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Bullet Not Passing Through Fired Brass necks Until 3rd Shot

13 Oct 2021
@ 11:07 am (GMT)

Mark Barrett


I read in Nathans reloading book, page 258, paragraph 4: "Thick necks can cause problems by trapping projectiles, resulting in pressure spikes and destroying accuracy. To check whether this is effecting your rifle take a fired case and try to pass a projectile through the case mouth. The projectile should pass relatively easily."

After firing my new 7mm-08 barrel the fired brass does not allow the bullet to pass thru until the 3rd shot. This happens after I have thoroughly cleaned the rifle copper free and polished the throat with a maroon poly, followed with the pad coated with autosol metal polish. The minimum SAMMI chamber specification is has a neck diameter of 0.316" and my loaded ammunition neck diameter is 0.309". The rifle is fairly accurate with 3 shot groups averaging about 0.65 inches with an occasional flyer about every 3rd group that opens the average groups to about 1".

Fired brass outside neck diameter is 0.315, which from reading countless internet gun blogs on this subject indicates that the chamber is cut to about .0.316' as the spring back of the fired round should be about 0,001".

So it seems that from 0.309" unfired loaded ammo neck to 0.315" is quite a lot of room for the bullet to not be trapping the projectile. Am I overlooking something? It seems that the necks should have 0.007 thou to expand to meet the neck chamber wall.

I shoot with a lot of experienced shooters and an old high reputation barrel maker that agree that a bullet should pass easily though a fired case neck. However in a famous bench rest shooter, Tony Boyer's: The Book of Rifle Accuracy, page 149 paragraph 1 states: After you have fired cases 3 to 5 times, you should be able to insert a bullet into them before sizing. If your necks are too thick, this will probably not work." He is using bench rest tight chambers with about 1 or 2 thou clearance between the loaded cartridge and chamber neck, which may be the reason it takes 3 to 5 shots to pass a bullet through a fired case neck.

I have the tools to turn the necks and kind of like doing it as it makes me feel like a pro, but I don't want to be a phantom fiddler and potentially f-up the brass.

Any insights into this topic would be greatly appreciated.

Thank you,
Mark Barrett


13 Oct 2021
@ 12:32 pm (GMT)

Nathan Foster

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
Hi Mark, good observations on your part.

From what you describe, the neck of the chamber is not caked or problematic, even though it is at minimum spec.

I believe what you are looking at here, is possible excessive neck tension and a lot of spring back, up until the third shot. I can only guess by internet though, so take that as you will.

A typical U.S 7mm cartridge will run around .312 to .313".

At .309, we are talking either a thin wall case or quite a bit of tension.

I also don't know anything about your loads or pressures. The loads could simply be very mild, again not fully obturating - again, only me guessing.

Its always such a ball of wax and advice is only as good as the information I have to work with. This may simply be a situation of a soft load that needs a wake up and some welly behind it. This wouldn't be the first time with regards to 7mm08 load data in general.

Yes so true about phantom fiddling. A lot of F-Class stuff going around at the moment which just confuses things.
14 Oct 2021
@ 09:02 am (GMT)

Mark Barrett

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
Nathan, Thanks for taking the time to respond with what I'd imagine to be a heavy work load for you.

The neck thickness is 0.13 so that was a spot on conclusion on thin neck brass.

I had purchased 250 pieces of brass from Starling so that's all I've been playing with. The barrel is a McGowan Remage and I suspect they must have used a tight reamer, but I have not cast the chamber and measured it. It is a slow fouler and I like it.

I sized some 308 Lapua brass yesterday and loaded dimension is 0.315. I'm using Trail Boss to fire form, but I suspect that may be a bit tight, though in spec. If it doesn't chamber I'll know. Maybe I need to fire it with a string from a safe distance.

The Lee neck sizer mandrel is 0.2815". It may be a bit tight, but I tried a custom 0.283" Lee mandrel and I could almost hand seat the bullets. So I think I'll try something around 0.282".

Thanks again for the feedback and giving me some things to think about.

14 Oct 2021
@ 10:20 am (GMT)

Martin Taylor

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
Mark l also sized down some new 308w Lapua brass but to 260rem in three steps. (At the time l just could not purchase 260 brass).

308w with no expander.
7mm08 again with no expander.
Then finally my Redding 260 Die.

Anyway, the neck wall thickness was to great as the sizing down bunched up the brass, even after doing it in small steps. I don't remember my measurements and its 6.5mm compared to your 7mm. Neck turning was a must for my application to gain the clearance needed.

So just wanted to offer this as a warning for anyone who may be resizing brass. It pays to know your reamer cut (mines a tight neck) and the Smith who did mine laser etched it on the barrel next to the calibre , which l think is a great Idea!
15 Oct 2021
@ 08:33 am (GMT)

Nathan Foster

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
Again, good observations on your part Mark and thank you also to Marty for your input. Yes, some Lapua brass is quite thick and also some Norma of late. As of 2021, Lapua are now producing commercial .300 Win Mag brass. There has been some excitement from shooters surrounding this. When asked about it, I have been advising guys to check the necks.

Ideally, it is good to have .003" or more neck clearance in the chamber. If using a suppressor, .004" can prove a better minimum.
17 Oct 2021
@ 07:28 am (GMT)

Nathan Foster

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
I just want to add for clarity, although it is common to see rather generous chamber neck dimensions for bottle neck rifle cartridges, the same is not true for rimless handgun cartridges. In the 9mm and .45 ACP for example, the case headspaces at the mouth due to the fact that the case has no rim or shoulder to arrest forward movement of the case during ignition.

The SAAMI spec for minimum chamber diameter is generally .001" wider than a loaded rimless case at the mouth. Max chamber is .005" wider than the loaded rimless case at the mouth (or .004" wider than min chamber). In practice, one may indeed come across a chamber that is only .001 or .002" wider than the loaded case. After firing a cartridge in such a chamber, a projectile cannot be passed (in some instances cannot even be started) into the case neck.

These are quite tight tolerances with regards to mass production. Both gun and ammunition manufacturers must be able to keep within this tolerance range.

Hope that helps paint a wider picture.
19 Oct 2021
@ 07:43 am (GMT)

Mark Barrett

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
Thank you Nathan and Martin.

1. I'm going to cast the chamber before shooting the necked down Lapua brass.

2. I will reduce my 0.283 oversized mandrel diameter to 0.282" and continue to investigate neck tension.

This is such a cool forum!

19 Oct 2021
@ 11:14 am (GMT)

Mark Barrett

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
Chamber cast 0.317" at front of neck and 0.318" just before shoulder.
Used Cerrosafe equivalent. Good stuff. Note to self: Should have done that before assembly of barrel to action.
27 Oct 2021
@ 08:46 am (GMT)

David Lenzi

Re: Clearance with suppressors
Nathan, et al,

Just got my first bolt gun suppressor... hadn't thought about neck clearance at all... what makes that additional thou desirable when running one?

I use a FL die with an expander ball to set neck tension... hadn't really thought about clearance, TBH. Guess I need to go re-read the Handloading Guide... good thing it's on the shelf.
27 Oct 2021
@ 10:05 am (GMT)

Martin Taylor

Re: Bullet Not Passing Through Fired Brass necks Until 3rd Shot
David the suppressor holds back some blast pressure which forces carbon back into the chamber/action. Overtime the build up closes the "gap" & why a good cleaning rescheme is even more critical.


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