New 7mmMag project rifle..... Is .311" a good no-turn neck size?

[VaniB]

I need to have a 7mmMag reamer made, and I'm looking to have a tight chamber reamed for precision shooting, but I don't want to turn case necks.

It seems most loaded 7mmMag ammo measures at .307-.308 diameter. Would it be safe settle at say maybe .311? My smith has a reamer that has a .315 neck. That would likely be fine for a standard hunting rig, but seems needlessly large for precision style shooting.

 

[TonyR]

I shoot a 284 Shehane with a 0.310 loaded neck and a 0.313 chamber neck. Some very good shooters believe that more than 0.003 clearance would be better with a 7mm. I have to turn the brass to get to .310 with the Berger 180 VLD, from about 0.014 or 0.015 on a side to about 0.0127 per side. According to Quick Design, the SAMMI spec for the 7mm Rem Mag cartridge loaded neck is 0.315 and the chamber neck is 0.316.

 

[VaniB]

I shoot a 284 Shehane with a 0.310 loaded neck and a 0.313 chamber neck. Some very good shooters believe that more than 0.003 clearance would be better with a 7mm. I

Tony, did you mean to say "Some very good shooters believe that no more then .003 clearance would be better with a 7mm" ?

But, if precision shooters do think that more then .003 clearance would be better, would you or anybody else like to venture to guess how much I should go with and whether my smith's reamer with a .315 neck is good enough for precision shooting? I sure don't mind saving the cost of NOT having to buy a $150 reamer.

Thanks.

 

[280man]

If all the other deminisions of the reamer are to your liking I would go with it. I shoot a 284 with a .313 neck. With turned Lapua brass and Berger bullets my loaded rounds measure .309. I think .315 is right on for a no turn neck I would be more concerned with the throat and whether I could seat my bullets where I wanted or not.

 

[TonyR]

I mean more than .003 clearance and 280man's 0.309 is giving him 0.004 clearance. As 280man says, you also need to think about the throat with the 7mm if you are shooting the 180 grain bullets. The standard 284 OAL is 2.800 while mine are 3.200+ to keep the 180 VLD boat tail away from the donut. It would probably be a good idea to make up a dummy round seated the way you want the bullet you plan to use and let your smith take some measurements. My gunsmith has a throating reamer and he can give me a little extra freebore if I want.

 

[watercam]

It really depends upon the choice of brass. If one has cases with .015" neck wall thickness one then has bullet dia. + case neck wall thickness (x2 of course) + .003" clearance. For my 7mm Rem. mag that comes out to .317" chamber neck diameter for no turn chamber using Nosler brass.

 

[Stonewall]

Email Jim Borden on the 7mm Rem reamer specs subject.

He has written several articles in Precision Shooting one that I remember was about 7mm Rem reamers.

If you turn the necks on Winchester brass so that they clean up you have a neck diameter at .310 loaded or twelve and three Quarter thousands each side of the neck .


Glenn

 

[TonyR]

It can be useful to measure the diameter of the bullets you plan to use because I have found different diameter. The 162 A-Max has been very close to 0.284, the Bergers have been 0.2845 and the Cauteruccios have been a bit bigger than the Bergers depending on the lot.

 

[VaniB]

It can be useful to measure the diameter of the bullets you plan to use because I have found different diameter. The 162 A-Max has been very close to 0.284, the Bergers have been 0.2845 and the Cauteruccios have been a bit bigger than the Bergers depending on the lot.

I had obtained about 5 hand loaded 7mmMag cartridges at the public firing range which utilized once fired winchester brass and are measuring .307-.308 at the neck. I also obtained fired cases from the same person which is measuring .315 But, I was surprised to pull one of those cartridges apart tonight and discover the bullet is .283 (not .284) The fired case walls vary from case to case but for example, one of them measures .014 on one side and .015 on the opposite side. Sooo.... if you add .283 + .014 + .015 it equals .312 Yet, all the loaded cartriges measure .307 to .308

I don't know who or how these cartridges were loaded, or if it's normal to have this much tension so they measure a tight .307", Nor do I know if it is common to find .283 diameter bullets. (??) (yes, I'm using a quality Mitatoyo dial caliper)


I've decided to proceed and buy a Redding body die, a lee collet die, and the 162Amax bullets that I will try to use as my bullet of choice. I will make a couple of dummy cartridges and measure them myself. THEN.....I'll confidently consult Dave at Pacific and listen to what kind of reamer he advises I use to utilize my 300 once-fired Remington cases that I intend to hand load for precision paper punching.

