unexpected neck tension question

[Hengehold]

I have noticed that in one of my rifles my fired case necks are not large enough to allow a bullet to slide freely through the case mouth. In other words, if I hold a bullet above the case mouth of a fired case and drop it, it will not pass through the case mouth opening and fall through to the bottom of the case. All of my other rifles allow a bullet to pass freely through a fired case mouth.

- I am reloading for a 6.5x284 with Lapua brass.
- brand new reamer used marked as .297 neck diameter.
- loaded round diameter is .294 which gives a total of 3 thousandths clearance in the neck or 1.5 thousandths clearance on each side of the case neck.

My initial conclusion is that my chamber neck is about 1-2 thousandths too tight and I will need to trim the necks on my brass. Has anyone else had this experience with an overall neck clearance of .003?

Thanks,

Trevor

 

[Catfish]

My first thought is that you have a doughnut forming at the bottom end of your case neck. It comes from brass flowing down from the shoulder of the case. You will need to get an inside neck turner to get rid of the doughnut.

 

[CaptainMal]

So??? That reads to me like the neck portion of the chamber is rather tight to the case. On firing it expands out a little to free the bullet and then springs back for extraction.

If you get decent accuracy and no pressure signs I read it as a good thing. You are minimally working the brass.

 

[dedogs]

So??? That reads to me like the neck portion of the chamber is rather tight to the case. On firing it expands out a little to free the bullet and then springs back for extraction.

If you get decent accuracy and no pressure signs I read it as a good thing. You are minimally working the brass.

+1 Captain!

 

[potatoe]

Both what catfish and Mal said. If you bullet doesn't slide at all than that's not really an issue as long as you trust you loaded diameter measurement and really have a few thou clearance. But if the bullet slides down to the neck shoulder junction and stops than you have a doughnut. That might also not be a problem if your seated round's bearing surface is past that point but if your doing mag length or short throat etc. and seating the bullet's bearing surface past that point then you have to ream it out.

 

[Dshooter]

I have experience with this...take your calipers and check the opening. Then slide your ID measuring side of the calipers in the mouth past the opening to where your measuring the case neck in past the very opening. My 300 ultra was the same way. The immediate opening on fired brass measured .307. In past the opening it measured .311. The brass grew forward to the edge of the chamber and was curling the mouth over enough to close the brass. My chamber was cut with a .343 neck so it had .005 clearance on a loaded round. And you could visually see it on mouth opening. If yours measures tight farther inside the mouth, then you have a tight chamber and need to turn brass if your not comfortable with it. In my case, I sent the barrel to Rayhill to rechamber

 

[JohnKielly]

Occasionally crimping at the neck mouth can be caused by cases being overlong for the chamber. Might be worth checking, though the circumstance mentioned previously is more likely.

 

[Dusty Stevens]

Skim turn your necks til the bullet slips in. If its goin in then hitting the donut thats a different story but if theyre not even starting just skim turn til they do.

 

[chevytruck_83]

Does this happen on the first firing or does it get worse the more times you fire a case? I think you can attribute it to the brass flowing forward if they get tighter each firing. My first concern would be pressure spikes if the brass isn't able to expand enough to let the bullet out freely. I imagine if you put a round in the chamber and push it forward lightly with say a cleaning rod or something and see if it drops out or if it gets stuck that will let you know as well. Best of luck.

 

[spclark]

Trevor I've seen that on my Holliger-sourced service rifle cases. Other chamberings I'm inclined to use turned brass & haven't seen it.

Whose 6.5-284 chamber profile you using? There are many... some may be a tad short in neck length but I doubt that's your problem. Typically chamber necks are cut around 0.015" - 0.020" longer than max. case length to be used, but with the gauge (case plug) Sinclair sells it's easy to check.

Otherwise I'd think either 0.004" clearance (one of my 6.5-284's has 0.300" neck, fine with un-turned brass) may be better, or skim-cutting case necks so there's a little more breathing room for when things get rolling.

BTW I hope you're well! Missed seeing you again at Winnequah last year, glad to see you're still 'in the game.' You were right several years ago, I finally cleaned 900 in Palma last year for the first time there. Did it more than once too, one time at Perry for the Worlds!

 

[Hengehold]

I have experience with this...take your calipers and check the opening. Then slide your ID measuring side of the calipers in the mouth past the opening to where your measuring the case neck in past the very opening. My 300 ultra was the same way. The immediate opening on fired brass measured .307. In past the opening it measured .311. The brass grew forward to the edge of the chamber and was curling the mouth over enough to close the brass. My chamber was cut with a .343 neck so it had .005 clearance on a loaded round. And you could visually see it on mouth opening. If yours measures tight farther inside the mouth, then you have a tight chamber and need to turn brass if your not comfortable with it. In my case, I sent the barrel to Rayhill to rechamber

Your experience may also be my issue. My load manuals are a little dated and do not have 6.5x284 load info. Does anybody out there have the max case length and trim to length for the 6.5x284? Since I was loading virgin Lapua brass I presumed that my case length was in the safe zone but perhaps it was not.

Thanks,
Trevor

 

[lpreddick]

What does a fired case neck diameter measure? It should be .001-.002 greater than loaded neck. If so, you are safe. There is .0005-.001 spring back from neck chamber diameter after being fired. Occ a case mouth can "auto crimp" itself after firing making bullet insertion snug. Mid neck diameter should be .001-.002 greater than loaded neck. Spinning a nylon brush in the neck gets the particulate material out and occ a boat tail will slide in snugly.

