Does a crush fit change pressures?

Discussion in 'Reloading' started by tlk, Aug 3, 2010.

  1. tlk

    tlk Well-Known Member

    Messages:
    238
    Joined:
    Apr 11, 2008
    Does having a slight crush fit change the pressures or anything else when compared to the same load in that is .002 off the chamber datum?
     
    Last edited: Aug 3, 2010
  2. BobbyL

    BobbyL Well-Known Member

    Messages:
    188
    Joined:
    Dec 15, 2008
    There may be a difference but it wont change enough to affect your load. Is there a reason that you are asking? I FL size and bump for everything i do everytime. But that doesnt mean its the best but its what i have found over the years that keeps my rifles as accurate as they can be.
     
  3. tlk

    tlk Well-Known Member

    Messages:
    238
    Joined:
    Apr 11, 2008
    As a matter of fact, there is a reason.:D

    Loaded some brass (everything the same) with my rifle's favorite combo, the exception being whether or not I had a very slight crush fit or not. The results were interesting: ~100FPS faster and MUCH (MUCH!!) more accurate than the same load that was not a crush fit.

    Wondering if anyone else has observed the same thing.
     
  4. BobbyL

    BobbyL Well-Known Member

    Messages:
    188
    Joined:
    Dec 15, 2008
    AH... I cant see it causing 100 fps difference but if thats what works then its what i would use. Whats the run out on the stuff you are bumping? That might be causing the accuracy difference. Of course thats if everything is exactly the same.

    Alot of the old timers i have talked to like to do 3/4 neck sizing so that everything stays alot more centered in the chamber. It might be the fact that you have a loose neck in the chamber. A bushing die might help by reducing the difference in the fired and resized round. Running brass without bumping shoulders will work just fine untill they dont chamber anymore. I would try to find a different way then not bumping but thats just me. If it works much better then stick with what works....

    This is another case in where custom chambers and dies made to spec help alot. As far as i can figure thats how i have got away with doing what i have been doing and not getting accuracy problems. I make the dies to the chambers that i use with a resize reamer cut just for them.
     
  5. tlk

    tlk Well-Known Member

    Messages:
    238
    Joined:
    Apr 11, 2008
    Yeah, the uptick in speed was what I thought would be goofy. Every charge was weighed with a regular scale then verified on a electronic scale, all bullets weighed, runout on all round was .003 max with most in the .002 area. Going to rerun the same load with the crush fit again to make sure it wasn't some light issue, etc that was messing with the Chrony. Accuracy was for darned sure a sight better, so I think I will stick with the slight crush fit from now on.
     
    Last edited: Aug 3, 2010
  6. MTBULLET

    MTBULLET Well-Known Member

    Messages:
    1,068
    Joined:
    Jul 16, 2007
    by "crush fit" are you refering to "crushing" the bullet into the lands on bolt closing ?
     
  7. tlk

    tlk Well-Known Member

    Messages:
    238
    Joined:
    Apr 11, 2008
    No, I am not seating onto the lands. What I am talking about is the case being "pinned" in between the bolt face and the chamber datum. IOW, the case length from head to datum is slightly longer than the chamber (bolt face to chamber datum).
     
  8. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,460
    Joined:
    Aug 10, 2003
    Possibly primer ignition?
    Maybe your firing pin strike isn't right, or your primers aren't seated at the right crush, combined with looser pockets.

    Longer cases have slightly more capacity. If there were an extreme here, your velocities would drop rather than go up.
     
  9. royinidaho

    royinidaho Well-Known Member

    Messages:
    8,901
    Joined:
    Jan 20, 2004
    As BobbyL says, what ever works!

    I'm one of those that neck sizes at least a caliber if the neck is long as in '06 family of cases. Or 3/4 or so on such things as a 338 RUM.

    The crush fit your are speaking of, to me, indicates the distance between the case's head and shoulder is a bit too long. It would seem to be to be difficult to maintain a consistent crush fit such as you describe. However it works go for it.

    The crush fit that I stumbled on that really changed things in one of my rifles was seating the bullet way too long, crimping with the seating dig and "CRUSHING" the the bullet back into the case with the bolt. I switch bullets and they shot super, super, super great when I seated to the same length but did away with the crimp. As they say, "go figure".:rolleyes:
     
  10. phorwath

    phorwath Well-Known Member

    Messages:
    5,779
    Joined:
    Apr 4, 2005
    Try as I might, I can't think of any reason MV would increase 100 fps with crush fit casings versus .002" shoulder setback resized casings. I could understand how one might shoot better than the other out of a particular rifle. But I can't figure any explanation for an extra 100 fps MV provided your using the same brand & lot of casings, the same primers, the same charge and Lot of powder, the same case trim to length, the same bullets, the same bullet seating depths, the same fouled bore, fired at same temperature.

    What brand chronograph is giving you an extra 100 fps? And how many shots has that extra 100 fps been registered on your chrono with the crush fit cases?
     
    Last edited: Aug 4, 2010
  11. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,460
    Joined:
    Aug 10, 2003
    I can't either.
    My thinking here is that velocity is changing some amount with headspace, because of it's potential affect on primer strike.
    Tight cases strike more solid than loose.
     
  12. BobbyL

    BobbyL Well-Known Member

    Messages:
    188
    Joined:
    Dec 15, 2008
    Yes primer strike sounds like it might be a cause at least its a good idea. There is also less gas leakage with a crush fit then there would be a normal FL resized round. Maybe its a combo of both thats causing it but its still gets me that its as much as 100 fps. Maybe the chrono its just hard to tell.
     
  13. MTBULLET

    MTBULLET Well-Known Member

    Messages:
    1,068
    Joined:
    Jul 16, 2007
    a "crush fit" that is so tight as to not allow the neck to expand on firing would increase pressure, and velocity.
     
  14. Fitch

    Fitch Well-Known Member

    Messages:
    478
    Joined:
    Aug 5, 2009
    I have to agree completely. With everything else the same, and the bullets not in contact with the lands, pressure is determined by the size of the chamber, not the fit of the brass. The brass will expand to chamber size well before peak pressure is reached so it shouldn't have much if any effect at all, certainly not enough to cause 100 fps difference in MV.

    Brass brands can definitely make that big, or even bigger, difference. Seating the bullets into the lands, or not, will also make that much or more difference, but not shoulder bump.

    Accuracy improvements might be do to the brass being more uniformly concentric with the throat in the crush fit case.

    I can't really account for the 100 fps.

    Fitch