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Bzone
Average Member
130 Posts |
 Posted - Aug 10 2012 : 12:14:57
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| Anyone know the neck wall clearance of a standard SAAMI spec chamber? |
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Bobo7mmmag
Advanced Member
2396 Posts |
Posted - Aug 10 2012 : 19:47:28
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| Go to saami.org/specs and look at the drawings. It's different for each caliber. |
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Bzone
Average Member
130 Posts |
Posted - Aug 13 2012 : 15:19:09
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| i dont understand the diagrams will try |
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Bad Flynch
Senior Member
USA
267 Posts |
Posted - Aug 13 2012 : 18:28:36
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If you have no training in reading the SAAMI drawing, that is OK. Not everyone does.
Neck clearance is relative to what you are using the cartridge for. For example: a military rifle. likely to shoot dirty ammo, will have coniderable clearance. Accurace is only expected to be a 12" cirle at 200 yeards, so the extra clearance does not matter much.
A benchrest rifle, meant for shooting groups in the 0.001" category, may only have 3/4 of a thoushandths on a side (0.00075"_ for a total clearance of 0.0015". Benchrest shooters may clean their chambers after each group of 5 shots so that carbon build-up does not affect ammo functioning. Benchrest shooters will clean their cases of soot and firing crud after every shot.
Its a relative thing. |
B.F. |
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Bobo7mmmag
Advanced Member
2396 Posts |
Posted - Aug 16 2012 : 17:28:33
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It is the neck diameter minus the bullet diameter divided by two.
7mm Rem Mag is .315" neck diameter. The bullet is .284"
.315 - .284 = .031" dividing by two equals .0155"
What Cal are you looking for info on?
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Bobo7mmmag
Advanced Member
2396 Posts |
Posted - Aug 16 2012 : 17:35:16
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It is the neck diameter minus the bullet diameter divided by two.
7mm Rem Mag is .315" neck diameter. The bullet is .284"
.315 - .284 = .031" dividing by two equals .0155"
What Cal are you looking for info on?
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Kansas Ed
Advanced Member
USA
1186 Posts |
Posted - Aug 16 2012 : 23:05:50
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What Flynch says is pretty accurate IME. Let me explain my take on SAAMI.
Quite a few years ago, I decided to play around with my own cartridge design. I knew what I wanted (short necked .224 caliber with a minimum body taper and 40 degree shoulder based on 25-35 brass. Very similar to the 22/30-30 AI but with a shorter neck.
Having access to the equipment at my place of employment, and the access to a Monoset to custom grind a chamber reamer I started in on the research. Using the SAAMI specs for 25-35 brass as a base, and chamber neck and base dimensions for the .219 Zipper I proceeded to cut a reamer and chamber a barrel.
After building the rifle, first firings proceeded to bulge the brass just forward of the web. Chamber dimensions at the base were verified per SAAMI drawings and tolerances. All was good. BUT...I ended up cutting an inch and a half off of the chamber and rethreading the barrel, rechambering to a dimension smaller than what was called out in SAAMI.
What I found was, that brass is manufactured to the lower end of SAAMI tolerance. Chamber dimensions for the base case were published by SAAMI beginning at the top end of the brass specs. So the tolerance overlap and stacking created a functionally oversized chamber.
IMO, the best thing you can do, is to build the cartridge first, Mic (no calipers, micrometers only) the neck dimensions based on your stock of brass which has been sized and dummy loaded, and cut the chamber to that....plus a little.
SAAMI is a general guideline IME and isn't a set in stone blueprint for success.
Ed |
Are we there yet??? |
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Bzone
Average Member
130 Posts |
Posted - Aug 22 2012 : 17:26:15
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| Sorry been away for so long. This is good info here. The specific caliber I am looking at is the 7 SAUM. I need a chamber reamer with no neck turn but not SAAMI spec as well. I have all parts for a build except for reamer. Still haven't talked to a smith yet. When hunting season is over my build will start. Any help on this topic is always a plus. |
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Shastaboat
Advanced Member
USA
2790 Posts |
Posted - Aug 22 2012 : 18:02:55
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| I like to outside turn all case necks to .010 to .013. I have one Ruger .243 that requires the .010 or pressure increases too much. I outside neck turn about every 3rd firing on this rifle. Other military chambers don't require me to turn necks at all but one 8x57 requires ne to turn brass made from 30-06 brass after every 4-5 loadings. No two chambers are identical. They are all individual to themself. SAMMI specs are a guide but any machinist will tell you there are acceptabe tolerances in all machine work. Might be only .0001 or less but tolerance measurements exist. |
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Bobo7mmmag
Advanced Member
2396 Posts |
Posted - Aug 25 2012 : 16:25:24
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From the drawing:
.322" at the shoulder junction
.284" deduct bullet diameter
.038" difference divided by two equals 0.019" case wall thickness.
I have a nephew and a friend of his that have a .300 Ultra and a .300 WSM and I have noticed that they have thicker than usual case walls. |
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Shastaboat
Advanced Member
USA
2790 Posts |
Posted - Aug 25 2012 : 20:16:16
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| Bobo, Why would you measure neck OD at the shoulder junction instead of the middle of the neck? |
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lightman
Advanced Member
1050 Posts |
Posted - Aug 26 2012 : 09:25:37
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| Bzone,I have used Pacific Tool and Gauge to supply reamers for my builds,and have been very happy with them.You can call them and speak with Dave Kiff,the owner.He can offer very good advice on most anything to do with reamers,chambers,or tooling.Tell him what you want to do and ask his opinion.There are other good reamers,but I like the fact that I can talk with the owner at PTG. Lightman |
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Bobo7mmmag
Advanced Member
2396 Posts |
Posted - Aug 26 2012 : 16:59:56
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Shastaboat
Off the drawing:
It's .321" at the front of the neck which averaging would give it .3215" at the center of the neck. |
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Topic |
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SAAMI neck wall clearance
