Potentially stupid gunsmithing question

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Racalman
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Potentially stupid gunsmithing question

#1 Post by Racalman »

It may be a daft question but I'm not proud so will ask anyway kukkuk

If a rifle chambered for .308Win causes the bullet from a magazine fed round to jump by 120 thou is it possible to modify the chamber to reduce the jump to 20 thou or is a new barrel required?
Duey
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Re: Potentially stupid gunsmithing question

#2 Post by Duey »

I would suggest you first try loading bullets to SAMMI spec regardless of the jump to lands and see if they group in your rifle
It is not possible to reduce a chamber size, at best, if there is sufficient metal you can get the barrel removed, the chamber cut off then rechambered with a tighter spec reamer but most likely it will have to be a new barrel
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Re: Potentially stupid gunsmithing question

#3 Post by kennyc »

Racalman wrote:It may be a daft question but I'm not proud so will ask anyway kukkuk

If a rifle chambered for .308Win causes the bullet from a magazine fed round to jump by 120 thou is it possible to modify the chamber to reduce the jump to 20 thou or is a new barrel required?
as above, what makes you think the rifle won't group as is? or would be better with less jump?
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Re: Potentially stupid gunsmithing question

#4 Post by Laurie »

As my old wood and metalwork master used to say to us hamfisted boys back in the 60s ......... "Alaways bear in mind if you take too little wood/metal off, you can always take some more off to attain correct dimensions, but if you remove too much, the result is only fit for the scrap bin and a 'Fail' rating." I can still remember the results of a tenon or some such jointing woodwork exercise that saw two pieces of wood joined at 90-deg, where there were some pretty undersize concoctions turned in, but the class idiot's exercise looked like two twigs stuck together, he'd planed so much wood off trying to get the pieces 'true'.

If you have 120 thou' bullet jump in a 308 and that's a result of round count / wear to what started as a modest freebore, you need a new barrel period - unless it still shoots well enough for you. You'll lose significant velocity though and that's a major issue if using factory ammo. If it's a low round count barrel that was chambered that way from new, you either live with it or get a gunsmith to chop a bit off the rear end ('set it back' to use the proper phrase), re-thread and rechamber it with a shorter freebore. In any event, bear in mind that a very long FB set-up reduces chamber pressures and MVs significantly and that you can safely use much heavier powder charges in handloads. If you are a handloader though, you have to remember that if and when the rifle is rebarrelled, these charges will produce pressures way over maximum and safe levels in the new barrel with a shorter FB.
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Re: Potentially stupid gunsmithing question

#5 Post by Laurie »

Oh ........ and I should have added that if it is a factory rifle such as some Remington 700s that come from new with massive freebores (anti-litigation initiatives), just live with the situation or rebarrel with a quality match version. Trying to set a hammer forged factory barrel back and rechamber it is rarely worthwhile, and some gunsmiths won't even try because of the risk of reamer wear or damage from cutting the very hard steel in these barrels.
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Re: Potentially stupid gunsmithing question

#6 Post by DaveB »

I used to have a friend who shot a lot. He went through a barrel every 18 months. He decided to start buying his barrels over-length. When the throat started to erode too much, he cut off the chamber and had it rechambered. He bought them long enough he could do that twice and still have reasonable velocity from the shorter barrel. Then they became tomato stakes.
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Re: Potentially stupid gunsmithing question

#7 Post by Laurie »

DaveB wrote:I used to have a friend who shot a lot. He went through a barrel every 18 months. He decided to start buying his barrels over-length. When the throat started to erode too much, he cut off the chamber and had it rechambered. He bought them long enough he could do that twice and still have reasonable velocity from the shorter barrel. Then they became tomato stakes.
That practice was pretty common in the USA when 6.5-284 was the norm in F-Class and other long-range disciplines. People found that if the expected life was 1,100 rounds, a set-back and rechamber at around 850-900 rounds would prolong the life to maybe 1,500. With gunsmithing costs as they are in the UK plus the cost of a re-proof, it's more cost-effective for most people to simply shoot the barrel out and replace it.
Racalman
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Re: Potentially stupid gunsmithing question

#8 Post by Racalman »

Thanks everyone for the info. It's a quality barrel with only 1500 rounds through it but I didn't measure the jump when it was new.

I wanted to explore the effect of varying the jump on ES but I can't load them to more than magazine length (2.9").
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Re: Potentially stupid gunsmithing question

#9 Post by Duey »

The other option is to single load and forego loading from the magazine to get a load that preformed better

To be honest I have a 308 remmy with a sporter barrel that had exactly this problem I couldn't get a decent group with anything from the magazine. I put up with this for a while and single loaded using a cheak riser that carried bullets, however this is my stalking rifle so I went to see Norman Clark and after a little discussion I decided to get it setback and had a palmer spec chamber cut. Still have it as my stalking rifle as it's nice and light to carry around and I load slightly below SAMMI spec from the mag and get good groups when I do my bit

It all depends what you want from the rifle and what your prepared to put up with in whatever you use it for

Just as an aside the timing on the barrel is out and the front sight fixings are no longer on the top of the barrel, not an issue for me as it's a scoped rifle and as I've had this rifle in this configuration for 15 yrs it never will be
Racalman
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Re: Potentially stupid gunsmithing question

#10 Post by Racalman »

Duey wrote:The other option is to single load and forego loading from the magazine to get a load that preformed better
I could certainly try that to see if reducing the jump makes a difference but it's not a practical solution as I use the rifle for snap shooting where a high round count and rapid reloading are needed.
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