bullets and loaded cartridges were true when loaded. The punch mark on the ogive is from the Forster Benchrest seating die, not the lube-sizer.

The only time the bullet got crooked or distorted was when it was actually fired.

The base is cupped but it is difficult to tell if it maintained itself square to the center of original form or to the crooked form after entering the rifling.

 

[Ian]

Damaged alignment from magazine to chamber?

Good guess and it does happen, but this is one of the first things I check with a new load in a gas gun. Ejecting a self-fed cartridge for examination eliminates a lot of head scratching. These did fine as have all others in this platform once I got the magazine lips sorted.

My M1A is another story, if the nose isn't just so, it will bend the cartridge slightly at the neck/shoulder juncture and shoot neat buckshot patterns.

 

[popper]

magazine lips sorted biggest problem in SA guns. Not just the space between but the length also.
'No trailing edge on the most front band' the bore part of the nose should have some alloy pulled into the L.G. It doesn't, therefore, it is too small for support! Lands will impress into the alloy differently. Cutting appears in the first as double edge groove on the first drive band.

 

[Spindrift]

That bullet has a very long bearing surface. With that tough alloy, the force required for engraving the bullet would be relatively high.
I would suggest either
- longer jump, to give the bullet a longer «running start». It should self-align well enough. Or
- a powder with a gentler pressure rise, usually this would mean a slower powder

I would try the longer jump first.

Edit
Very interesting post, by the way! Thanks, Ian!

 

[Bret4207]

I don't know what your theory is Ian, but I would have put $ on it being somehow abused in an auto loading gun on chambering. Maybe the round behind it not fitted in the mag right or something like that. Happens with bolt guns.

 

[Ian]

Spindrift is on the same track I'm on with this. I switched to IMR 4320 from Alliant Reloder 7 and the problem virtually went away. The 4320 load also happened to be about 200 fps slower (2250-ish chrono'd, I don't recall exactly), so that may have had an effect too.

I also theorize that seating this particular bullet made from this particular alloy considerably deeper would have helped with the RX-7 load.

The neck clearance, like with most .308 loads, is right at ten thousandths in this rifle with this brass and bullet combination. My thoughts are that the hot load of fast powder just blew out the neck to the chamber neck walls right away, leaving the main part of the bullet hanging out in space with no support before it ever moved forward (it was seated to touch the throat). All that good static fit was totally negated by using the wrong powder for the SITUATION. I shoot sub-moa groups with the same exact charge of RX-7 and an even softer bullet that self-aligns and is powder coated, and has a considerable amount of jump.

 

[fiver]

gravity.

 

[freebullet]

Hmm, I was kinda on trail. Neck tension & neck clearance definitely correlate.

To attempt a solve for the issue using the same previous powder...decrease neck tension=decrease neck chamber clearance...worth a shot. Diminishing return for accuracy if'n more crimp is needed to hold'em.

Figuring may keep 200fps & fix I dunno. Line I take I guess.

 

[Bret4207]

That's an awful lot to happen in a very short time. Do the markings support that theory as far as measurement of the difference between side? IOW- If it was left unsupported could it have tilted that far?

 

[Snakeoil]

Interesting discussion. Hard to conceive the order of occurences in the chamber. Theories are interesting.