How to measure neck diameter in chamber.

Quick neck sealing is desirable. Consider the hole through which gases leave the case, and potential benefit of that hole being the same, at the same timing, every shot. IMO, a perfect setup seals immediately, causing no carbon soot on the outside of necks.
Diametric neck clearance is a contributor to carbon sooting, but so is chamber end clearance, attributes of bullet engraving, neck thickness/hardness, and overall pressure curve amplitude & timing.
With this, a sooting rule of thumb for neck clearance would often be precarious.

The notion that bullets should drop freely into fired necks stands without rational basis.
I have one chamber set at 1/2thou neck clearance. From this, bullets do not drop into fired necks. They have to be seated with a die,, so I don't size these necks at all,, and never need to anneal them.
Everything about it works just fine, and it allows me to dismiss a good bit of reloading folklores.
So basically you are saying, no neck expansion doesn't result in premature pressure?
What you are describing sounds like case mouths buried into a legit carbon ring.
 
The easiest way to measure your neck is straight after firing then adding .001” to determine ACTUAL chamber dimension.
Otherwise your reamer print will tell you.
All of my reamers are cut with .0035” on the neck, except those that are .338” or larger and they are cut with .004” clearance.

Also, there is a phenomenon that curls the case mouth inwards after firing a shot that can confuse you, putting a bullet into such a case after firing feels tight, but it is a deception. I cannot get a good photo to show this, but a lot of my rifles do this, my 264WM is a prime example.

Cheers.
 
Also, there is a phenomenon that curls the case mouth inwards after firing a shot that can confuse you, putting a bullet into such a case after firing feels tight, but it is a deception. I cannot get a good photo to show this, but a lot of my rifles do this, my 264WM is a prime example.
Good to hear this. I have a few rifles that do just that. For a while I thought there may be a problem.
 
So basically you are saying, no neck expansion doesn't result in premature pressure?
What you are describing sounds like case mouths buried into a legit carbon ring.
There is expansion: 1/2thou. There is no upsizing
W/regard to low -vs- high neck clearance, there is no ill pressure affect. No tension difference.
There is NO problem running low neck clearance with cases prepped for, and fire formed in, a chamber set up for this.
 
I wouldn't turn any brass until you know. You could open another can of worms by doing so. One is too much clearance results in carbon ring ring factory.
If this is a custom chamber, the smith will know the specs of the reamer used, could be a simple phone call-email.

Here is a pic of Alpha 6.5x47 brass necked down to 6mm, new chamber. A bullet will not go back in the case w/o force.
Yet if you look at the carbon ring on the necks, they are sealing off 2/3 of the way down, which IMO means that I am fine. If that ring was close to the case mouth, I'd be concerned.
View attachment 580310
Can you explain the below? I genuinely want to know . I’ve always been under the belief that it is more desirable to have less carbon buildup on the outside of the necks. Because to me, it meant that it sealed off in the chamber and instead of the gas is going back towards the shooter, there is more gas pressure to push the bullet out.

“Yet if you look at the carbon ring on the necks, they are sealing off 2/3 of the way down, which IMO means that I am fine. If that ring was close to the case mouth, I'd be concerned.”
 
Can you explain the below? I genuinely want to know . I’ve always been under the belief that it is more desirable to have less carbon buildup on the outside of the necks. Because to me, it meant that it sealed off in the chamber and instead of the gas is going back towards the shooter, there is more gas pressure to push the bullet out.

“Yet if you look at the carbon ring on the necks, they are sealing off 2/3 of the way down, which IMO means that I am fine. If that ring was close to the case mouth, I'd be concerned.”
Oh boy, by the carbon ring on the neck, IMO, it did seal off in the chamber. Now if carbon reaches the shoulder, issues arise, of course you don't want gas coming back at you. No indication that is happening on my brass.
One note, these were test rds, I shot more on Thursday with more powder, I bet the seal off happened sooner, not sure if I can sort through my brass pile to find out.

In a perfect world, we would have all our components ready, and make a dummy rd or 2, send to reamer making co and have a custom reamer made specifically for the dummy rd. Not always the case though, we use what reamer the smith has. I have had 2 custom reamers made, brass still had carbon on the necks.
Maybe I am wrong, but too little neck clearance can result in too much pressure.
 
Carbon ring, carbon on the outside of the case neck, Carbon ring, hard carbon at the front of the chamber or the entrance of the rifling,, or perhaps the ring inside of the case neck at the shoulder preventing consistent seating pressure. How much clearance after firing is needed for a bullet to drop into the case?
"I opened a can of worms and every worm was holding a can."
 
Maybe I am wrong, but too little neck clearance can result in too much pressure.
Pressure-wise, too little = interference
You should feel that with chambering of a round.

