Re: How many reloads are you getting in your .300WM?
Bart Bobbitt 2006 responding to me about how to custom headspace a 300WM:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">When the 30 caliber belted cases were king of the 1000 yard prone
matches, folks got best accuracy with new cases headspacing on the
belt. Accuracy was based on shooting at least 20 shot strings; 25 was
common. New cases in a minimum SAAMI chamber shot Sierra Match Kings
no worse than 7 inches at 1000 yards. Which means sometimes a 5-shot
group would be 3 to 4 inches and a 10-shot group sometimes in the 4 to
6 inch range.
Both military and civilian top marksmen tried full-length sizing fired
cases such that they would headspace on the belt, but accuracy wasn't
as good. They tried neck only sizing; same results, often worse. Yes,
I know the long range benchresters liked to neck size their cases, but
they also got groups ranging from 4 or 5 inches up to over 12 inches
evidenced by their 3- to 5-group aggregates (averages). The problem
was the fired cases had an extra step in front of the belt. When the
firing pin drove the case forward to stop with its belt against the
chamber headspacing step and burning powder expands the case, the case
shoulder gets pushed forward against the chamber shoulder. Then the
case head gets pushed back against the bolt face taking the belt with
it and the case body immediately in front of the belt expands against
the chamber wall.
Now there's a step in front of the belt that regular full-length and
neck sizing dies won't get rid of. As the extractor in the bolt face
pushes the case head to one side when the round's chambered, that step
interferes with the chamber headspacing step so the belt doesn't easily
seat against it. When the firing pin strikes the primer and drives the
case forward (if the case isn't already as far forward it will go from
ejector spring pressure), that step on the case sliding off the
headspacing step in the chamber sets up unwanted vibrations. Depending
on where that step contacts the chamber, the resultant changes in
barrel whip cause bullets to be launched and greater and more varying
angles than the interference-free headspacing new cases have. It's
much the same thing as an out of square bolt face contacting a
previously fired case at their combined high points; groups open up
quite a bit.
Someone figured out that a way to get rid of that extra step on belted
cases was to cut off the bottom 3/8ths of an inch of a full-length
sizing die and to top of the die just below its body-shoulder junction.
Clean up and slightly radius these cut off areas, then set the die in
the press such that a fired case could be body sized all the way to the
belt reducing that horrible step back to almost new case diameter.
This "belted body die" is best used after running a fired case through
a regular full-length sizing die such that the shoulder's set back
about .005-inch ensuring the case will headspace on the belt. Set the
body sizing die such that its bottom just touches the fired case belt.
If the sized case diameter immediately in front of the belt isn't back
to no more than .001-inch over new case diameter, cut another 1/8th
inch off the bottom of this body die, clean it up then try again.
All the Winchester belted cases I've used are about .217-inch from head
to the front of the belt. Minimum SAAMI headspace is .220-inch;
maximum is .223-inch. Which means the fire case shoulder's got to be
set back at least .003-inch when resized so it'll headspace on the
chamber step; more if your chamber is at the long end of the
headspacing range. In measuring head-to-shoulder headspace on new
belted cases, they're at least .006-inch shorter than SAAMI minimum.
Those new cases do indeed headspace on their belts.
I don't think any extra belt relief on any H & H style cartridge is a
good idea. It'll only let a larger step show up in front of the belt
when a new case is fired. And I think the bigger that step is, the
more accuracy will be degraded.
I asked RCBS to make such a die in the early 1970's but they said there
would be no market for it. Now there's a collet version; checkout
www.larrywillis.com. </div></div>
I am now custom headspacing my rifles at .215".
Bart Bobbitt 1994:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">: I also heard that you can only stuff belted mags two or three times.
I also have heard that comment. And here's the reason why it happens.
The case is full-length sized too much such that the shoulder is set
back too far. When it's fired, the case blows forward and thins the
brass just in front of the belt. Seeings that many sizing dies for
belted cases are positioned in the press to set the shoulder back many
thousandths of an inch. Definitely not a good thing to do. I've seen
several dozen rifles for belted cases have this happen with reloads.
If you back the die out of the press just a tad, then size the cases to
where the shoulder is set back no more than one or two thousandths of
an inch, this problem goes away. But sometimes, another one is caused.
The body in front of the belt isn't sized down enough and then the case
can't be chambered. If your rifle's chamber is not too long in head to
shoulder headspace, you probably won't have any problems. If it is
too long, then your case life may well be short. An RCBS Precision Mic
is a great tool to measure both fired and sized case head-to-shoulder
headspace. After the first firing from a new belted case, it should
headspace on the shoulder in the chamber, not the belt, if long case
life is important.
I've a batch of .30-338 cases that I've full-length sized sixteen times.
They've been trimmed about every 5 loadings. Maximum loads are used each
time. And I know lots of folks who get 20 to 25 loads from similar cases.
But they've set their die correctly for the chamber the cases are used in.
And the chambers are of correct dimensions, too.
So, belted case life depends on three things:
* Chamber dimensions.
* Die dimensions.
* Die position in the press. </div></div>
