Bore - Sizing Fit

28 November 2003, 07:19

BrushBuster

Bore - Sizing Fit

It's my understanding that the matching of the cast bullet to the actual bore of the rifle is critical to accuracy. I have also read that the sized bullet should be .001-.002 over bore dia. to assist in providing a good seal.

The novice might be forgiven for thinking that since lead is such a malleable metal, why not lean toward a fatter bullet, say .003 or 4 over and have a real good seal?

My question is, what is wrong with this line of thinking, and at what point do you run into trouble?

28 November 2003, 07:33

grumble

Brush, at some point, you end up getting boolits too large to either fit the throat or to seat in the caseneck. Another problem with way-too-big boolits is that as they go down the bore, the extra lead has to go somewhere, and the most likely place is smearing back into the lube grooves or the base of the boolits. Not good.

And, rather than fitting a cast boolit to your bore, most here would suggest fitting to the throat. You want the round to chamber snugly, and barely engrave the rifling, but not be so tight that you can't remove the round without pulling the boolit. You should be able to feel it chamber and hit the rifling, but not where you have to use a lot of force to either close or open the bolt.

28 November 2003, 07:38

FAsmus

BrushBuster:

B: The novice might be forgiven for thinking that since lead is such a malleable metal, why not lean toward a fatter bullet, say .003 or 4 over and have a real good seal?

F: There are lots of CB shooters who firmly believe that bullet diameter (fit to barrel) is critical to accuracy. They will come on this thread and say so, take my word for it.

However! I have done some considerable shooting with "oversize" bullets, going, as you say, as much as 0.004 over groove diameter with excellent results. One must only tailor the bullet to the way the load is assembled and the mechcanical considerations such as oversize throats and so forth. The only thing really deadly to accuracy is UNDERSIZE bullets.

For example in a revolver of mine, groove diameter 0.452, I have to shoot bullets of 0.455 (or larger) in order to match the chamber throats of the cylinder. If I do not do this the pistol not only leads to one degree or another but accuracy falls off as well.

Good morning,
Forrest

28 November 2003, 11:15

aladin

quote:
Originally posted by BrushBuster:
It's my understanding that the matching of the cast bullet to the actual bore of the rifle is critical to accuracy. I have also read that the sized bullet should be .001-.002 over bore dia. to assist in providing a good seal.

The novice might be forgiven for thinking that since lead is such a malleable metal, why not lean toward a fatter bullet, say .003 or 4 over and have a real good seal?

My question is, what is wrong with this line of thinking, and at what point do you run into trouble?

BrushBuster you can get into trouble shooten oversized slugs. This being a lack of clearance between the loaded neck dia of the cartridge and the chamber neck cut. If the loaded round is too big and you shove it in by force, you might well blow up the gun. Neck clearance is a must..

As for leading the throating with oversized slugs, I haven't seen this in rifles but I'm sure it's possible. A slug does start from 0 fps just like it's passage thru a sizing die, so in effect that throat sizes the oversized slug just as a sizing die. I've shot 312's hard in my '03 which has a 307 bore with excellent accuracy.

Consider the case of bullet obturation of a relatively soft alloy using a faster powder in a rifle case like the 30/30. Likely that bullet base has swelled to the point of filling the expanded neck case before it exit's said case which is held by the chamber neck dia. Usually those chambers have 5-6 thou of clearance and often more. So the throating is again doing the first reduction in bullet size.

29 November 2003, 15:37

beagle

Brushbuster...Fit the throat. Normally about .002" to .003" is enough and will do no harm as long as the loaded cartridge chambers freely.

I'm shooting .310s in my .30s, .359s in the .35s, .432 in the 44s and .460 in the 45/70...and .462 in the 458 Win Mag.

Now, one problem that you do encounter is that some seating dies are not made for oversize bullets. I ran into this is a .30 Carbine Ruger and also in the 458 Win Mag. The crimp shoulder would actually seat the bullet. So be aware of this with O/S bullets.

You just have to find the diameter that works with the equipment and the rifle that shoots best. Ain't this hobby great!/beagle

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29 November 2003, 15:41

aladin

quote:
Originally posted by beagle:
Now, one problem that you do encounter is that some seating dies are not made for oversize bullets. I ran into this is a .30 Carbine Ruger and also in the 458 Win Mag. The crimp shoulder would actually seat the bullet. So be aware of this with O/S bullets.[

That's a good point beagle. How do you solve it with OS slugs?

I turn the collet style seater out and the seating stem down-- which seems to still seat the bullets straight.

I figure you might have a clever way of dealing with it.

30 November 2003, 05:58

NotRicochet

Yesterday I encountered something new, the crimp shoulder shaving the sides of round nose Lee 429-214-1R bullets in .44 Special. The SWC bullets I've used before had a rather minimal (if any) amount of bore diameter lead in front of the case and didn't show this. Guess I'll have to back off on the seating die body and use a separate Factory Crimp Die for these.

30 November 2003, 06:16

grumble

My solution to the too-small seating dies has been to open them up a thou or two with some sandpaper on a stick. The advantage of this is that it's quick and easy; the downside is that non-oversize bullets/boolits may not be held exactly in line with the case neck.

But, with Lee dies, it isn't too costly to have a spare set.