Neck diameter clearance on tight neck chamber

[JJMoody]

Bought a .308 necked 7 mag a while back from the late( here anyways),great @Remmy700 and I'm slowly putting together the necessaries for loading for it. Wanting to know best practices for bushing selection to give neck clearance needed in a tight chamber. All brass will be neck turned, of course. Then I'll be using either Whidden or Redding FL bushing dies, both of which are in hand. Needing help on where to go from here. Thanks

 

[MagMan]

I'd turn your brass first then worry about a bushing. In case you go to far. You'll probably be taking .004-.005 off the necks. If you end up at .307 with a loaded round I'd get a .304 & .303 bushing.

 

[J E Custom]

Bought a .308 necked 7 mag a while back from the late( here anyways),great @Remmy700 and I'm slowly putting together the necessaries for loading for it. Wanting to know best practices for bushing selection to give neck clearance needed in a tight chamber. All brass will be neck turned, of course. Then I'll be using either Whidden or Redding FL bushing dies, both of which are in hand. Needing help on where to go from here. Thanks

You need .003 to .004 thousandths for expansion to release the bullet correctly.

Check the neck diameter of a fired case in that chamber and then add the bullet diameter + the wall thickness of each side and you should have .003 to .004 difference. If there is less than .003 difference you will need to turn the necks more.

Once you have the chamber dimension, and the loaded dimension then reduce the neck by .0015 or .002 smaller than the total dimension of the bullet and the wall thickness x 2 for the bullet grip Example; ( .308 +.014 + .014 = .336 - .002 = .334) .334 should be the bushing size as long as the brass is annealed and does not have much spring back. You might have to go with a .333 bushing
to account for Spring back.

I Recommend removing the expander ball when using bushing dies to get the right sizing of the neck.

J E CUSTOM

 

[JJMoody]

Ok. That relates to another part of my question. What's optimum clearance? Is .001 enough or should I be shooting for more?... or is this a scenario where I'm not smart enough to have the conversation....the rifle will most likely not be a hunting rifle.

 

[JJMoody]

JE, I was typing while you answered

 

[JJMoody]

I do plan to remove expander balls. I am also wondering if there's any advantage to neck turning, using an expander mandrel and bushing dies all in conjunction with each other. Am I just duplicating processes for establishing neck tension? Also, while working this backward in my head, can you check my math??? starting with the .308 chamber, minus .003 min. for bullet release= .305 minus .284(bullet diameter) =.021 divide x2 = .0105 case neck thickness per side. Am I making sense, or causing confusion?

 

[J E Custom]

Ok. That relates to another part of my question. What's optimum clearance? Is .001 enough or should I be shooting for more?... or is this a scenario where I'm not smart enough to have the conversation....the rifle will most likely not be a hunting rifle.

I recommend .003 to .004 clearance to allow the case neck to cleanly release the bullet. On many rifles that are magnum cartridges to little clearance will produce unnecessary pressure. Smaller calibers have less trouble with this phenomenon.

The reason you want/need .003 minimum is that you already have several thousandths bullet grip and without one or more thousandths larger than the neck is sized, the neck really doesn't release the bullet enough. So when you add the .002 under size for bullet grip then add another =.001
you end up with .003 total effective clearance in order to release the bullet without adding more unnecessary pressure. That's why most like .004 (It gives you.002 thousandths total release) .001 thousandths on each side.

Some cartridges use .006 and dangerous game rifles will even go to .008 total clearance for dependability and chambering.

J E CUSTOM

 

[tim_w]

I recommend .003 to .004 clearance to allow the case neck to cleanly release the bullet. On many rifles that are magnum cartridges to little clearance will produce unnecessary pressure. Smaller calibers have less trouble with this phenomenon.



J E CUSTOM

What is your theory on why this is?

Thought maybe you noticed some correlations?

Had noticed that very tight necks on small vol cases seems to work best with seating into the lands.

 

[J E Custom]

What is your theory on why this is?

Thought maybe you noticed some correlations?

Had noticed that very tight necks on small vol cases seems to work best with seating into the lands.

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

 

[Mikecr]

A neck expanding so little as 0.000,000,040"(say ~10atoms) will FULLY RELEASE a bullet.
That is, the bullet is just swinging in space, with nothing holding it by then.
ANY neck clearance(which is not interference fit), is enough, and does not increase load pressure.

Next; You only get spring back for grip, and spring back is usually 0.0005-0.001 max.
Any amount of downsizing leaving interference beyond that is no more than excess, and doing nothing more for bullet grip.

JJMoody, you decide what neck thickness and clearance you want. Basically, what you're comfortable managing.
Necks around 12thou works, 1thou neck clearance works. That's just my baseline where I have control over it.

