Why rifle safety features are important - AI-AW50

[MCMXI]

I have, well, quite a bit of more recent LC brass. I'll be in touch re: the analysis.

OK ... if you plan on keeping the LC brass I'd suggest that you measure the case necks and sort out batches based on neck runout. I'd start off by using cases with tight case wall tolerances (as inferred from the neck measurements). Maybe there isn't a good correlation between that one case failure and its inconsistent case wall thickness but at this point it's all you have. If after the SEM/EDX analysis a clear defect is observed then perhaps you can chalk it up to an anomaly and go about your business. The question is, what do you need to do to erase this failure from your mind every time you pull the trigger? You may be wondering if a case will explode in your face and you may also be thinking that you'll miss your target if a case fails.

 

[gunnie]

1858,

he said the failure was about 90 degrees out from the thinnest point. if you want to check wall thickness beyond a shadow of doubt, make up a simple test fixture like the link in posting #30.

i searched for a copy of the original article i had read about this in. was an american rifleman circa '70's-'80's. the author also included a test for case head perpendicularity to center line, by centering neck bore on a ball bearing while locating case wall in a simple "V" block type set up, and indicating off of outer edge of the base beyond stamping indentions. said accuracy gains to be the driving force for its case wall usage, but case rupture prevention is a byproduct, though same seems non issue in Zak's case.

gunnie

 

[Boba Fett]

I went to school for EE.

My best friend is an EE working at Rockwell Collins.

EE and ME are fascinating to me....makes me wish I'd chosen one of those paths in college...ah well

At least I get to learn a lot about it from my friend.

 

[MCMXI]

he said the failure was about 90 degrees out from the thinnest point. if you want to check wall thickness beyond a shadow of doubt, make up a simple test fixture like the link in posting #30.

gunnie, I realize that the failure was between the thickest and thinnest sections of the case (the transition region I suppose) but the point is that a case with inconsistent wall thickness has failed. Did the inconsisent drawing of the brass result in inconsistent work hardening? Maybe and maybe not.

I agree that the fixtures shown in post #30 are the way to go to be absolutely sure of wall thickness througout the majority of the case but I'd hardly call them "simple" even if you have access to a machine shop. The Redding Case Neck Gauge is readily available and fairly inexpensive as is the RCBS Case Master Concentricity Gaging Tool (see links below).

http://www.midwayusa.com/viewProduct/?productNumber=311160
http://www.midwayusa.com/viewProduct/?productNumber=310955

 

[gunnie]

..."Did the inconsisent drawing of the brass result in inconsistent work hardening?"...

the simplest way to find this out would be to check wall thickness on the cases that didn't fail. dunno LC spec for thickness variation, but wouldn't be very surprised to learn that 15 thousandths is within max run-out.

gunnie

 

[Zak Smith]

I'll be shipping the failed case and another LC 82 case to 1858 shortly.

 

[MCMXI]

dunno LC spec for thickness variation, but wouldn't be very surprised to learn that 15 thousandths is within max run-out.

Zak,
If you still have the rest of the failed case could you measure the neck wall thickness at 3, 6, 9 and 12 o'clock. It'd be interesting to see if the ratios are the same when compared to your earlier measurements of wall thickness near the case head. This is a rather obvious attempt by me to see if there's any real science behind Redding's Case Neck Gauge.

 

[gunnie]

1858,

get a cheap-o mag base for your dial indicator:

http://grizzly.com/products/Flexible-Magnetic-Base-Holder-130-lb-Force/G9625

or, score a cheap-o indicator with it to save repeated indicator removal from your redding:

http://www.grizzly.com/products/Dial-Indicator-1-2-Range-x-0-00

bend about 1/8" dia round stock until it resembles the side profile of a folding knife belt clip. make sure the bend is enough to clear the offset from neck to case wall I.D. of your particular brass, and contact the round stock on the small radius that would be the initial contact point of above mentioned knife clip. make sure round stock is long enough to get to bottom of casing, just above webbing.

have a welder tack same onto disassembled flex neck lock handle of mag base. welding current could kill magnets. put lock collar back on mag base and bend flex-neck to where indicator plunger meets bent round stock just barely on the inboard side of interior contact radius of bent round stock. remember that the smallest reading you can obtain by moving the brass around will be the correct one.

not as easy to use as designated fixtures for this, but an inexpensive, accurate method for testing wall thickness. an added advantage to this will be the ability to test for unsafe thickness the full length of brass due to multiple reloadings. same will begin just ahead of base webbing due to brass flow from ignition pressures.

gunnie

 

[gunnie]

..."dunno LC spec for thickness variation, but wouldn't be very surprised to learn that 15 thousandths is within max run-out."...

i think i was way off with that statement. my thinking was untill the US purchased some barrett 82A1 rifles in 1990 for desert shield/storm, there was no real need for the 50BMG to be all that accurate, from a belt fed Ma Duece.

got two empties to test same theory. one was an LC 79 (pre 82A1) the other an LC 03 to cover the post 82A1 need for accuracy:

measured both with the transponder of an ultrasound thickness meter located in a holding fixture @ 5/8ths inch from base plate. turned both MANY times, recording all of the readings i got through entire circumference.

the LC 79 read 0.084-0.087---three thous. run out.

the LC 03 read 0.083-0.087---four thous. run out.

feel safe saying LC holds tighter tolerances for the 50BMG than i thought!!!

something very non typical with your failed case.

gunnie

 

[gb0399]

Maybe they should put a hole like that on this M4. Thats the bolt carrier peeled back. The round had a catastrophic failure. The upper and lower both bowed out. The floor plate of the mag blew out. The shooter was fine. Always wear your eye pro

 

[Slamfire]

Brass doesn't normally degrade over time, but I am unsure if the corrosive primers would degrade the brass (I think it might). Better send it to me for proper disposal.

Powder sure does and it releases nitric acid gas which will cause brass splits. Usually first in the case neck area. But this brass is not that old.

As for this 50 caliber case, brass flaws happen.

Shooting bud of mine, he was using new LC primed cases in his M1a. He pulled the 150 FMJ's, put in a new powder charge and match bullets. The case head on one developed a pin hole brass flaw in the unsupported case head. Blew his magazine out and split the stock.

Brass flaws happen and there is nothing you can do to prevent them.

Due to the unpredictability of everything, always wear shooting glasses. My glasses have saved my eyeballs a number of times.

 

[Zak Smith]

I got a DVD in the mail with the SEM/EDXA images from 1858 over the weekend. I'll get something posted up here soon.