Another Bore Slugging Thread

[3584ELK]

This may be more of a gunsmithing question than cast boolit subject, but I have an intriguing set of circumstances related to bore slugging my various rifles. First, let me say I have limited machining experience, but would rate myself as an above average boolit caster, having engineered a load or two with which I harvested an elk or two. Also, have a few handguns shooting boolits cast from Lee six banger moulds. Next, I have a fair understanding of how to read and handle a micrometer down to .0001” accuracy.

While slugging the bores of all but two of the rifles, I noticed a tendency of the slug to loosen up and become almost free in the rifling as it passed the mid-point going down towards the chamber. Strangely enough, the rifles on which this occurred have some age and erosion present in the bore. I did end up with slug measurements that made sense in all but one instance (an 8x57 Mauser slugging .3316”).

Is this a case of bore erosion, or am I seeing something else- perhaps the slug fits and begins to wear enough so that it slides more freely? It’s hard to believe its bore erosion, as some of the “loose” bores are also very new.

104752

[.22-10-45]

Had the same experience when slugging an 1898 Krag & 1917 dated 03' Springfield. It seems the early military smokeless powders reached max. pressure just short of midpoint in bore. In my case, it was less than .0001". I cured both with careful fire-lapping.

[petroid]

If your barrel is worn this can definitely happen to where the measurements are looser at different points in the barrel. I would slug the throat. This is where the initial fitment of the boolit takes place. If its tight, you may struggle getting a good cast load. If loose, size to fit the throat. Idk if a .338 sized down may be a better fit for you rifle but a throat slug can tell you

[Doc Highwall]

Hopefully if the barrel has a taper that it gets tighter towards the muzzle. The measurement that will be most important to you will be at the throat where the bullet will be when the cartridge is fired, and the pressure that is contained behind the bullet will apply enough force to allow the bullet to seal the bore all the way to the end.

[Yank146]

When looking for a cast bullet fit, the best method of course is to slug the throat. But a simple quick option is to size the bullet to just slip fit into the mouth of a fired case and see how it shoots.

[3584ELK]

Good input gentlemen, thank you!

[montana_charlie]

It's also possible for there to be a 'tight spot' in the middle portion of a barrel.
When found, it (most usually) is associated with the rear sight mounted in a dovetail cut.

If the dovetail is a very tight fit on the sight, hammering the base into position can actually 'bulge' the barrel inward a tiny fraction.
When a soft lead slug goes past that 'restriction', it gets 'sized down' to that dimension ... which is probably smaller than the remainder of the bore.

Instances of this are not real common, but they aren't rare, either.

CM