Need Gunsmith's answer re. Leade-Freebore-Throat and something else?

[Taxman1002587]

I am looking at a blueprint for a 6mmBr fininshing reamer. On the print immediately after the .272 neck portion ends there is a short.0142 area where the chamber tapers down to the bore diameter of .2435 - is there a name for this area? Then after this portion there is a area .080 long called "Lead"- is this area = to freebore? Finally after the "Lead"
there is a area .200 long called "Throat" and to the very right there is a 1 30 symbolizing I believe (but am uncertain) the angle of the throat. I would think, based on the print I am looking at, that the ogive of the bullet would actually first come in contact with the bore somewhere in the first 5% of the area called throat however I do know this is really just a guess on my part - and- I do know with complete certainty that I am NOT a Gunsmith.

If I provide a Gunsmith with the specifications off a reamer print or a copy of the print could the Smith tell me within +/- .005 where the ogive would engage the rifling (assuming a new barrel)

The reason for the question in the first place is I have a opportunity to buy a recently rebarrled 223AI 1/14 twist and I have a copy of the print for the reamer used to finish the chamber. I quit shooting .223 6/7 years ago but I still have 900 pieces of new brass and 1,850 Fowler 52 grs and 1,200 Shilen 52 grs and I don't want to buy the gun unless it has a reasonble chance of liking the bullets I already own. The print was faxed to me but the gun itself is three states away from where I live or else I would just go check the seating depth.

If these questions are to basic for this Forum I apoligize in advance.
Thanks for any insight or help.

 

[jpb]

I'm not a gunsmith by trade, but I'll have a go at your questions.

As for the part of the chamber/reamer where the diameter steps down from the neck diameter to bullet diameter, I've not heard any special term.

The (more-or-less bullet-diameter) cylindrical section ahead of there is called the lead(e) or freebore, usually interchangeably. IIRC I've seen someone claim that it's not technically correct, but I can't remember what distinction they were trying to make; it might have been that lead(e) = freebore + throat. Also, I've heard there is some variability in how the freebore length is defined, i.e from where to where.

The tapered region forward of freebore is typically called the throat, and yes the 1-30 is almost certainly the angle of the taper (1.5 degrees = 1 degree, 30 minutes). Sometimes the angle will be specified by the angle from each side to the centerline, sometimes the total included angle (i.e. twice the amount per side).

If things work normally, the bullet will engage the rifling at the part of the bullet where the slope of the ogive matches the slope of the throat taper. (There can be a problem here though if the bullet has a very long ogive and the throat has a steep taper; then the bullet could end up touching at the point where it's diameter matches the diameter of the rifling lands --- i.e. it runs up to the corner where the throat ends.)

Many (most?) bullets have what's called a "tangent ogive," meaning that the curve of the ogive blends smoothly into the cylindrical portion behind it. With a "secant ogive" (common on "VLD" bullets) there's meant to be an abrupt break in the slope of the bullet profile at the point where the ogive starts. If that starting slope of the secant ogive is steeper than the throat angle, the bullet will engage the rifling at the point where the ogive starts.

Let's say, though that you have the normal situation where the bullet has a tangent ogive and the chamber has a compatible throat angle. Where does the slope of the bullet profile match the throat? It depends on the radius of the bullet ogive. Say you have bullets with a "7R" ogive; IIRC this means that the ogive radius is 7 times the bullet diameter. So a 7R .224 bullet would would have an ogive radius of 1.57 inches; theoretically this would result in a slope that matches that of a 1.5 degree-per-side throat at a distance .041 inches forward of where the ogive starts. (This comes from ogive radius x Sin[angle].) The bullet diameter here will be about .223".

How accurately could a 'smith judge all this? Hard to say, because there are so many things that may not be quite ideal. At the very least he'd need to know something not only about the chamber specs but also your bullets. That said, I'm _sure_ there are folks here who would know what freebore length and throat angle are appropriate for a .224 benchrest bullet.

Instead of telling us about the 6mmBR reamer print, what are the figures on the 223 AI chamber you actually care about?

 

[Taxman1002587]

jpb - thank you so much - I had to read and study your response about 5 times but I believe I have got it. The reason I listed the 6mmBR specs is I have a new 6mmBR unfired - so with your help I am now going to attempt to compute where a specific bullet will touch within the throat of the 6mmBR - then I am in fact going to measure where that specific bullet does come in contact with the throat and compare to the predicted point ( I plan on using a VLD that will have a slope steeper then the throat) If I get within +/- .005 I will count it as a win. I then plan on doing it with what I will call a normal (non VLD) bullet, predicted vs measured.

jpb-I could certainly have made the question simpler but then I would have had to assume the responses were accurate and I really would not have increased my understanting. With your response it will now probably take me quite a few hours to work this out and diagram on paper and then additional hours to measure and compare predicted to actual. The winters are long here in Minnesota. I took a pass on the 223AI as I added .010 for the case length short of where the neck ends in the chamber + .015 for the transition from neck to bore + .090 leade (on print) so just to reach the throat was .115. The bearing surface on the Fowlers is about .320 and the case neck on the 223AI is only .200. So I reasoned (many time my reasoning is wrong however) that a bullet seated to the shoulder would have its bearing surface just reach the throat. Then I just guesed that if you want the bullet to touch the rifling or engage to .010 you would have to add .040. In which case the bullet would only be seated in the neck .160. I thought after 500 rounds or so I would run out of neck. From discussions with the seller I know he thought that the the leade -which he called freebore was the exact distance between the end of of his brass and where the bullet would touch the rifling.

Really apreciate your insight.

 

[WayneShaw]

If you have the chamber print of the 223AI, and want to use the 52 range bullets, it shouldn't have much of a freebore length. Somewhere around .010 to .020, assuming the hasn't been shot out.