Think about it. The only thing gripping your bullets is neck springback force against seated bearing (neck tension).
If your bullet bearing measures 0.264000000" and the neck expands 0.000000001" the bullet is swingin in the wind at that point, with zero neck force holding it back. So ANY expansion releases bullets just the same.
I've tested this some and conversed with others who had as well. There is little to no pressure/MV difference.

Why & when would there be any clearance difference to MV?
With big neck and chamber end clearances carbon laden gasses can turn back around to get between the neck and chamber before sealing.
Carbon is then deposited in the chamber neck, and builds up (a ring).
At some point the carbon can build to cause interference fit with necks. This slows neck expansion to occur at a higher pressure point.
The bullet will still release, but pressure/MV goes up.

If there is significant area for the back-gas intrusion, it can prevent more case sealing(than just necks) and also contribute to higher SD.
MV can inconsistently go up or down from varying gasses between the case and chamber due to changing case expansion energies to the powder charge.

By tightening clearances to seal faster, you can mitigate back-gasses. This is easy to plan for & do.
You measure new necks and decide on trim length and thickness (as turned to), and fill out your reamer print.
You don't need ~8thou of neck clearance,, a couple thou is reasonable (even for factory ammo or terrible reloading).
You don't need some arbitrary trim length from a book, if it leaves you a mile from chamber end.
 
Shot a few today here’s the necks..
To me it looks like it’s sealing at neck/shoulder junction? Again loaded rd = .2935.. Directly after shooting = .2960.. 5 hrs after .2950.. So I’d say I have .0035 clearance? If it sprung back 1thou from exiting chamber.
 

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Shot a few today here’s the necks..
To me it looks like it’s sealing at neck/shoulder junction? Again loaded rd = .2935.. Directly after shooting = .2960.. 5 hrs after .2950.. So I’d say I have .0035 clearance? If it sprung back 1thou from exiting chamber.
Don't turn those necks. I'd add some powder if possible, if carbon builds up on the shoulder in your chamber, it will compound the situation.
 
Don't turn those necks. I'd add some powder if possible, if carbon builds up on the shoulder in your chamber, it will compound the situation.
Yeah that’s my thought after learning to look for “where it seals”.. can’t go up any in powder. I started load development 1st time about 6 months ago.. , starting w/ powder first. (Done this about 5 times now due to “learning after the fact”, I had brass from different manufacturers, bought new, learned I had to fire form first, chose to high of window/ earlier pressure than expected, etc haha. Its been a learning process.. I thought I had finally found the upper end of a powder node, (also same charge as builder chose) but went out today 85° to test seating depth and had pressure right away (heavy bolt) so I’m backing powder off a lil or may redo powder charge test again. It was 50° went I did powder test last 85 today. Cleaned chamber with rubbing alcohol before test cause I’ve had earlier pressure than most guys I read about. That’s kinda why I thought I had tight neck clearance.
6.5 Saum, ADG, cci 250, H1000 59.5gn. Defiance/22” proof. Barrel cleaned after every outing. 250 rds.
Thanks for everyone’s input. Lots to learn.
Or maybe that loads ok but the few shots could have been with brass that’s seen pressure before? 🤔
 
Yeah that’s my thought after learning to look for “where it seals”.. can’t go up any in powder. I started load development 1st time about 6 months ago.. , starting w/ powder first. (Done this about 5 times now due to “learning after the fact”, I had brass from different manufacturers, bought new, learned I had to fire form first, chose to high of window/ earlier pressure than expected, etc haha. Its been a learning process.. I thought I had finally found the upper end of a powder node, (also same charge as builder chose) but went out today 85° to test seating depth and had pressure right away (heavy bolt) so I’m backing powder off a lil or may redo powder charge test again. It was 50° went I did powder test last 85 today. Cleaned chamber with rubbing alcohol before test cause I’ve had earlier pressure than most guys I read about. That’s kinda why I thought I had tight neck clearance.
6.5 Saum, ADG, cci 250, H1000 59.5gn. Defiance/22” proof. Barrel cleaned after every outing. 250 rds.
Thanks for everyone’s input. Lots to learn.
Or maybe that loads ok but the few shots could have been with brass that’s seen pressure before? 🤔
Lot of questions.
What bullet are you using? Unless you are into 150gr class bullets, 59.5gr of H1K doesn't seem warm, but I will have to check in QL.
How far are you bumping brass back?
Do you have access to a borescope to look at the chamber?

Rubbing alcohol won't do jack as far as cleaning carbon out of anything rifle.
I'd start by cleaning the chamber and neck, and clean your bolt face. I'm not beyond chucking brushes up in a drill to clean necks with C4, or cutting scotchbrite in big enough circles and forcing them onto an oversize brush to clean the shoulder of a chamber, again with C4.

You need a fresh start
 

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