Neck diameter clearance on tight neck chamber

[Rflshootr]

Now I'm curious........what size exactly is the neck in your chamber?

 

[JJMoody]

It's a .308 NK 7mm Rem. Mag

 

[25WSM]

I personally start at .003 and fire the case. If a bullet slides back in I call it good. If the bullet is too hard to put back in by hand I take more off my neck. Most cases are going to have a slip fit bullet around .003 to .004 which is exactly what most guys are saying they use. Problem with fitted brass that is .001 clearance is that every time you fire your brass a little bit of the shoulder flows into the neck. You would be neck turning your brass every 3 shots to maintain your clearance. I shoot 1000 yd comp and usually get 12 to 14 shots on each case. I still have to neck turn my brass again after 5 to 6 shots to maintain my clearance. Those necks do get thicker as you shoot them. At the 1000 yd range I don't know one person using fitted necks.
Shep

 

[Jud96]

@Mikecr how often do you anneal?

 

[JJMoody]

Hey JJMOODY look into some winchester brass first buddy off the top of my head most of mine measure .306" so you may not need to take off as much and honestly it will run with any of the new hotness just needs a little more work.. i always shoot for at minimum .004" clearence but like .0055" better .. all my
7mags have liked .001" neck tension

Already got me 100 pcs of ADG 7 mag. Thanks for the info though

 

[25WSM]

I anneal every 2 firings.
Shep

 

[25WSM]

I trim and check neck thickness every 2 firings also. Remember this is what I do to shoot small at 1000 yards. Most can do this every 5 firings.
Shep

 

[Rflshootr]

Most bench rest guys use .002-.003 clearance with no problem. Saying that, most also have the throat cut so that the base of the bullet is above the neck/shoulder junction to avoid the doughnut that can form in that area after repeated firings. Myself, I like .002 if I can get it, .003 tops. The more clearance that exits, the more the brass is worked when it is resized which equals less case life. I would determine where the bullet you will be using will sit when touching the lands. If the base of the bullet is .040-.050 above the neck/shoulder junction, then .002 clearance is enough. This will also give you some room to tune the load if your rifle likes the bullets off the lands. And just as an added note, minimum full length size your brass to headspace off the shoulder and not the belt. This will also help increase brass life.

 

[JJMoody]

All good stuff here fellas. A bit more info on the rifle and it's use may be in order.
BAT HRPIC single shot action
Bartlein 9 twist ( maybe a straight cylinder, at least close to it!)
Manners E4 adjustable ( think e4, not totally sure)
.308 neck chamber as mentioned 7 mag
Huber 2 stage
Smithed by QMM
She's pretty heavy and I can't see packing it over my 28 nosler in the field, but I do want to get some serious trigger time behind it. Mostly target stuff, maybe competition, but honestly, time likely won't be there for serious competitive shooting. Just wanna be able to function reliably in modest environments and help me to really develop skills at the reloading bench.not worrying about barrel life, as she'll probably get turned into something more exotic after the barrels done.

 

[DUSTY NOGGIN]

you have a 308 necked 7mm rm chamber (( have you measured ?? ))

first things first , id measure to see exactly what your chamber measures -

but to assume you have and have determined the neck inner diameter to be .308

.308 neck chamber as mentioned 7 mag

magman caught the 28 caliber

if you gave that chamber .003 clearance .. leaves you with .305 left
305 - 284 = .021 divided by 2 + .0105 -- ( .303-304 bushing )

if you gave that chamber .001 clearance .. leaves you with .307 left
307 - 284 = .0213 divided by 2 + .0115 -- ( .305-306 bushing )

.0105 is a very thin neck !! but so is .0115

i would carefully inspect the once fired cases for a slight radius in the neck to shoulder junction... as that will be your area that will give a little leaway . when you make your cut you will want aas big of a transition angle from thick brass to the thin brass as you can get ( that radius is your as big as you can get measurement) , too sharp of a cut at the end of your turn can create a breaking/ shear point when the case expands to seal at the chamber ( that small radius in the corner will give you that )

safety aspect of owing that rifle ... id find a way to label, stamp, engrave ... ect. "TIGHT NECK ONLY" in that gun ... not for you but for the ones who dont know ( enter what if's here )

 

[JJMoody]

".308 NK" is etched on the cartridge label of the barrel.

