Gunsmithing Belted Magnum Headspacing

[bronco9588]

Hi all,

I posted this conversation in the reloading section, but I think gun smiths might have a different opinion.

I have a 300 win mag. I purchased some go and no go gauges from PTG and some cheap PPU ammunition for break in. I did a comparison of the 3 head-spaces of the brass, the go, and the no go gauge. Notably, Head space for the belted magnum is from the bolt face to the bore end side of the belt. It is not the shoulder for belted magnums. This is somewhat a new concept for me as I have just gotten familiar reloading the 308 Winchester which is based off of the shoulder. Anyways, went to do a visual inspection of their belt heights. For comparison with the eye, I took computer printer paper and shimmed the brass to the go gauge and the go gauge to the no go gauge and checked their level.

Interestingly, I have a hard time telling the difference between the head space gauges. By the test, the difference is 1 slice of paper. Now the prvi brass took three slices to get to the go gauge and therefore 4 to get to the no go gauge.

1) Is it typical to have really similar set of gauges yet somewhat far off brass? I mean if min and max is within a sheet of paper and the brass is 3-4 papers off, wouldn't that be somewhat harmful to the boltface/ overwork the base of the brass? Is is possible that the brass or the gauges are out of spec?

2) Since the rifle is a bolt action, and the head space is not based off of the shoulder, I think that fire-forming/ just neck sizing the brass will allow me to have a custom fitting shoulder to the chamber. Theoretically less cold working the brass for longer life, and potentially more accuracy. Moreover, redundancy with "headspace" as the belt and the neck would keep the brass from moving around. Is that typical of belted magnums or do people still like to full length die them?

 

[jasonZ]

Re: Belted Magnum Headspacing

To the best of my knowledge the difference between a go and no go gauge is minimal. The 1pc of paper doesn't sound out of spec. I wouldn't be concerned about the brass being different. As is with all bolt actions, at least that I'm familiar with, the bolt will absorb all the recoil from the round. The bolt is what locks up and retains the round when fired.

As far as fire forming and then neck sizing. That's what I do with all my once fired brass. It should provide a longer life span of the brass. You will have to take into account for the neck tension, providing that you have enough and not too much. In most cases it's not much of an issue, but it is possible that you wouldn't get enough tension, or too much tension depending on how hard or soft you size the neck.

 

[LRI]

Re: Belted Magnum Headspacing

You CAN headspace a belted magnum off the shoulder. It's called a shoulder gauge. I use them for everything.

The only ones you CANT use them on is the WBY line of cartridges. The double radius shoulder is just too difficult to grind a gauge for. For those, resort back to the traditional belt gauge.

I like the shoulder gauge because now the case is qualified like everything else and I have a handy tool for setting up dies. Just plop it in the shell holder like a case and screw the die down till it touches. Remove, rotate a pinch more, and now you know your bumping shoulders without really hammering on them.

Hope this helps.

C.

 

[bronco9588]

Re: Belted Magnum Headspacing

Thanks for the info on the shoulder gauge, I would feel much better doing that than going off of the belt. I feel like it is a moot point after fire forming, but don't understand why someone would be so concerned with the belted head spacing gauge when the brass is 3X shorter in magnitude than the difference between the gauges.

Any brands that you would recommend?

 

[BlackOps Tech]

Re: Belted Magnum Headspacing

As Chad mentioned, the 300WM can be headspaced off the shoulder. The datum line on the shoulder measures .420", perpendicular to the centerline. The datum line will measure 2.279" from the boltface for the "GO" gauge and .006" longer for the "NO GO" gauge.

At one time, what kept me from using a shoulder gauge for the 300WM was the availability of quality brass. The issue was that I saw a variance by as much as .020" of the datum line on the shoulder of the brass.

I recently measured some Hornady brass with a comparator I made, and out of 20 pieces, I saw a variance of .0015" across the 20 pieces of brass.

When compared to the "GO" gauge, the shoulders of the Hornady brass were bumped back by about .003". This is a huge improvement over what I saw a few years ago.

 

[jasonZ]

Re: Belted Magnum Headspacing

That's very interesting. I was not aware of the shoulder head space gauge either! Wish I had known that, but then again I was using a 7rem mag. Hope I didn't give bad information and sorry if I did.

 

[bronco9588]

Re: Belted Magnum Headspacing

Any sources for these gauges? Never came up with one on the google search.

 

[LRI]

Re: Belted Magnum Headspacing

PTG. One stop shopping.

C.

 

[Rprecision]

Re: Belted Magnum Headspacing

I worked with a good friend to devolop a load for a 7mm Win Mag.

This is what I learned:

-Virgin brass is best fire formed for max accuracy

-We could neck size the brass only once before it would start getting to tight.

-I could onll Full length size 3 times before tossing the brass.

-We started to get failures in the case right above the belt, no uncommon, cracking, etc.

-We settled on 4 total loadings, all full length sized, ditched the neck sizing thing.

-Bottom line Mag's are hard on brass

Mind you this was all with a factory Savage chamber. I think a better chamber would give longer life.

 

[eddieharren]

Re: Belted Magnum Headspacing

You should seriously consider setting your die up so that it only "bumps" the shoulder back .002". I think you will find that your brass will last much longer without case head cracking.

 

[njohn890]

Re: Belted Magnum Headspacing

RCBS makes an item called the Case-Mic (? not near mine, in Hawaii right now) that has a case headspace micrometer function. The 300 WM version measures headspace at the shoulder as is done in a non-belted cartridge. It also includes an attachment for measuring loaded cartridge OAL but I use other methods for that. The main feature of measuring headspace at shoulder is useful though.

Neil Jones Custom Products also made a 7/8 press style neck die for me that uses a replaceable bushing for neck sizing a 300 WM as well. The Jones die bushing will not interchange with the Wilson/Redding style however-they are unique. The benefit is that the bushings incorporate both the entire case nack and shoulder and allows me to bump the shoulders,size the necks and leave the belt area alone on my 300.

When required I full length size in a traditional 7/8 press FL die and use the RCBS gauge to slowly creep up on the fired case headspace (measured at the shoulder) and size to .002"-.003" under fired dimension.

I bought the die back in the 90's, so hopefully Neil still makes them. The bushings are ordered just like Wilson/Redding bushings in .001" increments.

Hope this helps,

Nick

 

[bignada]

Re: Belted Magnum Headspacing

Redding makes .300win Type S FL and Neck dies that take their standard bushing. Redding and Wilson bushings are all .50 OD so interchange.

The RCBS Precision Mic is easy to use, as mentioned above. Wilson Case Gauge is also a similar option.