Is Neck Clearance a Safety Issue?

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  • Last Post 05 January 2016
John Alexander posted this 02 January 2016

      In the thread “Revolver Question” gpidaho asked if .001” clearance between the loaded round and the neck area of the chamber in a revolver was enough to provide enough expansion for safety when using cast bullets. This is a reasonable worry given the conventional gun lore about the danger of the neck of a case restrained by the chamber holding on to the bullet and causing a pressure spike.  I can believe that in some extreme cases as when the jacketed bullet is bound to the neck by dried sealer or some kind of chemical bond, the force required to break that bond might cause higher pressure. It seems like Hatcher had something to say about this but my copy of his Notebook is packed for shipment and I'm not sure in which box.

    However, I have often wondered if there is any real danger of this happening with a cast bullet and it seems doubtful to me.  Even if the neck of the cartridge had no room for expansion at all the slick lead bullet shouldn't have any more resistance to being pushed out of the case by the expanding gas than it took to push in in (seat the bullet.) This force needed to push or pull the bullet out of the neck is tiny compared to the force needed to accelerate the bullet to muzzle velocity (perhaps 50 pounds vs. several thousand pounds) and given the variation in chamber pressure, the force needed to overcome friction between the case neck and bullet must be in the round off error.  Do we have any evidence to the contrary?  By this I mean pressure test data not just some expert's opinion about what might have blown up a gun after the fact.     If someone equipped to measure chamber pressure has observed a pressure spike when there is no clearance between the neck of the cartridge with a cast bullet and chamber that would be convincing.  Does anybody know of such a test producing such a result?  Unless there have been such tests maybe we should question this bit of conventional wisdom.

John 

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gpidaho posted this 02 January 2016

Thanks for your thoughts on this John. I too have doubts (no proof yet, lol) that gun damaging pressure would occur when firing a CAST bullet out of a tight chamber, In the “seems to me” line of thought the pressures involved in shooting cast bullets (malleable little wads of bubble gum that they are) that even wedged tightly in the pipe they would squirt out just fine. What all this abuse would do for on target performance is a whole different story. That said, Always best to not push luck or poke the good idea fairy. Gp

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billwnr posted this 02 January 2016

I know that in jacketed rifle rounds that if the neck becomes too long it affects the grouping. Years ago I had a rifle that would chamber with resistance and also “two group” in that it put two rounds in one group and three in another right next to each other.

I trimmed the necks after figuring out what the problem was and the chambering went back to easy and the “two grouping” disappeared.

Same issues should exist with too long of necks and cast bullets. To add to the safety issue in question, if a Heavy class shooter were able to chamber a loaded round that exceeded the diameter of the tight chamber and fired it I would think a high pressure situation would arise.

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Ed Harris posted this 02 January 2016

I have experienced blown primers in tight necked .30-'06 loads in which I ignored hard bolt closure and the oversized bullets ripped the necks off the cases. Gun was a Winchester Model 70 and the primer pockets of the Lake City brass expanded to .215” similar to a proof load.

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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Scearcy posted this 02 January 2016

The 3006 has a .003 tighter neck than that of the 308 (Saami specs) With Lapua or Lake City brass the fit could get tight with bullets sized 312 or so. Winchester brass would likely give .003 or .004 more clearance. I have never worried about it as I have used Winchester brass in an 03 Springfield barrel. This year I am shooting a commercial chambered 3006. I think I'll be a little more careful.

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John Alexander posted this 02 January 2016

Ed Harris wrote: I have experienced blown primers in tight necked .30-'06 loads in which I ignored hard bolt closure and the oversized bullets ripped the necks off the cases. Gun was a Winchester Model 70 and the primer pockets of the Lake City brass expanded to .215” similar to a proof load. Ed,

That sounds like pretty strong evidence.  I assume you are talking about a cast bullet.

I wonder what the mechanism is even when the neck squeezes the lead bullet a thousandth or two to make it so hard to expel?

John

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Ed Harris posted this 03 January 2016

Yes, I was shooting cast #314299 with 21 grains of SR4759 with Federal 210 primers.

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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Tom Acheson posted this 03 January 2016

I was shooting the exact same load Ed mentions above in August 1986 in a “sporterized” 1903. The results are for another story, another day.

A “newer story". Last CBA match of the year, 2011, using my XP-100 chambered in 30 PPC. The gunsmith who assembled it recommended a inside neck clearance on each side of the loaded round of 0.001". The necks had to be turned (bumped 175-grain LBT bullets are 0.311” dia.) so I did what I had done on previous chambering projects with this gun....0.0005” clearance per side. I shot the same 200 cases in that gun in 30 PPC 2005-2011. Even set all of my CBA NR's with it. Each case had been fired about 24+ times, modified Lapua 6 PPC.

Back to the last match in 2011, the last round shot that day. Ejected the fired case and put it in the box with the rest. Got home, put them into the vibratory case cleaner. Upon sorting after cleaning.....here is one case with no neck! A look in the gun's chamber and sure enough, there is the remains of the neck in the chamber....last shot of the day, what luck!

I'll leave it to the jury to decide.....neck clearance too tight? I don't know and won't speculate. But today I'm a believer in case annealing. These cases were never full length sized so case “working” was minimal and were never annealed. But 24 firings at about 1950-2100 fps may have finally taken its toll over time.

Tom

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Ed Harris posted this 03 January 2016

I also shot the necks off some .30-30 brass shooting #314299 with a compressed caseful of W760 before I learned to turn the case necks. Frank Marshall was watching my 200-yard group formation in the spotting scope, and when I extracted the neckless case and looked at it, Frank laughed and signalled the line officer, “Ed Harris just DQ'd, his last shot had a jacket!"

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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John Alexander posted this 04 January 2016

OK. Shooting the necks off cases does sound like bad practice.  Almost all my shooting is with production rifles with twice as much neck clearance as I would like but if I start shooting my tight necked bench gun again I will make sure the loaded necks go into the check die cut with the same reamer.

Thanks for your experiences Ed.

John

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billwnr posted this 04 January 2016

I shot the necks off of a batch of 6PPC brass but it was used even before I got it.

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Ken Campbell Iowa posted this 05 January 2016

i have been thinking that wouldn't happen with heeled bullets .... as per my long-delayed 6 Super Rook .... i was going to chamber an interference neck .... and may still do so .... but i will certainly be checking every brass as it is extracted , for missing parts .

ken

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rockquarry posted this 05 January 2016

I've had to turn many case necks for rifle cast bullet loads, particularly in .30-30, .30-40, and .30-06. Most .30 caliber rifles will handle a .309” bullet without neck-turning, but quite often the procedure is required with .310” & .311” bullets.

The general rule-of-thumb for checking neck clearance using a bullet and fired case seems to apply here.

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Tom Acheson posted this 05 January 2016

The smiths who have done my barrel/chambering projects always have stamped the neck inside dia. on the outside of the barrel. Then its up to me to mic a loaded round and do the math to make sure there is clearance.

Tom

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