 

[Kenny474]

To get reliable neck thickness you need a tubing micrometer. Calipers will not give you an accurate reading as they are not designed to measure a curved surface. You are getting bad measurements which is why the numbers don't add up.

Using your calipers, measure the thickness of a piece of .015 brass sheet, and then bend it around a pencil and measure it again gently so as not to flatten the radius of the bend . You will have 2 different numbers. Unless you flatten the metal the radius will not allow the calipers to sit flat and get a true measurement.

 

[TonyR]

There are also small ridges inside the neck of many cases that cause different reading with the tubing mic from place to place even with a turned neck unless you inside neck ream. Lot's of ways to get confusing numbers.

 

[Steve Blair]

My .284 Win has a .313 neck and .220 throat. I turn my necks to .0125 with Winchester brass. Performance with Berger 180 VLD and Hybrid bullets has been excellent.

I agree with Tony that you would need to be around .318-.319 to guarantee no problems without turning. Below image is from QuickLOAD.

 

[VaniB]

To get reliable neck thickness you need a tubing micrometer. Calipers will not give you an accurate reading as they are not designed to measure a curved surface.

Yes, I took note of this fact, and so I outside reamed and inside reamed the straightest looking necks and took a couple of dozen measurements with the caliper grabbing maybe only 1/16" of the neck. The constant measurements I got was .014 -.015 on more then one fired winchester case, moving it around the circumference of the case mouth. I agree with you that without a ball head michrometer this is still a half assed way of trying to get measurements.

I personally don't care to own a michrometer, and rely on two different Sinclair concentricity gages on my shelf used for each and every handload I make. First I measure the concentricity of the sized case, then in "assembly-line" fashion I move the completed cartridge to the other Sinclair gage to check the concentricity of the completed cartridges. Suspect/problem cases are culled and checked on a third gage; my Redding case neck gage to check for inconsistancies in case neck thickness. (the case is often discarded or used for "fouler shots") I also have the new red Hornady "Concentricity tool/fixer tool" which I will use on occassion to straighten inconcentric rounds.( I like this tool much less and don't depend on it to make top-notch rounds) I magic-mark completed cartridges by their uniformity and place them in marked spots in my plastic ammo box accordingly. The most concentric cartriges are the ones I reserve for my 5 shot 1/4" to 3/8" groups. Cartridges with concentricity readings of .001- .004 on the Sinclair gage are premium rounds. (as you know, a .004" reading on the Sinclair gage is actually a round with .002 inconcentricity.)

 

[VaniB]

My .284 Win has a .313 neck and .220 throat. I turn my necks to .0125 with Winchester brass. Performance with Berger 180 VLD and Hybrid bullets has been excellent.

I agree with Tony that you would need to be around .318-.319 to guarantee no problems without turning. Below image is from QuickLOAD.

Sleepy,
Thanks for that file photo. I was in the middle of posting while you were providing that post and file photo. I have come to believe that a .315 neck is standard SAAMI specs. If this is so, would it be safe to assume that most factory cases will size back to .315 or less using standard reloading dies and teqnique? ......and anybody using just about any set of standard 7mmMag dies and fired brass should be able to get their reloaded cartridges to fit in a factory rifle with this sized neck?

 

[wapiti25]

I'm in process of building a 7mm mag and the win brass that I have is marginal in a .315 neck, I am going to turn my necks so I get at least a partial clean-up and will be more uniform. I plan on a .315 neck. The brass has about .002 or .003 variance in neck thickness which I want to be less than .001. I would buy a lot (100 0r 200 cases) of brass that you plan to use and see what neck size you need. I have some rem brass that has thinner necks than the un-fired win which I plan on using.