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300 RUM saami min headspace to datum line?

 
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  #1  
Old 04-09-2012, 09:55 PM
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300 RUM saami min headspace to datum line?

Guys, I have a 300RUM that just came back from the smith a week ago.
Ran 40 rds. through it with no issues.
Then had several random cases stick in the chamber in the next 20.
I checked several fired cases to determine fired headspace at 2.471 +/-.001. I set up my sizing die to resize to 2.468 and loaded a bit more.
Had a couple more random cases stick...... went through a bunch of brass i had sized in the same pass and found that some of them were measuring 2.471, 2.470 after sizing in the same setup.
Had a couple shellholders so I took .002 off one and seem to be getting consistent sizing to 2.468 +/-.001.

Think this will be a good solution to this issue?
From what I'm seeing it looks to be chambered pretty snug but I can't seem to find a datum line measurement anywhere to confirm. Any thoughts/suggestions?
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Old 04-10-2012, 08:13 AM
Edd Edd is offline
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Re: 300 RUM saami min headspace to datum line?

The SAAMI chamber spec is 2.4742"- 2.4842".

The SAAMI cartridge spec is 2.4712"- 2.4782".

Cases stick in the chamber because of their diameter.

Last edited by Edd; 04-10-2012 at 09:24 AM.
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Old 04-10-2012, 08:22 AM
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Re: 300 RUM saami min headspace to datum line?

Quote:
Originally Posted by Hookturnr View Post
Guys, I have a 300RUM that just came back from the smith a week ago.
Ran 40 rds. through it with no issues.
Then had several random cases stick in the chamber in the next 20.
I checked several fired cases to determine fired headspace at 2.471 +/-.001. I set up my sizing die to resize to 2.468 and loaded a bit more.
Had a couple more random cases stick...... went through a bunch of brass i had sized in the same pass and found that some of them were measuring 2.471, 2.470 after sizing in the same setup.
Had a couple shellholders so I took .002 off one and seem to be getting consistent sizing to 2.468 +/-.001.

Think this will be a good solution to this issue?
From what I'm seeing it looks to be chambered pretty snug but I can't seem to find a datum line measurement anywhere to confirm. Any thoughts/suggestions?


SAMMI chamber spec is 2.478

SAMMI loaded ammo is 2.474 min.

It does sound like you may have a tight chamber(Measuring can be difficult and is not always
exact so it is best to measure a fired case and with the same setup measure a sized case for
the difference.

Reamers are normally very precise but after one is used very much they may make a small/
tight chamber. this is still OK but you have to make adjustments with the dies.

The only other possibility that I Can thing of that would cause this is excessive pressure, so look
at bullet seating depth or back off on your powder charge just a little because of the smaller
chamber.

J E CUSTOM
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Old 04-10-2012, 01:09 PM
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Location: Utah
Posts: 1,377
Re: 300 RUM saami min headspace to datum line?

I saw a similar issue a couple years ago. Turned out our "match ground" ultra mag reamer needed a couple more thousandths body diameter. The clearance we used for other calibers just wasn't enough for easy extraction with a yard of brass case and high pressures. If your chamber was cut with one of these early reamers this could be your problem.
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Old 04-10-2012, 01:11 PM
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Join Date: Jan 2010
Location: Western Pennsylvania
Posts: 201
Re: 300 RUM saami min headspace to datum line?

Well, I have run this gun from 89.0 gr. of retumbo at .010 off the lands up to 94.0 gr. and didn't see any pressure signs until 93.5. At 94 the bolt lift was sticky.
I attained my fired case dimension by averaging 12 fired cases from the first batch of 30 rounds that had no extraction issues.
I'm loading it at 90 grains currently and still having issues.
My numbers on the fired cases haven't changed over several setups of my caliper with the hornady gauges and repeated measurements so i think I'm good there.
When I took the .002 off the shellholder I also removed the firing pin from the bolt and chambered about 15 pieces of brass with the new sizing spec and had no resistance when closing the bolt. Seems to me that headspace shouldn't be a possible issue at this point, I'm measuring .004 shorter than fired with my resized brass.
I'm going to take a shot in the dark and put a new extractor in it and see if that alleviates the issue. The current extractor has around 1800 rounds on it and looks like it may have a slight twist to it.
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  #6  
Old 04-10-2012, 02:15 PM
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Join Date: Jul 2004
Location: Texas
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Re: 300 RUM saami min headspace to datum line?

Quote:
Originally Posted by Hookturnr View Post
Well, I have run this gun from 89.0 gr. of retumbo at .010 off the lands up to 94.0 gr. and didn't see any pressure signs until 93.5. At 94 the bolt lift was sticky.
I attained my fired case dimension by averaging 12 fired cases from the first batch of 30 rounds that had no extraction issues.
I'm loading it at 90 grains currently and still having issues.
My numbers on the fired cases haven't changed over several setups of my caliper with the hornady gauges and repeated measurements so i think I'm good there.
When I took the .002 off the shellholder I also removed the firing pin from the bolt and chambered about 15 pieces of brass with the new sizing spec and had no resistance when closing the bolt. Seems to me that headspace shouldn't be a possible issue at this point, I'm measuring .004 shorter than fired with my resized brass.
I'm going to take a shot in the dark and put a new extractor in it and see if that alleviates the issue. The current extractor has around 1800 rounds on it and looks like it may have a slight twist to it.

Two more things that may also be the problem, The RUM was designed with .309 free bore and the
.010 may be a little close for that much powder. I would recomend at least .020 to .025 to start
with and pressures should drop off.

Also Retumbo is one of those powders that can go critical from one load to the next with just
a small increase. 95 to 96 grains seem to be about max with bullets seated to magazine length.
So with the bullets seated close you powder charge may be over max.

I use between 102 and 104 grains of 50BMG to get top velocities without any pressure signs
and 95.2 grains of Retumbo with 200 grain ABs for just over 3120 ft/sec.(Also no extraction
problems)

While working up the Retumbo loads I found that 95.4 grains showed signs of pressure and 95.6
was over the top and bolt lift was stiff so I backed off to 95.2 and everything was good.

There are ways to get accuracy without seating the bullet close but the rifle and ammo has to be
set up for it to yield great accuracy. (Below .200 hundred Thousandths MOA).

Just a thought. I hope it helps.

J E CUSTOM
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  #7  
Old 04-10-2012, 02:24 PM
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Re: 300 RUM saami min headspace to datum line?

Factory extractor? Sounds like you're onto the culprit.
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