300 win mag head space

Hi all,

I just installed my first barrel on my 116 300 win mag.. my question is, as I see it, the headspace seems to be based on the case belt rather than farther up like i had assumed.. that being said, and assuming that those belts are of a fairly standardized size, would it be possible to set head space off an old piece of brass from the first barrel since it seems to fit in the chamber? or take the firing pin out and use a live factory round? and if i have to use a head space guage do any of y'all know where i could rent one and do i really need a no go gauge?

I know lots of questions but all of you always rock in the advice department so i wanted to know from you guys.

Thanks, Jordan

The right way to do it is to set headspace off of a gauge, no I dont know where to rent one. In theory, your factory loaded brass should be usable to set headspace, but If I did that it would be checked with a no go and a go before I fired it. The problem that comes in to play using brass is that it can be sized slightly smaller than saami minimum specs (so it can fit in very tight but saami compliant chambers) and you could run into chambering issues with other manufacturers. I would feel comfortable setting it off a once fired and sized piece of brass, but I reload and can guarantee the uniformity of the brass I'm feeding the rifle... I still used a go/no go gauge on my rifle.

Yes, that belt has a very specific measurement.

Jordan, The gauge is fairly important here. Belted magnums need to be fairly close at the belt, like you stated. If you don't have a gauge and you want to set your headspace anyway... Go ahead but make the belt close. Usually belted magnum shooters know the first time the brass is fired it will grow between 5 and 10 thousandths at the shoulder. From there, neck sizing is good untill the bolt will no longer close. Using a case comparitor of some sort is a valuable tool to learn about your chamber.

No worries guys... I ordered some gauges from reamerrentals.com... Should be here in a couple days. I assume the head space procedure is the same as any other round?
Also all i have is full length resize.... How vital is the neck sizer? Never needed one for 308 and thats all I've loaded

jordan............

fl sizer is just fine, just set it up to just bump the shoulder a couple thou on resizing. You pretty well have to have some sort of measuring ability, like the Hornady gauge but that will allow for smooth chambering without excessive working of the brass. Most all belted mag chambers are btw anywhere from 10 - 20 thou long in the shoulder. You should also make sure to check out the freebore/length to rifling with whatever bullet you shoot as it's pretty common for them to be "long".

will definitely grab one of those from hornady and probably a neck sizer when i start reloading for this... I was just gonna shoot some factory for a while to build up the brass collection.. chamber should be just fine as far as factory goes... then i will have a bunch of once fired from this chamber specifically...

as far as headspacing procedure though which is what i was wondering.. its just as simple as it looks? insert go gauge, spin barrel til the gauge bumps the bolt, unscrew just a touch, insert no go, if it doesn't close we are good, if it does screw in just a tad til it doesnt and then go back to go gauge... tighten nut, check with both again, un clamp from action wrench, check again....

all that easy? being belted round shouldnt change that right?

also, the barrel that it is replacing wouldnt really expand the cases any to speak of.. sizing die didn't even touch them when i put them through and they came out of the breech still in spec? is that normal? it seems like that would be what i should be aiming for since i wouldn't have to work the brass much at all?

Maintain <.002 on the belt headspace. As said before the first firing the shoulder on the case will extend to the neck of the chamber and all will be good. As far as full length sizing go's, magnum cases have a much shorter life span if you do.

so if it fits the go gauge but not the no go gauge, is that within .002? sorry for all the newbie questions but i've never done this before and i want to make sure im doing it right before i mess something up..

The 300 WM is normally chambered where the belt is .220 to .227 from the bolt face. That is a .007 tolerence. The Datum diameter of .420 has a tolerence of .010 and measures real close to 2.279 +.010 in length.

The .002 tolerence I shoot for is for the particular brass I will be using. It is my method and i do it that way on belted cases to extend the life of the brass and shoot as accurately as possible. You can go anywhere between the go and nogo and be good. I like to have the belt close and let the neck follow the chamber then size, you may have better luck doing it anywhere in the gauge tolerence to be honest with you.

Research articles on the web and in books rather than just people like me posting opinions. You will get good stuff by doing that but it is always good to see if someone is full of chit or not.

It didn't take much to find this

http://www.larrywillis.com/headspace.html

Awesome... Got my guages in today and everything went great... Bolt closes on go and not on no go... So from what I've been reading, i should be good to go from a safety stand point... That sound right?

Jordan,

Im glad that you made this post. I am waiting on a 264 win mag barrel to get here. I have been worried about the headspace on it as well. I have done a 6.5x284 and a 220 Swift and they were no problem using the technique you described. Was it any harder than that? I have my guages.....

Ryan

haven't had a chance to shoot the darn thing yet but rounds and brass chamber just fine and if I'm not mistaken i should be good to go.. no one has told me otherwise.

Judging by what the headspace gage is telling you ,you are good to go.