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Bullet fit in revolvers

Discussion in 'Handloading and Reloading' started by 16in50calNavalRifle, Feb 6, 2012.

  1. 16in50calNavalRifle

    16in50calNavalRifle Well-Known Member

    I have read a fair amount on this topic here and elsewhere and think I understand, but would like to confirm that.

    In revolvers, to test for bullet fit, you first slug the bore. With this slug sized to the bore groove diameter, you then check the chamber throats.

    If the sized slug passes through the chamber throats easily - indicating that the chamber throat diameter is equal to/greater than the bore diameter - then lead bullets should not (other things such as powder charge and bullet hardness being OK) cause much leading in the cone or bore. The bullets are not "under-sized" by the chamber throats and thus do not allow gases to cut and blow by the bullet base in the cone and bore.

    If, on the other hand, a sized slug does not pass easily through the chamber throats, a lead bullets may be "under-sized" as it leaves the chamber, which means the bullet base has not obturated as it enters the cone, resulting in leading in the cone and bore.

    I am going to run this test again today, but on my Model 66-2 I believe a slug I sized to the bore passes easily through 5 of the 6 chambers, but is a fairly snug fit as it passes through the sixth chamber.

    So a related question to my overall query here is: how loose/snug a fit is OK for the slug passing through the chamber throat? Must it pass with gravity alone, i.e. does a slug requiring a push through the throat indicate a chamber that is under-sized?

    Trying to identify and correct the causes of the leading I have had in this revolver with every bullet and charge combination I have tried so far. Thanks to all for your help.
  2. chhodge69

    chhodge69 Well-Known Member

    Lead bullet diameter should be at least your bore diameter +0.001" Larger is ok as long as they chamber properly.

    I made the mistake of buying some 0.357" hard cast bullets for my 66-2 and they soldered up my barrel like mad.

    It's a balancing act of pressure, size and hardness. I do not own the proper instruments to diagnose the problem scientifically, so trial and error had to do. Leading at the muzzle? bullets going too fast (reduce pressure). Leading at the forcing cone? Bullet is too small, too hard or not enough pressure.

    0.361" soft cast (BH=12) 150g SWC from Mike at MasterCast bullets worked for me.

    http://www.mastercastbullets.com/ call him up he'll send you a free sample.
  3. mdi

    mdi Well-Known Member

    I understand your post, but I believe you're missing sumpin' here. Slugging the barrel and cylinder throats is for information to size the bullet to fit the gun. Slug the bore, measure, then slug the throat, measure (two different slugs). The groove diameter should be smaller than the throat diameter. The difference varies from manufacturer to manufacturer. (I'll use my Ruger SBH as an example) My Ruger's throats measure .431", measured with a pin gauge, and the barrel groove diameter slugs at .429", checked with a micrometer. The idea is to size the bullet to fit easily through the throat, slight pressure while pushing, yet be larger in diameter than the groove diameter. If the throats in my Ruger were .429" and the groove diameter measured .431", I would most certainly get leading in my barrel; the bullet would be swaged too small for the groove dia. and allow gas blow-by and leading. I have found it best, for my revolvers (5) to size the bullet the same dia as the cylinder throats (431" for my Ruger and Dan Wesson). A too large bullet (.433" in the Ruger) has had very little effect in my experience, but I do get cylinder face leading and frame leading, but I cannot say for sure this is the cause. Using the cylinder throat as a determination of bullet size has worked for me with very little regard for bullet hardness and normal powder choices (I have shot 9 BHN in my .357 Magnum, magnum velocities, with no leading)...
    Last edited: Feb 6, 2012
  4. 16in50calNavalRifle

    16in50calNavalRifle Well-Known Member

    Thanks for the replies.

    mdi, not sure what I'm missing - you provided the exact confirmation I was looking for. I have not slugged the throats - was planning to do it, as I have the lead slugs for it - but otherwise my understanding of the issue was as you explain it.

    One related issue is measurement. I only have my calipers to measure the slugs - regular linear calipers, not the "V" variety I have seen mentioned in connection with groove measurement. My S&W has the odd groove number, and as I recall it was a bit dicey trying to get a good read on the calipers.

    I think I will call an LGS with a 'smith and ask whether he would measure my bore and throat slugs - I assume any decent shop would have the right sort of calipers and micrometers for this. For my other handguns, I am able to get (at least what appears to me) a better reading on groove diameter with my regular calipers.
  5. USSR

    USSR Well-Known Member

    Nope. A lead bullet smaller than the chamber throats will cause leading. The throats are the critical dimensions. Assuming your throats are larger in diameter than your bore (which they should be), you want a snug fit in the throats (0.000" to 0.001" greater than throat size). Upon the bullet exiting the throat, the bullet enters the forcing cone, whose job it is to reduce the bullet diameter to bore size. This is not a problem, as lead is very malleable. I size my cast .45 Colt bullets to .455" to match my throats, and these bullets being reduced to .451" - .452" by the forcing cone produces no leading.

  6. Walkalong

    Walkalong Moderator

    Size the bullets to fit the throats. If the throats are not .001 to .0015 over groove diameter, have them reamed out. If the throats are undersized you will get leading, no matter what.
  7. 16in50calNavalRifle

    16in50calNavalRifle Well-Known Member

    Well I took another run at slugging and measuring, and so far things look good.

    To start I took two lead bullets I have been using (125 MBC RNFP and 158 Penn SWC) and checked them in all six chambers. In each case, they had to be tapped through the throat using my brass squib rod and a few careful taps from a rubber mallet. Quite a snug fit for all chambers in the cylinder. Each bullet is .358 diameter.

    I then tapped a slug meant for the purpose through one of the chambers - best I can tell, it measures .357 after being tapped through.

    Then I slugged the bore one more time (got a much cleaner specimen than the first time). As mentioned earlier, with 5 grooves in the S&W bore, I find it very difficult to get a square measurement of the slug with the normal calipers (my 1911 and XD, with 6 grooves, are easy). So my best estimate, based on something slightly off from a true diameter measurement, is .354-.356 for the bore.

    So it appears that the revolver has the correct dimensions for lead bullets: .358s are snug in the chambers, which appear to measure at .357, and the bore looks to be smaller than .357.

    I am going to swing by a gun shop later this week and ask the gunsmith to measure all my bore and cylinder slugs from all my guns (also have a Colt snub with 5 grooves).

    Many thanks to the expert commenters here, as always a great resource.

    The leading I've experienced so far hasn't been spectacular - I just would like to reduce it.

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