Hi,
I’ve been hanging ”off-line” for a while and this is my first post in here. This topic is so informatic, because there are real numbers, so you don’t need to read “between the lines”. Sorry my English, it’s not my native language.
For me, there are some information I ‘m not sure what they mean, or how those are measured. If anybody or Kevin himself could give me a good explanation with some measurements what I’ve been trying to figure out.
Also, I do want to carry my part of examination into this nice writing.
Reason why I’m trying to figure out some things is bringing into focus how small I can squeeze freebore without losing accuracy (If the best accuracy lies with smallest freebore). I’m looking the best accuracy for benchrest game (for about 50 shots).
Kevin wrote on his posts #1 and #82, that .030” engraving set actual length is .075” when measured over live ammo. From where .030” should be measured, or where should I set that dimension?
There are setup dimensions’ writings on posts #1 and #63 through .580 "-. 660". Between what two points this dimension has been measured or should be set? Is that a dimension from the point where the bullet shoulder makes contact on the leade up to the face of the barrel or what?
In topic Lapua vs Eley post #26 Kevin has been mentioned engraving length .40”. Is that measured from live ammo, or is this some setup which makes some XX- length live engraving.
http://www.rimfireaccuracy.com/Forums/showthread.php/10593-Lapua-vs-Eley/page2
On post #82 Kevin wrote: I like to set chambers engrave up to .090” (live ammo?). Is that the result of post #26 with .040” engraving setting length (for Lapua?), or is this one Eley (live?) engraving length?
When bullet is its contact point, bullet has always (99,9% of all cases) some way to go before rifling starts to engrave. When people talk “ogive” contact, I'm little confused, because for me ogive won’t hit to the lands. For me the contact point is the foremost outer corner of the bullet.
I drew one example of a graphical format (Picture #1)
Sorry, I must use another site than Photobucket, it didn't want to play with me.
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Grey area is part of bullet, straw-colored is part of case. Everything else than rim area is on scale. This describes a setting where bullet has visual engraving length as .075”. This is true only on the picture marked values. Bullet has same diameter from “nose corner to case” in drawing, but diameters marked as they are.
Dimensions may have some odd numbers, because those have been converted from meters to inches, and I was too lazy typing enough numbers.
I typed on picture “Leade starts to the left from this point. Straight part of chamber starts to the right at this point up to front of rim”. I mean that rim is located at same position as the face of the barrel, when ammo is loaded.
In example drawing, live ammo engraving length will be realized when ammo is set carefully to the leade contact and dimension from case head to the barrel face is .1235”.
Calculation formula: Distance what bullet must travel from its contact point to groove diameter .0084” + live ammo engraving travel .075” + rim thickness .0401= .1235”. That should be pretty close in real world (only drawing dimensions).
What affects measuring? What do you think what diameter is your bullet? It is not simple to look and measure engravings length from live ammo.
When we take a closer look at Lapua ammo and its bullet diameters. Bullet straight part has not same dia in every part, it is “variable” (Picture #2)
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Bullet’s 0,01mm (.000394”) diameter change has 0,1905mm (.0075”) length on 1,5-degree leade. If bullet is this small on front corner, before first greasing “groove”, it is probably deeper than we imagined. With this small 5,65mm (.2224”) front dia, bullet has still 0,213mm (.00084”) travel to go before it is on groove, where engraving starts.
If bullet has same diameter all over its straight part, for example dia 5,69mm (.2240”) bullet, it has 0,99mm (.039”) travel to groove 5,6388 mm (. 2220”), from its contact on leade.
Estimating engraving lengths, it needs microscope, or magnifying glasses with a big magnification. I use light bringing contrast, usually I take picture. I enlarge picture and check engraving length. Cheap smartphone is better than nothing. It is still not a piece of cake. (Pictures #3-5)
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It’s not the most reliable place measure setup length from case end to the barrel face, especially with live ammo. Ammo may (or will) pass deeper than its real contact point is on leade. I made a dummy brass ammo with Lapua dimensions, it should give more reliable readings. It is way better to compare chamber length with this tool than try to figure out it visually from live ammo (Pictures #6-7)
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It does not have to be exactly same dimensions than your live ammo. It is more tool you can compare chamber lengths, it gives more sensitive feeling. If you use tool like this, you must know exactly groove diameter, chamber leade angle, your tool dimensions. You can use math to find out what affects to where and how much.
Part of this is hair splitting, but it shows you can’t trust some other “seating depth setting”, unless your parameters are exactly the same.
Regards
Juha
ps. When comparing this setup together with PTG Eley ESP reamer chamber (with Eley ammo), what has quite small clearances, this example has much way bigger freebore (Picture #8)
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Just think of this from another point of view. Freebore volume what ESP reamered chamber has, could you shrink Lapua chamber freebore down to the same? If that small volume freebore works on ESP, would it work on Lapua?
If you only focus on compare volumes, it is not necessarily the same thing. Maybe carbon ring needs its length (and height), whatever they are.
