Hame-made method for determining breach-face to lead distance..

[jjrothWA]

Have a number of T/C barrels [10" Octagon & 2 21" rifle barrels], started to load ammo but wanted to determine the lead to breach-face distance.

So I took a 168gr SMK bullet dropped it [base first] into barrel and inserted a sized deprimed case into chamber and closed the action.
Open the action and removed the assembled "dummy" and measured the case head to bullet diameter using a Sinclair [hex] comparator, subtracted the dimension of the hex flat and result I believe is the lead to breech - face distance.

Repeated using 80gr SMK for the .223 barrel.
Checked some commercial rounds [should be minimum to fit all chambers] and the result
follow:

30/30: Win 150gr soft point = 2.132" case head to bullet diameter
T/C 30/30 barrel measured= 2.229" free bore => 2.229-2.132= 0.092"

for .223 Win commercial 55gr MC = 1.870"
T/C .223 barrel measured= 1.930", free bore => 0.168"

Does this sound valid??

Thanks for responses.

[Parashooter]

1. If you write "leade" (a preferred spelling) fewer readers will be confused. "Lead" has too many other meanings and pronunciations to be really helpful.

2. You are measuring the position of a cone with one angular dimension by inserting a cone (the bullet base) with a different angle. This is fine for comparative purposes, but actually tells you very little about the form and position of that portion of the chamber throat where the lands are reamed away to form the "leade".

3. To learn where the lands actually begin, use a flat-ended steel spud of groove diameter and measure how far from the bolt face it stops.

4. Make and measure a chamber cast to get a better idea of the leade angle and actual length of freebore - also the major throat diameter, often significantly greater than groove diameter..

5. Be careful not to confuse "free-travel," "free-run," or "bullet jump" (distance from seated bullet to land contact) with freebore (distance from chamber mouth to beginning of lands. These are very different dimensions (unless we're shooting flush-seated wadcutters).

[jjrothWA]

yes, "leade" is proper!

What I did was to basically get me into proper seating situation.

Later I tried some seated cartridges that enter but had to be force seated to allow closing.

So I gave a full turn on the "seater" stem and now the cartridges lod easy and extractor with minor resistance.

I have learned that the Contender can "benefit" by having less jump when firing a round thru it.

[raymeketa]

It's actually the throat but let's not quibble over semantics.

Measuring using the base of a bullet tells you absolutely nothing. Each bullet has it's own particular ogive and you need to measure from case base to ogive. That's a measurement that you record. Using a comparator, you can always go back to that dimension for that particular bullet and come pretty darn close when switching bullets.

[Dollar Bill]

It's actually the throat but let's not quibble over semantics.

Shoot, that's half the fun, Larry

According to the SAAMI definitions:

THROAT See Leade
LEADE (LEAD) That section of the bore of a rifled gun barrel located immediately ahead of the chamber in which the rifling is conically removed to provide clearance for the seated bullet. Also called Throat or Ball Seat.

Then you have:
FREE BORE A cylindrical length of bore in a firearm just forward of the chamber in which rifling is not present. Associated with bullet jump, it's between the chamber and leade.

Hornady's website illustrates the pressure changes with seating depth/distance to lands:
http://www.hornady.com/ballistics-resource/internal

You can buy a tools to help but the "old school" method is to seat a bullet long (so it will not freely chamber), seat the bullet deeper 1/8 turn of the seater plug (the thread pitch is 28 so 1/8 turn = about .004) until the round just drops in. Now, you can refine that by starting over 1/8 turn out, smoking or magic marking the bullet and checking for marks from the lands and get it down to the nats a** but IME this makes little difference. Check it a couple times with made up dummy rounds and you will have a good cartridge overall length to just touch the lands. keep in mind that bullet tips, even within the same lot/batch vary, even the high dollar match bullets but we're just looking for a starting point.

I typically start load development .010" off the lands, (1/4 turn of the seater plug = 0.0089", fairly close to 0.010) work up to best accuracy and then vary seating depth by 1/4 turn or 0.010". Very rarely will smaller increments yield appreciable difference in accuracy. Most of my rifles seem to prefer about .030 or so off the lands but there each rifle seems a law unto itself.

[Parashooter]

"LEADE . . . That section of the bore of a rifled gun barrel located immediately ahead of the chamber in which the rifling is conically removed to provide clearance for the seated bullet."

SAAMI has the right definition of LEADE but errs by equating it with THROAT (which also includes the non-conical freebore - if any is present). As happens so often on the internet, questionable information expands to fill empty space. Here's a diagram that may help (and inspire arguments from those convinced some other nomenclature is the only correct one).



Hornady provides some excellent illustrations at http://www.hornady.com/ballistics-resource/internal - but their text implies the leade, throat, and freebore are the same thing, an oversimplification that deprives the three terms of some very useful individuality.



Referring to the SAAMI defintion quoted earlier by Dollar Bill, we see a critical difference between leade and freebore - "FREE BORE A cylindrical length of bore in a firearm just forward of the chamber in which rifling is not present. . . it's between the chamber and leade." If we accept this, it's clear the leade and freebore are not the same.

In the end, though, the words are useful only for discussion - especially when considering a custom chamber or throating reamer. The fit of a particular bullet in an individual throat is something to be established by trying that bullet in that throat - and since every shot erodes things in there, we sometimes can't even stay with our painstakingly-established seating dimensions for long.

[emmagee1917]

Hame ..... does that mean it's not as good as hand , but a lot better than home ?
Chris

[Dollar Bill]

".....and since every shot erodes things in there, we sometimes can't even stay with our painstakingly-established seating dimensions for long.

Chasing the lands, the bane of many a shooter. I think that's why some HP shooters rebarrel after 2K rounds or so or every season. I haven't noted much degredation in accuracy as throats/leads erode except with Berger VLDs. Those bullets can a PITA when it comes to determining the optimum distance they have to "jump to the lands".

[dogtag]

Close bolt, insert 36" 1/4 dowel into muzzle, push in until it contacts bolt. Mark at muzzle.
Next: insert bullet and push into lands- hold there with short dowel.
Inset 36" dowel at muzzle, push in until it touches bullet. Mark dowel at muzzle.
Distance between marks is bolt head to lands [for that bullet]