.31WTF bores

Discussion in 'Shooting, Hunting and Fishing' started by stoatman, Nov 3, 2009.

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  1. I was having some more "interesting" cast bullet fun with my number 4, namely in that a plain base bullet was shooting just fine at 1100 ft./s, but the same bullet with a gas check somewhat faster (not measured, but I assume around 1600 ft./s) was all over the shop. Apparently it is a known phenomenon for undersized bullets to shoot fine at low velocity but then for accuracy to fall apart at a more practical velocity.

    Given that my bullets were 313, which is after all the official groove specification (I checked the list of changes in Skennerton after having assumed for so many years that it was the same as the bullet, i.e. 311), I thought it couldn't have been that bad.

    Because the barrel is 5-groove, I don't have the equipment to measure a slug properly, but some bright spark on the cast forum suggested that I slug it and put the slug through a 314 die, which I have, and I would then at least know if it was under, around, or over 314.

    So, out come the 90-grain SWC 314's, un-sized, and with the calipers I pick the biggest one I can find -- 317. A bit of breakfree, an old rod and a lot of hammering later , I have an absolutely beautiful slug.

    The barrel on this rifle is shiny (it's certainly the best one I've ever seen on a used Enfield) with sharp rifling, so it hasn't seen much use. The leade is clear and smooth.

    The slug it produced was beautifully sharp and smooth on both lands and grooves. Lovely, thinks I.

    Down to the rock chucker with it I go, in goes the 314 sizer, and in goes the slug.

    Contact is significant, and it comes out significantly squished with the protruberances swaged down rather a lot.

    Aha! Thinks I, I have one of those magical three - one - oh-my-God's.

    So I've sent a couple more slugs off to a friend who has the relevant equipment to measure quite how large the "oh my god"-digit is, but I suspect it is going to be closer to a Mauser J-bore than to what it should be .

    So, here's the question:

    I can understand why the chambers were cut loose, that's obvious. But why on God's sunny green earth is the groove diameter so frequently over-spec even on new barrels? In any case, why was the specification .002" over the bullet size anyway? What was the obsession with requiring the bullet to set up?

    Sorry for the long post...
     
  2. If you get hold of one of the modern reprints of "Textbook of Small Arms 1929", there are about 100 pages of .303 ballistics, including about half a chapter on the firing sequence of the cartridge. There isn't a direct explanation for the choice of bore size (rather, I haven't read & remembered that bit), but it does go on at length about the setting-up of bullets - particularly the importance of flat base over any other shape. Lot of detail about gas-flow, etc.

    The answer to your question appears to be that the chamber, lead (sic "leed") and bore/rifling size were carefully designed to control peak gas pressures resulting from (a) different hardness and shapes of bullets, (b) progressive wear in the bore. I.e. the rifle was designed that way to ensure a decent service life combined with reliable performance over wide variation in ammo supply. (The ToSA goes as far to describe a "worn-out" specimen rifle held at Woolwich that continues to shoot c. 3 moa @600 yds with certain types of ammo - which confirms that any worn barrel will continue to shoot just fine with the right choice of bullet, and that "throat erosion" has little impact on accuracy).

    The impression you get from reading ToSA is that everything in an Enfield was engineered and tested to an incredible extent - they fired over 50 million rounds in proof and trials testing at Woolwich alone. The evidence in the book certainly puts in perspective the ignorance of much modern "common knowledge" that gets promulgated through internet forums!
     
  3. Thanks.

    I suspect that our modern knowledge is predicated on wringing the best performance out of boat-tail bullets, without real consideration of ragging a rifle around the Empire for tens of thousands of rounds while retaining service-acceptable accuracy, hence the prejudice that we have that the bullet should be precisely matched to the groove diameter.

    I also suspect that American thinking on the subject has a large impact.

    I actually have an original textbook of small arms 1929 -- I shall consult it this evening.

    Interestingly, in the field of cast bullets for modern cartridges the consensus has moved on from the 1991 Lyman manual, which was insisting that cast bullet should be matched to the groove diameter (and maybe a little larger). The consensus now is that they should be no less than .001 over the groove diameter, and best performance will probably be achieved when matched to the throat diameter, even if this is enormously over groove size (you hear of people loading .338 or larger to get a 8 x 56 Hungarian to shoot straight with a nominal groove diameter of .329!)

