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Question on jump

 
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  #1  
Old 09-08-2011, 07:53 PM
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Question on jump

First let me start by saying i have just taken delivery on a custom rifle from a very reputable smith (Beautiful rifle) in 280 REM.Now to the point, Upon delivery i measured the throat using a modified case and a 7 mm 160 accubond bullet.To my surprise i came up with 3.165" using calipers with a .284 comparotor .that gives me a C.O.A.L of 3.740" now to fit in magazine i load at 3.482" C.O.A.L. My Question to you is this .258" jump excessive? I mean the bullet touching lands is not even seated in the case.
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  #2  
Old 09-09-2011, 12:18 PM
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Location: Allen, TX
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Re: Question on jump

For starters, we commonly like to get the bullet near the lands, but rifles may still shoot well with a good bit of jump.

Nonetheless, it sounds like you have several issues.

(1) The Hornady modified cases aren't consistent. You should compare the length from case head to shoulder datum line of the modified case to a once fired case using the proper Hornady headspace adapter for your caliper. ..or, similar tool. The modified case headspace measurement is often much shorter than SAMMI and/or you fired brass. So, you may want to factor that into your measurements.

(2) If the 160 AB touching the lands isn't even sitting in the case neck, then something seems awry. ...excessive throat, freebore, and/or headspace

(a) You should ask your smith what he was using for a reamer and/or if he intentionally extended the throat/freebore

(b) You might try a longer bullet such as the 180g Berger provided your twist is fast enough.

(c) Freebore isn't necessarily bad, but it will lower pressure and velocity and require more powder to get the velocity back up.

(3) If after working up a load that's accurate, you determine it won't fit in the magazine...

(a) use the rifle as a single shot

(b) look into the Wyatt's or similar extended magazine well

Hope some of this makes sense.
-- Richard
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  #3  
Old 09-09-2011, 01:12 PM
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Re: Question on jump

+1 on what Richard said.

Additionally, if it is the throat.......personally I'd ask the smith to re-do it.

The Boltface to lands distance is only gonna grow longer as you break in the barrel. It'll grow even longer after a few trips to the range and more cleaning. They obviously never get shorter.

It may drive tacks as it is, but you'll probably quickly get to a point where it won't. Some rifles really do like some bullets close to or touching the lands, some are less picky. I'd feel really shortchanged if they gave me a long throat (designed for a bullet that I can't shoot because the twist is too slow) and then I had to re-chamber after only 1000 rounds............some competition shooters normally re-barrel this often, but it's not necessary in any type of hunting rifle.

Throat em short for the heaviest bullet we intend to use is my preference. Sure, any rifle may shoot better with a little jump, but nearly 3/10's inch seems excessive to me.

I've had a couple of factory 25-06's (1 remington 1 ruger) and I was able to have 115 Nosler ballistic tips (fairly long bullet for the caliber) touching the lands and still had plenty of magazine space. As you probably know, the 25-06 is basically the same case as the 280.

IMO, the long throat is only gonna leave you with a re-barrel or set back job that much quicker.
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Last edited by SBruce; 09-09-2011 at 01:16 PM.
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  #4  
Old 09-09-2011, 04:29 PM
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Re: Question on jump

Yeah who's plan was this? Yours, or your gunsmith's?
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  #5  
Old 09-10-2011, 11:37 AM
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Re: Question on jump

Quote:
Originally Posted by Mikecr View Post
Yeah who's plan was this? Yours, or your gunsmith's?
Mikecr,

Now you've opened the can of worms...
Bad plan? No plan? Or, incompetent smith?
Either one sounds expensive.

-- richard
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  #6  
Old 09-10-2011, 01:03 PM
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Re: Question on jump

Not bashing the OP, as this is pretty common.
It's an example of why 'custom' does not always mean 'better'.
With this, custom often means 'departed from gunbuilder design'.
The gunbuilder being factory, or another gunbuilder, which is NOT a gunsmith.

GunSMITHs fix guns, they don't design them (unless they are also gunbuilders).
Most of the better gunsmiths stay busy & don't have time to properly design, build, & test new designs.
GunBUILDERs focus on just that, and are not machinists or stockmakers(unless they happen to be).
Most of the better gunbuilders stay busy and don't have time & resources to be the best at making all things individually.
Stockmakers make stocks..
Machinists make parts..
Bulltsmiths make bullets..
Scopemakers make scopes..
Etc..
Bringing it all together into a proven design, and building guns to plan, is what gunbuilders do.

So many shooters go to the nearest gunsmith & have him make what they 'want'.
And while it's likely neither is knowledgible in designing guns, the result good or bad will be referred to as a 'custom'..
Most start off as donor Remington actions (gunsmith's friend), and merely depart from gunbuilder design.

Oops, that's a rant..
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  #7  
Old 09-10-2011, 02:56 PM
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Join Date: Apr 2010
Location: Allen, TX
Posts: 2,608
Re: Question on jump

I wasn't intending to bash either.

Your smith may be a builder/designer/engineer. I don't know.

Tell him about the issues and he may already have a solution.

It's a non-trivial challenge to measure everything correctly with precision instruments when you're holding the rifle. Troubleshooting over the internet is just going to get you a bunch of opinions based on whatever data points you happen to provide.

-- richard
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