Throat length, Pressure,& MV

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by winmagman, Sep 7, 2004.

  1. winmagman

    winmagman Well-Known Member

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    I've got a question for the technical guys out there.
    I've got a 6.5-284 that has a relativly short throat in it, my overall length is 2.965 on a stoney point comparator. I'm shooting a 140 sst over Re 22 and at 49.3 grains I start getting pressure signs.
    Now I know I can get the throat lengthened and load more powder before reaching max pressure, but given that I'm at max pressure at either length will there be a difference in MV with the added powder or is max pressure only going to generate so many FPS?
    I may be over thinking this but thought I'd ask.
    Chris
     
  2. Mikecr

    Mikecr Well-Known Member

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    There will be a difference in BOTH pressure and MV.
    This, assuming you stay the same distance from the lands, and add powder to maintain same fill ratio (given that you moved the bullet out of the case alittle).
    I'll throw out some hypotheticals here, using QuickLoad to illustrate.

    With a 6.5/284 pushing a 140smk, into the lands, .525" of total bullet seated, out a 28" barrel, 49.3 of Re22[89.1% fill].
    I'm showing 57Kpsi and MV= 2900. Keep in mind this isn't calibrated(I don't have a 6.5/284).

    Same conditions, only seated shallower @ .425"(throat extended .100"), still into the lands:
    55Kpsi, MV=2870, fill=87.1%

    Now adjust powder for original fill:
    89.1%fill=50.5gr, 58Kpsi, MV=2930
    BOTH PRESS AND MV WENT UP.

    To match original MV and Pressure:
    49.9gr, 88.1%fill

    [ 09-07-2004: Message edited by: Mikecr ]
     

  3. winmagman

    winmagman Well-Known Member

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    Thanks Mikecr
    I was giving some thought to lengthening the throat, but for such minimal increases I think I'll just chase the lands as the barrel wears with use. Appreciate the numbers.
    Chris
     
  4. shilen30

    shilen30 Well-Known Member

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    A question for you guys since mine fits into what winmagman states. I have a custom 300WM, and I have shot a few hundred rounds out of it so far (700 maybe). The lands have moved forward .07" approx since it was new. Is this much normal? I always allowed 3-5 minutes of cool time between shots. Occasionally, the barrel would get warm, but not hot enough I couldn't rest my wrist on the barrel for quite some time. If it is normal, is it time to turn the barrel back a couple threads, and if it is not normal, do I need to take it back to my smith?
     
  5. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    Shilen30,

    That is a tricky one, I have seen same brand barrels act completely different as far as throat erosion goes. barrel steel varies from lot to lot, even the good stuff.

    There are several factors that can effect how fast teh throat erodes, is ball powder being used or stick powder?

    Is the rifle being cleaned with a bore guide?

    What are the throat dimensions in the chamber?

    Stick powders tend to erode throats faster then ball powders on average because they are more physically abrasive and they tend to burn a little hotter then ball powders.

    If a bore guide is not used, this will accelerate the wash out of the throat.

    If the throat is loose, which on a custom rifle it had better not be but it is not uncommon, your throat will erode much faster then with a quality match grade throat cut 0.0002" to 0.0005" over bullet diameter.

    This is because it allows much more gas to escape around the bullet and this gas basically acts like a blow torch on the throat.

    For a 300 WM to have .070" throat erosion in only 700 rounds would be quite a bit with the care you have taken of the barrel. Still not unheard of depending on the factors listed above.

    Your working pressure of your loads also has a great effect on throat erosion.

    Having a 6 or 8 groove barrel will result in faster throat erosion then a 3 or 4 groove barrel.

    There are alot of variables!!

    Good Shooting!!

    Kirby Allen(50)
     
  6. shilen30

    shilen30 Well-Known Member

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    Kirby,

    Thanks a bunch! I e-mailed you a while ago about possibly doing a 6.5-284 for me. I got the job I was after and as soon as I finish up my defense you will be the one to build the rifle. I may also send this 300 up to you to rechamber and set back a couple threads. I have a 22-250 that has fired more rounds than the 300 and it has not shown near the erosion as the 300.
     
  7. D.P.

    D.P. Well-Known Member

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    Shilen30
    I had the same thing happen with a 300 win Sendero after 500 rounds. Found out it was not throat erosion. The batch of NBT's I got had the Ogive further from the tip about 15 thou. Nosler Rep advised this was within tolerance. Set em back out and it was as good as new [​IMG] Now for each rifle I keep an early bullet with an OAL measurement at lands for future erosion check.
     
  8. budlight

    budlight Well-Known Member

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    wingman - I went through this years ago and ended up using a .300 inch throated barrel. Pressure signs went down chrono speeds stayed the same. My .277 win AI case uses a very compressed load. It takes a tall drop tube to get all the powder in the case nearly level to the top before seating the bullet.

    I also went slower powder than R-22 to IMR 7828 and CCI250 primers.

    A few years back I started moly coating all my bullets and they were just about across the board a 50 fps drop in speed and even less pressure.
     
  9. BountyHunter

    BountyHunter Writers Guild

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    wingmagman

    1. RL 22 has a well documented history for lot to lot variations. you could have very fast lot. there were several pulled from the market two years ago.

    2. Bullets will vary on base to ogive. For example sierra 142s are .080 longer than JLK 140s in this area.

    3. Same bullets will vary as much as .20 from lot to lot with the same mftr. Always measure.

    3. You never mentioned what MV you are getting with that load. If it is 2950 or up, you are probably at the limit anyway depending on barrel length.

    4. Read up on the ladder method of load tuning (here on the board) and find the best load for your barrel, bullet and powder combo.

    BH
     
  10. LDHunter

    LDHunter Well-Known Member

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    FiftyDriver,

    "Stick powders tend to erode throats faster then ball powders on average because they are more physically abrasive and they tend to burn a little hotter then ball powders."

    You sure on this? Both Hodgdon and Accurate Arms techs have told me the reverse. According to them ball burns hotter and subjects throats to more erosion.

    I don't claim to know from first hand experience. The throats I've burned were all subjected to "stick" powders only.

    $bob$
     
  11. lynn

    lynn Well-Known Member

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    Winmagman
    You need the boattail to bearing surface length 0.050 in front of the neck shoulder junction in your cases to reach both peak accuracy and muzzle velocity.Seat a bullet to this setting and measure your overall length then chamber the round and measure your overall length on the same bullet.This will tell you how much longer your throat needs to be cut.By chasing the throat you'll never get good accuracy or velocity out of your existing barrel.
    Lynn
     
  12. winmagman

    winmagman Well-Known Member

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    Thanks for the input.
    I've been working with this rifle for a little while now and it shoots the 140 gr SMKs into the .2 to .4 range with a MV of 2980 (avg) and an ES of 16 (10 shots).

    I also tried the 140 gr SST in hopes of finding a high BC hunting bullet. Unfortunatly the rifle didn't seem to like them. I tried Imr 4831, H 1000, and Re 22 with MVs between 2800-3000 rarely did I get a group under .75 inches.

    So when deer season opens this Thur. I'll be shooting the MKs. Mainly cause I'm fond of hitting what I aim at.
    [​IMG]