What Cartridge Dimension Determines Bullets' Jump to Rifling?

[fguffey]

Murf, you did exactly what a friend did years ago trying to disprove my test results.

I agree Murf, every time this story is told I keep insisting there is something missing in the sequence of events. No one ever ask "WHAT! is missing?".

I have killer firing pins. My firing pins crush the primer before the case, bullet and powder know their little buddy, the primer, has been hit.

F. Guffey

 

[murf]

i don't know what to tell you, bart b.. pulled the trigger on a fired .223 rem case last night, no setback. did the punch and hammer test on a fully sized 270 win case, no setback.

heck, i hit that primer hard enough to collapse the side wall of the primer.

murf

 

[gamestalker]

Ok Bart, I'm gonna test this theory. BTW, I'm saying that shoulders don't set back from the firing pin strike. I'm just curious, I've never confirmed nor tested for such. I'm gonna push a fired primer back out to normal pre fired condition, fill the cup with some JB Weld, then measure a piece of brass head to datum line shoulder before, and after firing pin strike.

My primary reason for jamming, is simply based on my experience with shoulder setback, in that it allows the shoulders to reach my target forming dimensions, that being within .0015" - .000" of zero fit tolerance to the chamber. If it works because I'm defeating the effect of the primer impact, or because it defeats extractor button pressure, or defeats some other element of physics involved, I don't know, it just works.

But now you've got the science part of me curious, so I'm gonna remove the extractor button to eliminate that variable, insert a piece of measure brass that fits at zero, and see how much set back the primer actually creates, if any.

GS

 

[Metal God]

Ok Bart, I'm gonna test this theory

Me to and I say it will set them back .

Ok I took two 308 LC-14 cases and measured head to datum ( no primer when measured ) . They both were exactly the length of my Forster GO gauge

GO gauge measurement


This case represents both cases


I then seated a live primer in each case then chambered and fire one each in my Ruger American and the other in a Savage Model 10

This is the measurement of each after the test and primers removed




Well I was wrong clearly my firing pins did not set back the shoulders .

 

[fguffey]

The shooter/reloader brought 20 cases from the firing range, 5 failed to fire, 15 fired. The 5 that failed to fire had more than one opportunity to fire in the owners new rifle, then the 5 cases were given additional opportunities in three other rifles. I measured the length of the failed to fire cases from the shoulder to the case head. They measured minimum length/full length size when compared with new over the counter new factory ammo. I was surprised the fired cases fit my chamber gages, my L.E. Wilson case gages, my 30/06 chambered new barrels and home made gages with slight thump pressure.

We pulled the bullets and primers, then we weighted everything, most impressive, then we installed the primers that had been struck at least 5 times back into the cases they were removed from. Then we chambered the cases in one of my rifles with a killer firing pin, all primers fired. We measured again, the primers did not protrude and the case did not shorten between the shoulder to the case head.

Like Murf, I have driven cases into chamber barrels to determine the difference in chamber size and case size. I failed to lube the case once. I have used a Dake Arbor press to press a case into a chamber to determine how wild the wildcat chamber was. I have used drifts to drive cases into a L.E. Wilson case gage to determine 'how they do it'. I stood the gage on a block of lead then placed a case into the gage and drove in in with a drift that was the same diameter as the case head. Do not forget to lube the case.

The datum in the Wilson case gage has a radius.

F. Guffey

 

[murf]

fguffey,

so, the force of the exploding primer compound won't affect the case shoulder either. so much for that theory!

thanks to you and metal god for the info.

murf

 

[Bart B.]

My tests involved a 25 pound firing pin spring, a 2.1 ounce firing pin traveling at least 65 mph on impact with the primer, .060" pin protrusion from bolt face on Federal and Winchester .308 Win. cases, for the most part. LC match cases had the least of all shoulder set back.

Weak springs, insufficient pin protrusion from bolt face and hard brass will minimize shoulder setback.

 

[fguffey]

My tests involved a 25 pound firing pin spring, a 2.1 ounce firing pin traveling at least 65 mph on impact with the primer, .060" pin protrusion from bolt face on Federal and Winchester .308 Win. cases, for the most part. LC match cases had the least of all shoulder set back.

Then there is that .7854 thing I use, my firing pin crushes the primer before the bullet, case and powder know their little buddy, the primer, has been hit.

the bullet, case and powder

Total weight.

F. Guffey