6.5x55 neck inconsistencies??

[kawicrash]

I'm having a bit of a strange situation with 6.5x55 cases fired in my Schultz & Larsen M/69. It is a Brno VZ24 receiver and Schultz & Larsen barrel.
Some of the fired cases won't accept a bullet, while others will.
Lapua cases are 2.155 in length, inside neck is .263 or .265. Outside neck .300 or .302. Hornady cases are 2.153 in length, inside neck .263 or .265. Outside neck .300 or .302. My measurements are with a caliper and micrometer.
Why would some be tighter than others? I cleaned the barrel and chamber very well in case there was carbon biuldup or ?? in the neck area of the chamber.
Any ideas why this may be happening?

 

[Buckmastr]

Have they all been fired the same number of times?

 

[bigedp51]

I use a Redding neck thickness gauge below to check the case neck thickness like pictured below.

I buy bulk once fired military Lake City 5.56 and 7.62 cases and the 7.62 cases have thicker necks than commercial .308 case necks.

My guess is the Schultz & Larsen barrel may have a snug neck diameter and your brass may need to have the necks turned to a thinner neck thickness.

The SAAMI chamber drawing shows the neck diameter to be .2972 to .2974.

Proper neck clearance
http://forum.accurateshooter.com/threads/proper-neck-clearance.3772273/

Neck Clearance
http://benchrest.com/showthread.php?92717-Neck-Clearance

On the flip side, Saddam Hussein cat preferred very tight neck clearance.

 

[Hitzy]

Your loads look a little mild, lots of carbon on the necks.

 

[kawicrash]

The soot was something I noticed as well. This was the second firing for the Hornady brass. When it was virgin the load was 45.4gr of Reloder 22 and a 140gr. Hornady SST bullet. The entire case was sooty, but I put that down to maybe the brass being a bit undersize, as I believe US manufactured brass tends to be a bit smaller than European spec. especially in the case head and rim where they use the standard .473 as opposed to .480, which is correct for the 6.5x55. The case head measures .475 after firing. I cleaned the chamber with a brass brush and Hoppe's in case there was a build-up of carbon or whatever after the first time out.
The second loading on the Hornady was 36.5gr. IMR4320 with Hornady 140 HPBT Match and Nosler 140 RDF. Not ideal I know, but I have lots of it, so I was experementing. There was no soot on the bodies on the second firing. It actually shot very well, groups are at 100 yards, this being only the second time I had her out.
The Lapua cases were 41gr. IMR 4350 with a Hornady 140 BTHP Match. I did not chronograph any of the loads.
I'm sure I can bump the loads up being that it's a nice matching VZ24 action, and can handle a heavier load than my 96's so maybe I'll try increasing slowly and see what happens.
I wondered if maybe the neck in the chamber could be a little short and was crimping the case mouth, but all cases are trimmed under max length for sure. I'm confident it's nit a 6.5 SKAN chamber either.

 

[vmax204]

Try a Lee collet die. It will give you uniform neck sizing

 

[Eagleye]

I suspect your load may be a bit light, causing the tight neck on some cases.
This would account for the larger amount of "soot" on the necks as well.

You mentioned earlier that your beginning load blackened the whole case.
This is not an "undersized brass" issue, it is a low pressure issue. Move up
in charge weight carefully. I believe your problem will disappear. Dave.

 

[bigedp51]

kawicrash

Place a straight edge on the base of the case centered on the primer and see if the primer is protruding above the base of the case.

If the primer is protruding it is a sure sign of low pressure as Eagleye stated above.

When the primer is protruding it means the chamber pressure was not great enough to push the case back and contact the bolt face.

Below when the chamber pressure is high enough the case is pushed back against the bolt face and reseats the primer flush with the base of the case.

All my 30-30 cases at the max chamber pressure of 38,000 cup or 42,000 psi have the primer protruding.

 

[kawicrash]

biged, you were right, there is a little primer protrusion on some of the cases, I'll try bumping up the load and see how it goes.
Thanks to all for your help.

 

[bigedp51]

biged, you were right, there is a little primer protrusion on some of the cases, I'll try bumping up the load and see how it goes.
Thanks to all for your help.

Do not forget that Eagleye was the first to state your loads were on the light side. All I did was tell you a way to confirm it by checking the primer protrusion.

My first guess was thick necks and Eagleye hit the nail on the head with low pressure.

When you make a workup load starting at the suggested start load and work up, the primers will protrude from the base of the case to approximately the midpoint of the workup load.

And by making this workup load you will learn more about your rifle, the chamber pressure and reading your brass for signs of low pressure, and then higher pressure.

Below these 7mm Mauser cases tell me two things,
1. The primers are protruding a good bit meaning the chamber pressure was low.
2. The amount the primers are protruding is how much shorter the case shoulder location is than the chambers shoulder. This can mean the case was made too short or the rifle might have a headspace problem. And at normal chamber pressure the case would have stretched as much as the primers are protruding. And this is what causes case head separations and proper fire forming is required to prevent the case stretching.