7 SAUM Brass Prep

[AngryDropbear]

Gday Guys,

Just got my Norma Brass for a new 7 saum brass. Intend to build a dummy round for the gunsmith to calculate free bore. Was thinking about getting my first bushing type FLD, but reading online says to insert a projectile first to get correct bushing. I can see some of the brass is a little dinged around the neck. Do I need to /should I run these through and expanding mandrel first and then seat a projectile to measure the bushing size?

[JezL]

Surely not all the necks are dinged up ... my recent shipment of the same Norma brass wasnt ... just find one that isnt, yeah ? ... or if your worried, just run them all through a 282 / 283 mandrel ... you would likely still get the same result if you didnt expand.
If your also getting a Bushing Die for your first time ... I would recommend getting a couple of different size bushings. -0.002", -0.003", -0.004" etc ... just to give flexibility in your neck tension etc.


I would typically run all mine through an expander to iron out the dings, but also to lighten the tension, as factory new cases often are a bit on the tight side.
I also expand to neck turn, but thats a different topic.

[AngryDropbear]

Maybe a follow on question, my gunsmiths reamer states 7RSAUM .312 NK .90 freebore. So when ordering bushings can I simply do a bit of math to figure out desired bushing size.

Norma wall thickness average 0.016.
Projectile .284
Reamer NK .312

.312 - .284 = .028 /2 = wall thickness .14 so I would need to neck turn to this diameter
.312 - .003 = .309 Size Bushing allowing for spring back approx .002 neck tension

[AngryDropbear]

Surely not all the necks are dinged up ... my recent shipment of the same Norma brass wasnt ... just find one that isnt, yeah ? ... or if your worried, just run them all through a 282 / 283 mandrel ... you would likely still get the same result if you didnt expand.
If your also getting a Bushing Die for your first time ... I would recommend getting a couple of different size bushings. -0.002", -0.003", -0.004" etc ... just to give flexibility in your neck tension etc.


I would typically run all mine through an expander to iron out the dings, but also to lighten the tension, as factory new cases often are a bit on the tight side.
I also expand to neck turn, but thats a different topic.

Not all the necks were dinged up but maybe 1 in 5 were. I normally wouldn’t worry as they would come out when I seat the first bullets but with a possible need to neck turn (see above) I am a little more concerned.

[JezL]

Hmmm ... unless im reading your comment wrong ... I dont think you are accounting for Chamber Clearance.

I run a 313 NK ... my Neck walls are 0.0125" ... this gives me 4-5 thou of chamber clearance.

If you have a 312 NK Chamber .. I would assume you need your brass to be 12.5 - 12 thou ... pending your preferred chamber clearance. I would want 4 thou clearance as a MINIMUM.

Lets say you want 0.005" chamber clearance ... This is purely what I would do ... not what you HAVE to do.

.312 - 0.005 = .307 loaded round
.307 - .284 = 0.023/2 = 0.0115" Neck wall thickness.

Bushing sizes of 305, 304, 303 would then be my choice

[AngryDropbear]

Hmmm ... unless im reading your comment wrong ... I dont think you are accounting for Chamber Clearance.

I run a 313 NK ... my Neck walls are 0.0125" ... this gives me 4-5 thou of chamber clearance.

If you have a 312 NK Chamber .. I would assume you need your brass to be 12.5 - 12 thou ... pending your preferred chamber clearance. I would want 4 thou clearance as a MINIMUM.

Lets say you want 0.005" chamber clearance ... This is purely what I would do ... not what you HAVE to do.

.312 - 0.005 = .307 loaded round
.307 - .284 = 0.023/2 = 0.0115" Neck wall thickness.

Bushing sizes of 305, 304, 303 would then be my choice

I was not accounting for chamber clearance, thank you so much for reminding me. Will grab those bushings for now.

[JezL]

No worries! Happy to help.

Also dont take my preference for Chamber Clearance ... Everyone has there own preference with clearance.
For the 7mm's ... its typically 4-6 thou clearance is preferable from what I see and hear.