    Interestingly, my particular rifle did not like the indented-based Lapua 123 grain .311 spitzer
     
  4. Right. I have consulted the 1929 textbook, and have discovered the following interesting and potentially contradicting facts:

    In the section on ammunition, there is a chamber diagram which shows the throat at .3135", and next to it a cartridge showing that the bullet is in fact intended to be .312".

    But, in the section on the description of the rifle (SMLE in this case), it states that the depth of rifling is .0065". Doing some maths, we get: .303 + (2x .0065) = .316 for the grooves!

    This latter figure would appear to be the closest in reality for the majority of cases, including the No.4, which the list of changes says has .005" deep rifling (giving a total of .313").

    Intriguing...

    There are also some interesting figures on throat wear and how it affects the freebore after various numbers of rounds fired.
     
  5. ugly

    ugly LE Moderator

    Thanks for that, the art of casting for centre fire rifles is trully an art and we do seem to rely on internet rumours and some manufacturors research without enough true peer review. Manufacturors will only change their products when competition forces them to do so. They also will produce research to prove their design is best (we hope it is too) but we need to constantly challenge this. One of the problems with this ofcourse (and it is widly accepted) is that many barrels made under different contracts at different plants will throw up different dimensions. The LoC will show what dimensions are considered within tolerance along with various texts such as the ToSA.
    Publishing this data is important to allow such research to add value to shooters and also to force the remaining bullet and ammo plants to produce a viable range of factory fodder. The problems encoutered by those such as yourself when using the 125 gn Lapuas (I have a little experience in this field) shows why cast is the way ahead especially when loading for indoor range practice.
    Using original mil spec ball factory ammo where available should allow a degree of acceptable for all accuracy, this is important in a historic match when prizes are at stake and loading rounds just good enough to get to the bull at that distance is certainly not sporting. I have seen a shooter with boxes of cast or possibly moly lubed ball all marked for various distances. His excuse was barrel wear. I called foul but wasnt backed up despite the spirit of the original rule being in place.
     
  6. I finally have the measurement. The 2 slugs that I sent measured up as .3175 and .318!

    That would be a Mauser J-bore with deep rifling then...

    In the meantime I had paper patched a bullet up to .323 and rammed it in, so I got a nice impression of the throat and the start of the lead. The 1st 10th of an inch didn't contact at all! But after that, the throat was sharply defined.

    So, I'm currently thinking of 2 bullets that should do well in this barrel:

    Lyman do a 200-grain .314 intended for 303, which I can paper patch up to 319-320:

    [​IMG]

    And they also do an alleged 319 Loverin-type which drops at apparently 321, weighing 165 grains and intended for .32/40:

    [​IMG]

    So, custom sizers at 319 and 320 for me it is then!
     
  7. Now that I have my measurement, I'm just not going to bother with jacketed in this rifle at all, except on the odd occasion where I may have to purchase factory due to not being able to take ammunition on the Harwich boat.

    I suspect that Mark 7 bullets would probably shoot fine and dandy (at least within the 3-4 MOA specification), but I strongly suspect that the 123 grain hollow base Lapua just doesn't have the inertia to set up the 7 thousandths required, although maybe going to a faster powder (I used N130) may do so. This would also explain why my extremely shiny and smooth bore picked up jacket fouling enormously -- gas will have been escaping around the bullet.

    It certainly seems that my light cast loads shot acceptably well (160 grain .313 with 6.4 grains BA 10 at 1100 ft./s doing around 2 MOA) -- presumably the sharp kick from the extremely fast powder set them up enough, although going to a heavier charge of the slower BA 9 to try and get them to around 14-1600 fps was very much affected by the fact that they were undersized (I thought initially that it may have been the gas check, so I loaded some with gas checks and my light load which also shot around 2 MOA, with one 5- shot group coming in under 1 MOA, although the biggest group was in fact also bigger.) Although actually at those kinds of velocities, you don't even need the gas check, so I might try some faster ones without, although it is probably urinating into the wind now that I know what the barrel diameter is.