 

[Hengehold]

Does this happen on the first firing or does it get worse the more times you fire a case? I think you can attribute it to the brass flowing forward if they get tighter each firing. My first concern would be pressure spikes if the brass isn't able to expand enough to let the bullet out freely. I imagine if you put a round in the chamber and push it forward lightly with say a cleaning rod or something and see if it drops out or if it gets stuck that will let you know as well. Best of luck.

I started with Lapua virgin brass and have only fired it once. I wanted to identify this apparent problem before I try to fire another loading of the brass. The initial loading of the virgin brass was shot in Camp Perry Aug heat and did not experience any pressure signs. Thanks for the idea of testing to see if a loaded round will fall out of the chamber. I will have to give it a try.

-Trevor

 

[randy x]

I started with Lapua virgin brass and have only fired it once. I wanted to identify this apparent problem before I try to fire another loading of the brass. The initial loading of the virgin brass was shot in Camp Perry Aug heat and did not experience any pressure signs. Thanks for the idea of testing to see if a loaded round will fall out of the chamber. I will have to give it a try.

-Trevor

Carbon ring forming.

 

[Dshooter]

Trevor...trim length for your case is 2.160 according to Berger book. It probably won't have anything to do with your length because I'm sure the new lapua brass is within .002 of min trim length. And to lpreddick's comment; the fired case will actually be within .001 of the chamber neck. Has nothing to do with loaded round diameter. Regardless of what the measurement of the final loaded round, the brass will expand out and forward, hitting the neck wall and spring back within .001. On all of my chambers, I have .004-.005 neck clearance from loaded round. Once fired they are always .003-.004 more than loaded measurement.
In my case Trevor, I started turning necks on the Ultra and now the mouth is .309 which is enough to get a bullet in a fired case ever so slightly. I wasn't comfortable with that because I have a .343 neck and loaded rounds before turning were .338... .005 clearance. That's plenty. Now I have right around. 0065 clearance after turning and I don't like over working necks. Not to mention that's just putting a band aide on the problem. Brass still flows far forward and curls at the mouth. I just opted to cut a new chamber, as this one wasn't done correct in the first place(not enough freebore and had chatter marks on brass from smiths crappy cut) best of luck to you

 

[Dshooter]

Carbon ring forming.

Could be on to something here too...

 

[Hengehold]

Trevor...trim length for your case is 2.160 according to Berger book. It probably won't have anything to do with your length because I'm sure the new lapua brass is within .002 of min trim length. And to lpreddick's comment; the fired case will actually be within .001 of the chamber neck. Has nothing to do with loaded round diameter. Regardless of what the measurement of the final loaded round, the brass will expand out and forward, hitting the neck wall and spring back within .001. On all of my chambers, I have .004-.005 neck clearance from loaded round. Once fired they are always .003-.004 more than loaded measurement.
In my case Trevor, I started turning necks on the Ultra and now the mouth is .309 which is enough to get a bullet in a fired case ever so slightly. I wasn't comfortable with that because I have a .343 neck and loaded rounds before turning were .338... .005 clearance. That's plenty. Now I have right around. 0065 clearance after turning and I don't like over working necks. Not to mention that's just putting a band aide on the problem. Brass still flows far forward and curls at the mouth. I just opted to cut a new chamber, as this one wasn't done correct in the first place(not enough freebore and had chatter marks on brass from smiths crappy cut) best of luck to you

When you say trim length is 2.160, is that the minimum trim to length or the maximum cartridge case length?

Thanks for sharing your experiences.

 

[Hengehold]

My first thought is that you have a doughnut forming at the bottom end of your case neck. It comes from brass flowing down from the shoulder of the case. You will need to get an inside neck turner to get rid of the doughnut.

The bullet will not even begin to enter the case mouth which leads me to believe one of two things are probably happening.

1. the brass is not expanding far enough to the chamber to allow for a bullet to pass freely.

2. OAL of the Brass is beyond edge of chamber and possibly crimping on the rifling.

 

[spclark]

Cartridge dimensions posted here show 2.1701" as maximum.

Minimum's anything you think you can get away with though habitually trimming cases short can lead to problems later on if you start using longer (as in new) cases. Chamber throats get residue build-up just ahead of neck mouths; short cases leave this where longer cases then encounter it, invariably leading to pressure issues from neck clearance being reduced.

If you know what reamer was used to chamber that barrel you ought to be able to locate its dimensions. Otherwise it's fairly easy to make a casting using Cerrosafe (low MP alloy that permits taking accurate dimensions for a short period after cast is removed from chamber) that will tell you straight away what you're dealing with.

 

[spclark]

The bullet will not even begin to enter the case mouth which leads me to believe one of two things are probably happening.

1. the brass is not expanding far enough to the chamber to allow for a bullet to pass freely.

2. OAL of the Brass is beyond edge of chamber and possibly crimping on the rifling.

Maybe, maybe not.

1. Possible, particularly with minimal neck clearance. Case necks may seal effectively but still leave inadequate initial clearance for propellant gasses. Brass will spring back a bit leaving you with a reverse bell-mouth shape. Like I said I've seen this in 223 cases out of my service rifle.

2. You need one of these. Along with a sacrificial case into which one is inserted, it helps you measure where your chamber neck ends and the freebore begins. Once you know this dimension it's easy to keep your brass trimmed back enough for safe clearance.