General Rifle Questions

[Looking2Learn]

What is 'lapping a barrel'?
What is fire forming?
Which is more popular in rifling LH or RH, and why?

And how would you explain "Ballistic Coefficient" to a five year old?

I'll have PLENTY more...

Thanks!

Looking2Learn

 

[kudu]

Here goes,

Lapping a barrel is when you take an abrasive compound and adhere it to a lead plug and force it through the barrel slowly to erode any high spots or burrs or rough parts in the bore. They can also be fire lapped the same way with a low velocity load usually suplied by the manufacturer of the slugs. It is done just enough to help the accuracy, to much will hurt the barrel rifleing. If this isn't totally correct I'm sure someone will correct me.

Fire forming is taking a standard cartridge that has the right base dimensions and shooting it in a custom chamber to expand the case to the chamber. Example: I have a .375 Weatherby chambered rifle. The shoulder is a bit wider and longer than a .375 H&H cartridge, but the same length, and the same caliber. To form a new reloadable piece of brass I fire an H&H round through the gun and have a .375 Weatherby case, it is blown out to the chamber dimensions. Same for Ackley Improved cartridges.

I can't answer the rifleing question.

BC (ballistic coefficient) is how slick a bullet will move through the air. Kinda like divers in the olympics the less splash or disturbance the better the coefficient. The higher the BC the better it will cut through the wind and will have less drag.

 

[sumpnz]

Lapping a barrel can take a couple forms. One is hand-lapping, the other is fire-lapping. Both are essentially methods of polishing the lands and grooves of the bore. One is done manually (hand lapping), the other is done by shooting bullets with an impregnated abrasive (usually several continuously finer grits).

Fire forming means taking brass cases originally formed for one caliber and hot-forgeing them to a different caliber. It is also possible to cold form brass from one caliber to another if the change is not too great (e.g. .308 Win to .260 Rem). By heating the brass you can make bigger changes at once because the metal is softened by the heat. Another trick sometimes used is to cold form the brass but anneal it in between each step (e.g. .308 to 7mm-08 to .260 to .243 with an annealing between each).

Not sure on rifleing.

Ballistic Coefficient is a measure of how "slippery" a bullet is in the air. The higher the BC the more slippery it is. A higher BC will result in the velocity of the bullet dropping slower than with a lower BC. E.g. Two bullets fired at the same velocity, one has a "high" BC, the other a "low" BC. If you measured the velocity of each at, say, 500 yards the one with the higher BC will be going faster.

Hope that helps.

 

[Looking2Learn]

Thanks guys, exactly what I'm looking for! But you know you all have just opened up a can of worms

How often should you lap (or is it lapp) a barrel?

So you can only fire form to a smaller round/cartridge? And if you can fire form "up", don't you run a risk of damaged brass or excessive pressure? I'm not quite catching it.....

The B.C. explanations are perfect (in 5 year old terms ), here's the next one:

How do you calculate B.C.?

And I've often read "the .308 has the perfect B.C.", is this true?

Thanks!
(I'll be saying that alot, and it doesn't begin to express my appreciaton for everyone's help)

Looking2Learn

 

[JohnKSa]

Lapping a barrel is a job for an expert. It's not something that should have to be repeated once done.

LH vs RH rifling. No difference from a ballistic standpoint. It's probably smart to set up your rifling so that the counter forces don't unscrew the barrel though.

You can only fireform from smaller to larger. Firing a small cartridge in the large chamber blows out the small cartridge to match the large dimensions of the chamber. THIS CAN ONLY BE DONE IN SPECIALIZED CIRCUMSTANCES. Don't get the idea that any cartridge that will chamber in a given firearm is safe to fireform in that firearm. That's a good way to ruin a gun and lose digits & eyes.

You can calculate BC by measuring the velocity of the bullet measured at two points along it's trajectory. There's a formula but I don't have it. And it's not nearly as exact a number as it might seem. The same bullet can have a different BCs when fired at widely different muzzle velocities.

You can't make statements about BC as if it's a function of caliber. Two .308 bullets could easily have dramatically different BCs. An easy way to make a BC better is to simply keep the design the same and increase the weight and length of the bullet. Clearly then, if you make a bullet that is best in a given caliber, I can always make one a tiny bit better by simply lengthening the bullet a bit.

 

[Vern Humphrey]

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How often should you lap (or is it lapp) a barrel?
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You shouldn't -- lapping is done by the maker. If you have to lap a barrel, you should do it only once.

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So you can only fire form to a smaller round/cartridge? And if you can fire form "up", don't you run a risk of damaged brass or excessive pressure? I'm not quite catching it.....
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Fireforming is always to a LARGER round. It expands the case to fill a somewhat larger chamber (keeping the same head size and max diameter, of course.) A good example is my .35 Brown-Whelen. I use .30-06 brass, and when I have completed fire-forming, all the taper is gone from the original case, the shoulders are moved forward and are much steeper. Case volume is increased by about 12%. And, of course, the caliber has gone from .30 to .35.

You can also create a "wildcat" cartridge by simply expanding the neck of the case. For example, the basic .35 Whelen is formed by expanding the neck of the .30-06 (without fire forming.) The .25-06 is made by squeezing the neck down to .25 caliber.

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How do you calculate B.C.?
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B.C. is determined by firing a bullet through two chronographs, one near the muzzle and the other down range. The difference in velocity is used to determine B.C. There are ballistic computer programs that do this.

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And I've often read "the .308 has the perfect B.C.", is this true?
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There is no "perfect B. C." And BC is associated with the bullet (and velocity), not with the cartridge. A given bullet will change BC as velocity changes -- the BC figures given by bullet manufacturers are merely averages.

In .308 CALIBER (which covers the .308, the .30-30, the .30-06, the .300 Weatherby Magnum, and so on) you can get a wide range of bullet weights (from 100 grains to 250 grains normally) and a wide range of bullet shapes, from flat noses to Very Low Drag (VLD) forms. The BCs of these different bullets varies from very low to very high.

 

[Looking2Learn]

Thanks everyone. Lots to absorb there, in the mean time

I have a wooden stock and want to free float the barrel, how?
And how do you bed the stock?

 

[dakotasin]

bedding the stock - glass bedding, is an accuraccy enhancing move best left to people w/ a little more experience w/ gun-tinkering... but, everybody had to start w/ one. here's how: http://www.baitshopboyz.com/forum/forum_posts.asp?TID=5343&PN=2

free floating means that the barrel of the rifle touches nothing. this is a diy project, generally, and can be accomplished w/ sandpaper. basically just remove material from the rifle's forearm so that a once or twice-folded dollar bill can slide between the tip of the forearm all the way to the receiver w/o getting hung up. if you remove too much, there will be an unsightly gap between the rifle and stock. too little and the barrel won't float. you want to remove material evenly from both sides so the barrel stays in the center of the channel. so, grap some 60 grit paper and a deep well socket, wrap the paper around the socket so that your sanding stays even in the channel, and float away.