300 win mag chamber

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what do you guys recommend for a precision gun build as for the chamber of a 300 win mag? heard of 2 different headspacings one that headspaces on the shoulder and one that headspaces on the belt? whats your experiences and or opinions?

thanks..
 
Guntech has a min SAMI spec reamer with a fairly tight lead.
Give him a call.

My first load with Lapua brass, 190SMK's, and RL-22 went sub 1/2 minute at 200 yards (about 3/4 inch).
There is more accuracy to be had, but the 300Win is no 6mbr.

If you jerk with headspace too much you will have issues getting factory amunition to chamber.

After all, you control the headspace when you reload right? ;)
 
I think there are too many variables for the gunsmith to headspace a belted magnum using a shoulder.

I would recommend having it chambered to be tight on a steel go gauge.

For the strongest fire forming method, enlarge the neck of the brass and then size it down just enough so you can feel the bolt closing fairly hard on the brass. The will hold the brass hard against the bolt face and the shoulder will form ahead correctly.

After this has been done you can reload those brass using the shoulder for minimum head clearance.

Jamming a bullet out into the rifling causing the bolt hard to close will not fireform brass for the greatest strength. The firing pin will simply drive the case onto the bullet until the case stops either on a shoulder or the belt and then when the pressure increases enough, the case will stretch back to the bolt face... resulting in a weakened case ahead of the web.

Head space is set by the gunsmith when assembling the rifle.

Head clearance is adjusted by the reloading process.
 
A proper 300 Win Mag chamber will headspace on the belt, sort of. The nominal headspace dimension for a belted cartridge is .220". Factory brass ussually measures on the order of .214" to .216". It is proper to chamber to .220 and size reloads to allow headspacing on the shoulder.
I have known some 'smiths who have cut the chamber to headspace on the shoulder of new brass then cut the belt recess to ensure clearance. In fact, I have done this myself. It is not really a good way to do things though. At least not for a hunting rifle. A good hunting rifle should accept any ammunition which is made in that caliber. The same would apply to a tactical rifle buld. From an accuracy standpoint, there is probably no advantage to a non-standard system for headspacing when building the rifle. When loading for any belted magnum, it is advantageous to headspace on the shoulder. Regards, Bill.
 
I don't shoot a 300WM but several 7RM's. The process I use is the same for all my chamberings.

Fireform the brass to fit the chamber - I use a light charge of pistol powder, and cornmeal. For wildcats, I may also form a false shoulder to ensure the cartridge is properly held during this very important firing process.

Once formed, do basic case prep, neck size and load that bullet straight. As firings progress, you may find that the case doesn't chamber easily anymore.

You can use a body die to bump the shoulder which is the most common problem - you are now using the shoulder to headspace. If the area infront of the belt is also bulging, you may get lucky and have a FL sizer fit that area properly.

I am lucky in that my Lee FL sizer fits my brass/chamber very well so can use it as a FULL body die. I hogged out the neck so the brass is not sized. Both the shoulder and belt area is sized.

It is not easy to get both belt and shoulder to work for headspacing especially if the dies don't match up. In my Shilen, both the die and chamber match....very lucky.

But at worst, you will just toss the brass after 5 to 8 firings...

Jerry
 
Leeper said:
I have known some 'smiths who have cut the chamber to headspace on the shoulder of new brass then cut the belt recess to ensure clearance. In fact, I have done this myself. It is not really a good way to do things though. When loading for any belted magnum, it is advantageous to headspace on the shoulder. Regards, Bill.
Click to expand...

I read about one build that took this step further.

The chamber reamer was speced to headspace off the shoulder.

The belt area was cut very tight AND undersized/shorter.

The belt of the brass was turned to fit ensuring that ALL cases would be dimensionally correct.

WOW, what a royal pain

Why they didn't just turn the belt off the cases is beyond me?

Jerry
 
knockturnal said:
Huh. Learn something new everyday!!
Click to expand...

True!

Hell, even the Redding Instant Indicator says I'm reading headspace?

If I have it right the instant indicator is reading head clearance on the cartridge, and the headspace is a fixed measurement in the rifle?
 
X-fan said:
True!

Hell, even the Redding Instant Indicator says I'm reading headspace?

If I have it right the instant indicator is reading head clearance on the cartridge, and the headspace is a fixed measurement in the rifle?
Click to expand...

Yes.

All of the tools on the market that state they are for measuring head space of your reload are using the term incorrectly. They measure head clearance.

Head space is a commonly misused word, (even within the firearms industry) possibly because head space or head clearance both affect the brass similarly when fired.
 
If I was interested in a precision .300 Winchester build, I would take the short neck of the cartridge into account get the lead cut long. I would choose the heaviest bullet available which at this time is the 240 gr MK, and load it so that the shank of the bullet did not extend below the neck of the cartridge, but the neck is so short the base of the bullet would extend level with the bottom of the shoulder. Just to see what the OAL might look like, I put together a round with a 220 gr MK (I don't have any 240s) and I get an OAL of 3.618" compared to 3.340" COL listed in the Sierra manual. From the case head to the forward edge of the lead equals 3.120" with this bullet. I would guess that when loaded with a 240 gr MK the OAL could be as long as 3.813" compared to Sierra's suggestion of 3.535" with a standard chamber, and 3.600" with a custom chamber. Unless the rifle was built on a magnum length action, it would by necessity be a single shot, although the 220 gr MK round fits easily in my Brno 602 magazine.
 
I have spent quite a bit of time with the 240gr MK and they can be tricky to set up. The 300WM would be on the small side for this much bullet. I used a RUM and glad I had that much powder.

LR performance was very impressive and there was no denying when a bullet arrived at 1000m. I eventually ended up with the 220gr MK as they were far easier to dial in and less prone to blowing up - not the smoothest bore on the planet.

today, with a 300WM, I think the 208gr Amax and 210gr Berger VLD would get my attention.

Recoil would be more pleasant too.

Jerry
 
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