 

[Mikecr]

@Mikecr how often do you anneal?

With cartridges re-formed I anneal as needed during new brass preps.
Down the road, with chambers not of my control & having more clearances, I'll eventually need to re-anneal a batch back to baseline. This, when it gets to be a battle to produce matching seating forces.
With this I don't anneal all that often. I also don't trim beyond initial. Much of this because I keep clearances tighter and NEVER FL size. I can do this because I PLAN to do this, with every choice along the way(From bullet choice, to cartridge choice, to brass plan, to reamer design, to custom dies).

On another matter, I do not have issues with donut rebuilding, and I do not ever have to re-turn necks.
Where folks get there, it's due to their choices and not something that just happens to everyone.
Also, the notions that bullets need to drop freely into fired necks holds no basis. Tight neck chambers and necks turned for lower clearance in them can leave necks springing back into slight interference for seating. These would normally be further neck sized before actual seating. It's perfectly fine and normal for a lot of folks. For a fitted neck chamber, the fired necks spring right back to set interference so that no neck sizing is ever required. Works great, and nothing about those necks will ever change(never needing annealing).

Now if you choose a hunting capacity cartridge with high body taper and low shoulder angles, you will have to FL size. That sizing will roll brass thick towards thin up cases into shoulders, neck shoulder junction, and necks. You will eventually be trimming brass away, affecting case capacities. But if you would make those choices, your pockets and cases probably won't be lasting long anyway.
In contrast, if your choices lead to something like a 6Dasher or a 6.5x55AI, and you choose tighter clearances and tolerances, there is potential to burn out many barrels with the same handful of brass.

When you think there is no choice, and your basis is no more than what you think everyone else is doing, it means you do not understand it yet.
So if you want to do something (maybe go right) that a mob would persecute you for (while going left), step back to the crossroad choice and reconsider the possibilities. After all, You can always catch up with the mob, as they're not going anywhere..

 

[tim_w]

What is your theory on why this is?

Thought maybe you noticed some correlations?

Not much theory, just experience. I Have experienced pressure when shooting cases that had thick necks and were not free to expand when fired because of the neck chamber. After turning (Reducing neck wall) pressure went away. I have also found that the large capacity wildcats are very sensitive to neck clearance and work best with .004 thousandths.

When necks are turned or clearance is .004 on new cases, Pressure is not a problem. Reduced loads help control pressures, but with the trend of shooting max loads this clearance and freebore will solve this pressure problem. Anything that restrains the bullet takes energy and that equates to more pressure.

This is not a personal like, it is a fact when shooting big powerful cartridges fast while maintaining reasonable (SAMMI) pressures. Smaller diameter projectiles are less effected but still need at least .003 clearance to optimize the velocity and pressure.

Seating bullets to the lands has it's place, but for velocity without excessive pressure you need freebore and neck clearance.

J E CUSTOM

Thanks. Makes sense. Never felt it was personal preference on your part, as what I have seen of your posts, you seem to do what works best based on results ⅝. Was only interested if you might have found the direct cause of the different effect which seems to be a combination of differences.

No question about bullet seating into vs jump. I only mentioned it as a majority of the cartidges and guns combo I ran super tight neck clearances it was for target shooting where into lands seating was SOP. I thought maybe its effect on the pressure curve or lower charge weight vs a jump somewhat masked any effect of the tighter neck clearance.

I run my larger vol cases with more typical .003-.004 clearance. I still design the reamer so I have to turn the necks to get that clearance as I prefer the consistency and characteristics of turned necks.

 

[JJMoody]

Little more research and phone call to QMM on the rifle has turned up a few things. Original load development by the smith did indeed utilize brass with necks turned to .0105/side and even 9 years after building it, his comments were ( not exact quotes)
I remember that Was an extremely accurate rifle.
Make sure you know what you're doing ( I don't ... Yet) safety issues otherwise.
Among other things in a very informative conversation.
I'm a little nervous and a lot excited at this point. Keep the info coming, I am listening!!