Comet, the previous postings are correct. the purpose of this posting is to help you to establish the principle factor (there may be more than one).
Keyholing is caused by the bullet being skewed off-axis when it leaves the muzzle. This can happen when the bullet leaves the muzzle and receives an asymmetrical thrust while emerging.
So first, as jmoore suggests, first check out the muzzle + crown. If this is damaged, slightly bell-mouthed or worn unevenly (a common result of over-enthusiatic use of pull-through) then any competent gunsmith can fix it by recrowning, using a standard crown-cutter for the 7mm bore. A badly bell-mouthed muzzle may, however, be past that, requiring counterboring as a desperate measure.
I doubt that the general state of the main length of the bore is what causes keyholing. Damage that could cause skewing within the bore, such as a ringed bore, would be easily visible to the naked eye.
This leaves the No. 1 culprit for old service rifles: the deep throat syndrome, allied to the boat-tail illusion. The boat-tail illusion is the erroneous conviction that boat-tail bullets that produce excellent results with modern match rifles made to tight tolerances and SAAMI or CIP dimensions are also the best to use for old military rifles made for very differently shaped bullets decades before the standards bodies were even set up.
With the exception of my M1917 30-06 and Israeli Mauser in 7.62x51 (a.k.a. .308 WIN)
most of the other old service rifles in my possession show throats that are too long for optimum performance with boat-tail bullets. And
none of these were made to modern SAAMI dimensions. Many of them were made in the days when "spitzer" bullets were the latest thing, and they all perform better with bullets with a flat base, long cylindrical section, and round nose.
Why is this so?
Because in the deep-throated rifles, the boat-tail base loses contact with the case mouth before the bullet touches the vestigial lands at the start of the transition cone (the throat). During the brief period of free-flight between losing contact wth the case neck and being engraved in the throat, the bullet can skew. It is then rammed into the throat at an angle, and becomes engraved on the skew. After that, a perfect bore and muzzle will not save it from going off-course as soon as it emerges from the muzzle into free flight.
The only way to prevent this is to keep the bullet on the "straight and narrow" path as far as possible. Which means using a bullet with a flat base, long cylindrical section, and round nose, as already stated. It might seem plausible to use a thicker, cast lead bullet, but this option is usually severely limited by the simple fact that (to take the case of my
Argentine
Engineer's Carbine) a bullet that fills the throat expands the neck so much that it is impossible to chamber the cartridge. And, of course, there is the worrying aspect of a possibly hazardous increase in pressure caused by, basically, jamming in an oversized bullet.
To establish to what extent the throat may be a major factor in the keyholing shown by your rifle, and which bullets are likely to work better, I therefore invite you to read "Refurbishing an Argentine RB - Part 11" in the "Restorer's Corner" forum, which I shall be posting within the next 2 hours. I do not have a Chilean 1895, but I do have a 7mm Boer Mauser (1893 model) and it ought to have pretty much the same chambering and rifling dimensions.
See you later, in the "Restorer's Corner" - it might help you to solve the problem!
s. Not sure why. I cleaned the bore as well as I could by hand first.
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Originally Posted by Calif-Steve
