300 RUM saami min headspace to datum line?

Discussion in 'Gunsmithing' started by Hookturnr, Apr 9, 2012.

  1. Hookturnr

    Hookturnr Well-Known Member

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    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?
     
  2. Edd

    Edd Well-Known Member

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    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: Apr 10, 2012

  3. J E Custom

    J E Custom Well-Known Member

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    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
     
  4. grit

    grit Well-Known Member

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    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.
     
  5. Hookturnr

    Hookturnr Well-Known Member

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    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.
     
  6. J E Custom

    J E Custom Well-Known Member

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    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
     
  7. grit

    grit Well-Known Member

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

    Hookturnr Well-Known Member

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    Yup, factory extractor for a couple more days, soon as the parts get in we're dealing with that issue.

    As far as loading over pressure and to the lands, I think I've elimnated that as an issue. I ran initial load development .010 off the lands and found best accuracy on the light end of the load chart (89-90gr. .350 average groups) with no pressure signs. I then ran a seating depth test backing off in .030 increments with 90gr. charges and found exceptional accuracy at .070 off the lands and 90gr. It was during this stage that I had the first few cases stick. That's when I revisited the headspace issue. I took a long hard look at the extractor today and it doesn't look bad but it does appear to have a slight twist which agrees with what I was seeing on the rim of the stuck cases once I tapped them back out. Good excuse to change the extractor anyway, should have done it with the rebarrel but it just got overlooked.

    Once I get the extractor done, I'm going to revisit my load development to make sure I'm not over pressure but I'm not seeing any pressure signs on the fired brass now so I don't expect that to be any different. It does look to be a tight chamber from what I'm measuring on fired brass so we'll see what happens. I've had a few tight chambered guns but never run into this issue before so it's kinda throwing me for a loop.

    Thanks for all the input guys, I do appreciate it!
    This thing has been driving me up a wall for the last few days over this. I'd be looking for something more substantial if it wasn't shooting but it's still staying at 1/2MOA avg. through all this crap. That's why I figured I would ask here and get some outside opinions.
     
  9. Hookturnr

    Hookturnr Well-Known Member

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    Well, after waiting for backordered parts and a trip out of town, the rifle has a brand shiny new sako extractor sized for the lazzeroni class cases. Lots of surface area to grab the rim on this one! However, no joy on the sticky case issue. It does extract them everytime but not without some effort. The bolt lift is fine until you get to the last bit of lift before it pulls the case from the chamber, then it wants a tap to get the brass moving. I'm into a new batch of retumbo so i downloaded to 88.0, 88.5 and 89.0 gr. charges and still have the issue. My brass is coming out with a bunch of fine scratches on it though. Looking at the chamber with the bolt out and some light you can see that it's kinda rough looking...not happy... I've gone through all the normal culprits, think maybe the chamber needs polished? Best way to go about it? as always, any and all input would be appreciated.
     
  10. grit

    grit Well-Known Member

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    Frustrating from here. It'd take a minute to diagnose in hand. Sometimes, a reamer flute will pack full of chips and stick to the side, tearing a gouge around the chamber. When you fire, brass could flow into this and make the brass sticky. You would see evidence on your brass.

    I'm wondering how much extraction cam is active. The extraction cam is a shortcoming of the 700. If your smith had to take much to true the lug abutments and bolt lugs, this can minimize the amount of extraction cam left working.

    I polish in the lathe, after chambering. You could use a drill and a dowel. 320 grit is as fine as you want to go. Tape one end of your paper to the dowel, wrap the dowel until it's near chamber size. A bit of toilet paper sandwiched could provide a little cushion. You don't want it tight. The end of the paper will flap out and polish. Use some WD-40 for lube. Wet the paper and the chamber.

    Just spin it for a bit. You just want to polish, not open things up.
     
  11. Hookturnr

    Hookturnr Well-Known Member

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    grit, thanks for the reply. I thought about polishing it up myself and giving it another go then it occured to me yet again that i didn't get what i paid for...it's going back to the smith tomorrow so they can figure it out. I just wanted to be sure it wasn't something i was doing that was causing the problem before i went that route and I've reached the end of my chain, $70 in parts and around $70 in components and it still sticks cases..time for them to make it right. I figure I've got abotu $2200 into a rifle that shoudl drive tacks ( it does) and pull the damn brass out of the chamber so i can drive the next one! I'll post up an update once I hear from them and get it back, if its right, a good review will be in order, if not, well, we'll see how that goes..
     
  12. grit

    grit Well-Known Member

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    I know, it's hard to send one back, once you get it. My bet is, they'll fix it up and have it back, runnin' smooth in no time. Luck :D.
     
  13. Hired Gun

    Hired Gun Well-Known Member

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    I'm with Grit, I suspect yuor extraction cam is not sufficient due to loss of metal from bolt lugs and lug abutments during the truing operation. Probably just needs the handle knocked off and put in the correct spot.
     
  14. Hookturnr

    Hookturnr Well-Known Member

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    Rifle came back today..Had a talk with the smith and he tells me he shot it with 6 different powders at sierra max loads and had no issues until he put retumbo in it, then he got stuck cases. He also said he still had stuck cases with retumbo at 4gr. below max.. Tells me the scratches are from dirty brass but he polished the chamber again anyways. Not sure this sounds quite right to me but I'm going to give it another go. Got the Vortex remounted today, picked up some H1000 and loaded up 6 steps from starting load in 0.5gr. increments to take out tomorrow and see what happens. I'll drop a line back with some results..