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When a jump is jam! Or not?

 
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  #1  
Old 10-06-2011, 06:19 AM
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When a jump is jam! Or not?

I've been measuring AOL and jump to the lands with a Stoney Point AOL gauge and a Stoney Point Comparator from when these were Stoney Point.

I followed the instructions and what I'd read and suggestions. I thought I was doing a pretty good job of it.

Using the AOL tool with the modified case I arrived at a "dead length" number. I even allowed for headspace variance between the modified case and a fired case (again measured with a Stoney Point tool). I tried checking for marks from the lands after cleaning up the bullet with steel wool and sooting them up with a candle etc. I had no real joy with the last two methods, but I was pretty confident in my measurements.

I then had an idea to try marking the bullet some other way to look for marks from the lands. I figured if I painted the ogive with a thin (not that it is probably possible to do it THAT thin) layer of dark nail varnish.

On this basis I found that what looked like a jump was probalby a jam. The nail varnish marks easily and you can see when you are right at the jacket, or simply scraping marks in the paintwork (which is probably a really tiny bit of jump).

I do realise that the nail varnish has some thickness and also that all that we really do with these tools / methods is to arrive at a starting point.

The point is that if you work on what others tell you about a jump, or a jam, and how well a bullet shoots there, you may be doing something totally different from whoever measured it and is sharing this recommendation with you. And either one, or both of you may be wrong.

Granted, some of you are probably really good at this, but I just thought I'd share what I found. I've now checked most of my measurements and found my "dead lengths" or "zero point" to be a slight jam.

What shot best before will still shoot best now, but it's worth knowing where you are.
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Old 10-06-2011, 08:30 AM
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Re: When a jump is jam! Or not?

There is no doubt that when using these tools, the weak link in the machinery is the individual using the tool being able to feel the point of "contact" when the bullet touches the lands and grooves. I use an eraseable magic marker instead of nail varnish, but we are trying to do the same thing.

I have never tried to load anything to "touch" the lands, I either go into them at least 0.010 or jump roughly the same amount. Not because I'm not sure where they are, but they seem to work better at those distances. I do test seating depth separately from load as my final stage of load development, but varying 0.005 increments to see if I can tighten a load up even more...

good post; its good to see others are taking some of the same steps that I am in looking for that best load.

JeffVN
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Old 10-06-2011, 08:33 AM
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Re: When a jump is jam! Or not?

Thanks for sharing, good info. I use black magic marker which has a little thickness to it as well. I will try the nail polish next time and compare.
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Old 10-06-2011, 10:00 AM
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Re: When a jump is jam! Or not?

The first step I do with a new bullet is define where it touches the lands and the easiest thing I've found is to just give the bullet a little turn with some fine steel wool, you can pick up the slightest shiny spot from the lands touching. Much easier to pick up than marker and I seem to get more consistent number when checking a few times.
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Old 10-06-2011, 11:13 AM
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Re: When a jump is jam! Or not?

I think that all methods of measurement have drawbacks. I do not feel that a particular method is as important as the understanding behind its use. Every bullet in every box and every chamber is an odd-ball and should be treated as such. Every measurement I read about is unique to its self! -- A very large grain of Salt!
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Old 10-06-2011, 11:20 AM
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Re: When a jump is jam! Or not?

It can be measured EXACTLY.
Just NOT with a Stoney point/copies approach..

But all that matters is where the gun shoots best, and like JeffVN mentioned, just 'touching' is likely the worst. Either jam or get off it.
(that's what she said)
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Old 10-07-2011, 12:27 AM
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Re: When a jump is jam! Or not?

Thanks for the replies.

I agree with everything posted.

By the way, the steel wool polished bullet and the magnifying glass never seemed to work for me, but I know others have had success.

I think what I was headed at was to say that a reference point is great, but don't take it at face value that you are jumping or jamming as you may be wrong. There are many loads that are taken as "trite", like this or that bullet shoots best jumped so much, or jammed so much. If you try to duplicate that you may not be!

Mike, tell us your method for determining the exact number please, if you are happy to share it.
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