*0.308+0.024 = *0.332 loaded neck OD, subtract 2thou for 0.330 neck bushing, the necks will spring back to ~0.3305" OD. Mandrel neck expand for an interference ID of 0.3065 -0.3070.
*(measure actual bullet dia)
If you want to start at 1/2 tension, neck sizing length can be 1/2 seated bullet bearing length.
This because tension is NOT interference fit. Tension is spring back grip X area gripped (PSI).

 

[Elk Hunter 338]

Bought a .308 necked 7 mag a while back from the late( here anyways),great @Remmy700 and I'm slowly putting together the necessaries for loading for it. Wanting to know best practices for bushing selection to give neck clearance needed in a tight chamber. All brass will be neck turned, of course. Then I'll be using either Whidden or Redding FL bushing dies, both of which are in hand. Needing help on where to go from here. Thanks

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

 

[J E Custom]

Every single bit of this is dead wrong.

For one thing, a neck expanding so little as 0.0000000000001" will FULLY RELEASE a bullet.
That is, the bullet is just swinging in the wind, with nothing holding it by then.
ANY neck clearance(which is not interference fit), is enough, and does not increase load pressure.

Next; You only get spring back for grip, and spring back is usually 0.0005-0.001 max.
Any amount of downsizing leaving interference beyond that is no more than excess, and doing nothing more for bullet grip.

JJMoody, you decide what neck thickness and clearance you want. Basically, what you're comfortable managing.
Necks around 12thou works, 1thou neck clearance works. That's just my baseline where I have control over it.

*0.308+0.024 = *0.332 loaded neck OD, subtract 2thou for 0.330 neck bushing, the necks will spring back to ~0.3305" OD. Mandrel neck expand for an interference ID of 0.3065 -0.3070.
*(measure actual bullet dia)
If you want to start at 1/2 tension, neck sizing length can be 1/2 seated bullet bearing length.
This because tension is NOT interference fit. Tension is spring back grip X area gripped (PSI).

Sorry, I can't help anyone that believes that .001 thousandths is enough neck clearance for our type of shooting /cartridges. So you keep doing what you believe and I will keep doing things the way I have learned that not only work, they are safe and not hard on my equipment. And you are right about the bullet being released as long as the Id of the case neck is larger that the OD of the bullet. The problem lies with the pressure it creates.

I have shot cartridges that after you fired them you didn't have to size the case, but these cartridges were for precision shooting to 300 yards or less and powder charges were very small to prevent unwanted/needed pressure.

Sorry you don't agree but you are untitled to your opinion. and I am untitled to mine. so go for it.

J E CUSTOM

 

[Mikecr]

If I have an opinion it's excess this that & another is just that, and neither necessary nor 'good'.
Also, if I believed that tighter clearances are harder to manage, I wouldn't bring contrast to that.
But in reality, tighter is easier.

Keep in mind that the tighter you go, the straighter your ammo is made, so the easier it is to manage tight clearances.
The flip side holds just as true:
The looser your clearance, the tougher it is to make straight ammo, requiring looser clearances.
Closing the loop.

It is desirable to make straight ammo, not for safety, but so that you will not cause chambered tensions that affect results. With this, competitors have evolved to higher clearances, but not because they shoot better. It's really because they shoot with less problems, having more clearances -for their crooked ammo.
It's really no more than any issue leading to a bigger loop.

With a current gun I manage 0.0005" total neck clearance (0.00025" per side) with turned necks/clean chamber.
I've experimented way way tighter for testing, with no issue to load performance whatsoever. That's how I KNOW (not just believe, not opinion) that high clearances are not a functional requirement for small arms internal ballistics.
And logically, it makes no difference what cartridge we're talking about.
I developed loads with this, and I'm confident the gun shoots as well as anyone would hope for the cartridge.
In fact the entire chamber is fitted, and you know what's easier about it?
-No resizing needed
-No trimming needed
-No annealing needed
-Matched H20 capacities holding as such
-Tight as new primer pockets with over 50 reloads ->onward

 

[Varmint Hunter]

Super tight tolerances maybe be ok for expert reloaders who consistently recheck dimensions with the highest quality measuring equipment and use the most precise techniques, but that description probably wouldn't fit some of us.

I have been reloading for 40yrs. I own and use mid-level equipment; presses, dies, scales, cutting tools, gauges, etc. I have pretty much found a happy medium for what works for me and leaves me with a comfortable safety margin while producing sub 1/2" moa quite often. Remember - I'm an average reloading hobbyist and not a super-competitor.

I consider .003"-.004" neck clearance to leave some room for error (inconsistencies) while not degrading the type of accuracy that I desire. I use .002" (or slightly higher) neck tension to ensure a tight bullet grip which should aid in uniform combustion and is certainly enough to hold a bullet tightly even if rough handled or rapidly chambered.

Consistently shooting groups under 1/2 moa at 400yds from my "hunting" rifles makes me a happy camper and a more confident hunter.