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Re: marks on bullets
Posted by Larry Gibson on Friday, 29 July 2011, at 1:10 p.m., in response to Re: marks on bullets, posted by steven gabrielli on Thursday, 28 July 2011, at 8:51 p.m.
Not going to argue semantics on obturation. It does mean the expansion of the case sealing off gas leakage most often but also does refer to “set back”, “bump up” and “expand” as in relating to the bullet filling the bore to prevent gas leakage around and forward of the bullet.
Unknown if obturation of the 405 gr bullet was ‘expected”. They found out it did not and this led to incomplete powder combustion as Dick notes in his post. The 500 gr bullet had the inertia to obdurate, upset or bump up which sealed the bore and affected much better powder combustion. In Spence’s book on page 150 we find the official service description of the cartridge. It references “there is a dished cavity in the base of the bullet, sufficient to bring it to its proper weight without affecting its general form.” This is also referenced in the official platte drawings for the bullet. A much better description of the ammunition and bullets will be found in THE 45-70 SPRINGFIELD. I have also read but don’t recall exactly where that the change to a more HB instead of “dished” was to maintain the same bullet weight but lengthen the bullet to increase compression of the powder charge for better ignition.
You might also note in the cartridge specifications that the arsenals used a harder bullet cast of 12-1 lead-tin instead of the 20-1 recommended by Spence for rifle loads or 40-1 for carbine loads. That probably added to the lack of obturation with the 405 gr bullets. The 500 gr M82 bullets (also referred to as M1881 bullets) were of 16-1 alloy which was found to be best after lengthy testing at Frankford Arsenal. As Spence used the softer alloys and with correct loads they will obdurate to a degree is probably why not much further discussion was put into Spence’s book.
If one is departing from Spence’s methods to replicate service loads then we find many target type loading techniques work as well in TDs as they do any other rifle. I use a Rapine 460500 cast of 16-1. It is sized at .4615and is lubed with Spence’s olive oil/beeswax lube. I neck size the cases and leave the case mouth flared to additionally seal the chamber. Bullets are seated on a mildly compressed 70 gr charge of GOEX cartridge so that the front driving band is ½ engraved by the very short leade. The bullet is already .0005 larger than groove so no obturation is required or wanted. It also provides sufficient inertia so that powder combustion is very good. This load produces excellent accuracy through 12 -15 shots with just mild blow tube use. Substitute a duplex load using 7gr 4759 under 54 gr of GOEX Cartridge and accuracy is even better, no blow tube use is needed and accuracy remains constant for 100+ shots that I have tested.
Spence’s replicate loading techniques do indeed replicate the m1873 and m1881 ammunitions along with their accuracy potential as described in A COURSE OF INSTRUCTION IN RIFLE FIRING by COL. T. T. S. Laidley, 1879. I have and do highly recommend Spence’s book to anyone loading for the TD. However, with modern target loading techniques the TD can be a much more accurate rifle.
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