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Jump to the lands and pressure spikes

 
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  #1  
Old 12-01-2006, 12:38 PM
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Jump to the lands and pressure spikes

I have a stock grade 308 varmiter which has the typical long throat. I have read this is purposly done by the big gun companies to limit the possiblity of pressure spikes from rounds "jammed against the lands". Does this pressure spike occure anyway even when off the lands or is it proportional to the jump distance? Also, since I am looking for accuracy, what is the recommended (general) jump distance.

Thanks

Mule
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  #2  
Old 12-01-2006, 01:27 PM
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Re: Jump to the lands and pressure spikes

General jump distance. No such thing, each gun and caliber is a beast unto its own.

However, couple real general rules you can go with.

VLD type bullets generally do not like a lot of jump and often like to be seated in the lands as much as .030. that often means that for pure accuracy they are too long to use in a magazine.

Non VLDs and Sierras in particular generally like to be off the lands as often as far as .050.

Barnes TSX bullets are designed for a jump of .030-.070.

You pressure will normally less off the lands vs jammed in.

With the 308 and Rem varminter you should start with the SMK 175 and see if it will not shoot well enough for you. I think you will find something there with the bullets set to max AOL to fit in the mag.

BH
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  #3  
Old 12-01-2006, 04:55 PM
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Re: Jump to the lands and pressure spikes

[ QUOTE ]
"... I have read this is purposly done by the big gun companies to limit the possiblity of pressure spikes from rounds "jammed against the lands". Thanks

Mule

[/ QUOTE ]

This does not cause a pressure spike. Most accuracy loads have the bullet contacting the rifling. I have found that the difference in velocity can be from nothing, to an increase of 30 to 50 fps - this does NOT represent a pressure spike.

It's an old wives tale who's death is long over due.
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  #4  
Old 12-01-2006, 09:26 PM
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Re: Jump to the lands and pressure spikes

Thanks Catshooter

I suspected the answere and am glad to see that folks are patient with these questions on this website. Sometimes I like to ask questions even though the answere may be apparent because there are a lot of newbies whom want to ask them but might not and thus the "old wives tales" are perpetuated.

Now a follow on question.

If you have a long throat and seat the bullet out to touch the lands, is there any adverse effect from reduced contact area of the bullet and the case neck (assuming less than full length contact in the neck)?

Thanks

Mule

Thanks again

Mule
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  #5  
Old 12-09-2006, 09:03 AM
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Mule, the pressure does go up somewhat......

as you seat the bullet closer to the lands.

There are several issues here. First, if you work up your load jumping 0.050, then use that same load with the bullet touching the lands, you may see pressure signs if your origional load was right on the boarder line of being a max load.

The main issue is that most rifles prefer a certain amount of distance from the lands to touching the lands, and every gun is different.

308's are so accurate, I don't think that you will have much trouble finding a very accurate load.
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Old 12-09-2006, 01:42 PM
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Re: Jump to the lands and pressure spikes

[ QUOTE ]

Now a follow on question.

If you have a long throat and seat the bullet out to touch the lands, is there any adverse effect from reduced contact area of the bullet and the case neck (assuming less than full length contact in the neck)?


[/ QUOTE ]

Again, "an old wives tale"...

It is often said that the bullet MUST be in the case at least one bore diameter (for WHAT reason???).

Not true... I have several bench rest grade rifles that have the bullet seated less than 1/4th of a bore diameter, and they shoot in the 2's (under .3") all the time. One of them has the bullets seated in the case 0.08"

My 300WM match rifle has the bullets seated 0.14" in the case... that's less than 1/2 of the bore diameter.

.
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  #7  
Old 12-10-2006, 09:14 AM
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Re: Jump to the lands and pressure spikes

Mule, No such a thing as "Old Wivies Tale". On www.benchrest.com do a search on "Jam seating the bullet" read the posting and they will explain how to calulate jam seating depth and the different type of bullet jamming and what can cause a pressure spike in a factory chamber, neck thickness,neck tension etc. It's alittle bit more than just Jamming a bullet. You don't need to be a member as you can just click on search might take afew hours to read all the posting. Good thing is you get to see posting from Berger Bullets to Dave Tooley to F-Class shooters to just varmit shooters so it covers all. Well good luck
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