Some brass chambers hard, some doesnt- whats going on?

[mattri]

Riffle is a PTA with a CBI 6BR. Brass is Lapua.

New, unfired cases chambered easily. All new cases were trimmed for uniformity and chamfered before loading.

After initial firing cases were sized with Redding die to resize neck and bump the shoulder .002", cases were sized, trimmed and chamfered.

On 2nd firing I noticed that the vast majority of cases chambered properly, but a few seemed "tight", the bolt closed ok but seemed to have an undue amount of friction.

It was only on a few cases, thought maybe it was me. Fired all cases and formed/trimmed/chamfered again.

This time I seemed to notice some slight extraction marks on a few cases, just a hint of a rubbed look.

Sized/trimmed/chamfered again, rechecked all measurements- COL, length to shoulder, inner and outer neck diameter, body diameter, rim thickness- everything matched.

Chambered all the empty cases, a few were definitely tight. Triple checked measurements, then completely blacked out a couple of cases that chambered well, and a couple that were tight.

After extraction the only difference I could see was an extraction rub on the ones that were tight. If the overall length and length to shoulder bump are the same, where is this coming from?

[redlabel]

you might need a small base die

[mattri]

What would that change?

[olddav]

Are the tight cases egg shaped or arched?
I had three cases that appeared to have an arc in them, very minor one but enough to cause a problem. Resizing the brass would not fix them, everything would measure fine but problem persisted. Upon inspecting the case I saw that the line (left by the sizing die) extended further down on one side of the case. I inserted the case into a Wilson headspace gauge and it would not fit unless it was rotated into the same position each time. I assumed my problem was a flaw in the brass not the chamber.

[rbp75503]

It wasn't clear in your discussion whether you used a neck sizing die only to resize your brass. If so, you may need to use a full length neck bushing die or body die then follow up with your neck bushing die.

Typically RCBS recommends using small base dies in ARs, pumps, and lever actions to make sure the cartridge fully goes into battery. The small base die will bump the shoulder .003" more than the standard die.

[mattri]

Thanks for the replies.

If the die didn't size properly wouldn't that show up in the measurements?

[jonbearman]

What load are you using?

[olddav]

It didn't show up with mine, a least not in a way I could find.

[Hotolds442]

1.1495" is the max dimension from the datum line to the cartridge base. The datum line is at .375" diameter on the shoulder. Drill out a Hornady comparator bushing to .375 (3/8") and take your measurements there on your freshly sized cases.
If you're a bit long in that measurement, you may have to hit the face of your shellholder with some sandpaper and a piece of glass to remove some metal. I had to do this on my 243 AI to get it set up properly. It's a lot easier to remove from the shellholder than the die face. All of my cases were .001 long with the die hard on the shellholder. I'm not sure why yours are just a select few though.

[mattri]

Measurements to the datum point are all the same, that's what doesn't make sense.

Load is 28gr of H4895 with 105gr A-Maxs and CCI BR-4s.

[sharpshooter]

Measure your case heads with a micrometer, not a caliper. A micrometer will give you a truer reading. Make sure when you size the case you are consistent with case lube.

[mattri]

Will do thanks.

Have a full length die on order but hate to just swap parts without finding the actual cause.

[390fe]

After looking at your brass yesterday and doing some thinking, I suspect you have a tight match chamber and your brass is just right there - know what I mean?

How does your fired brass measure compared to the SAMMI blueprint?


http://www.6mmbr.com/cartridgediagrams.html

[redlabel]

If the chamber is a tight match chamber the small base die will size it a little smaller at the base.

[mattri]

Good news and bad:

The full length sizing die "fixed" the problem, cases chamber uniformly now. Shoulder bump is consistent, accuracy is the same.

Never found a measurable difference in cases.

[390fe]

Full length resized cases don't measure any different then fresh fired cases?

In any case, the important thing is that everything is consistant now, and you still have the accuracy.

[mattri]

Jason- that's a good question. At this point I haven't had the time to fire, measure, form (neck and fl), measure, compare and evaluate.

Hopefully over time I'll get to the bottom of this, something has to add up.

Side note, stay off the range since they burned, it's amazingly slick, buried the Ford and had to be pulled out.

[Three44s]

It's just the way it is when you are neck sizing .... your brass grows and you have to knuckle down and partially or completely FL size once in a while ...... with a tight match chamber, things get tight quicker ..... and whether you can measure it or not ....... your FL die tells you so.

Three 44s

[gotcha]

Your problem could just as well been caused by an incorrect or poorly machined shell holder. Can't say you'd be the 1st guy this happened to................... including me, that would be at least 2 of us. :)

[390fe]

Side note, stay off the range since they burned, it's amazingly slick, buried the Ford and had to be pulled out.

Gotcha - plan on some range time this weekend, if I need to place targets, I'll plan on walking.