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Calculating OAL

 
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  #1  
Old 12-27-2012, 03:33 PM
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Calculating OAL

Hi Folks,

this is my first post so be gentle ....

I have been reloading .308 for a few months now and have been sticking to reloading manuals for OAL. I have just tried to use an empty case with the neck sized down to hold the bullet (.308 Sierra HPBT Match #3156). I start the bullet in the case OAL is about 2.970 at this point - i then lock the bolt down (full battery?) and eject the round - i believed that the lands should push the bullet back giving an accurate OAL as I have seen in videos - but nothing happened ...... is this normal? The rifle is a Howa 1500 varmint which i bought new earlier in the year.

Any help would be great

thanks

Andy
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  #2  
Old 12-27-2012, 04:36 PM
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Re: Calculating OAL

Your rifle's chamber may well have a longer throat than normal.

Are you sure it's a Sierra 3156 bullet? Their web site lists the 155-gr. match bullet as a 2156. And that bullet will not reach the lands in many commercial sporting rifles even when barely held by the case neck.

Here's what you can do.

First....Seat a bullet in an empty sized case, then chamber it. Push a small rod down the bore until it stops against the bullet tip. Mark the rod at the muzzle. Pull the rod out then measure the mark to the end of the rod that stopped against the bullet.

Second....Remove the bolt, point the rifle muzzle down then drop in one of those bullets so it falls into the chamber and stops against the rifling at the front of the chamber. Put something in the chamber to hold the bullet firmly in place, then put that rod gently back down the muzzle until it stops against the bullet. Mark it at the muzzle.

Third....compare the difference between the two marks. That shows the difference between the seated bullet's tip in the test case and where the bullet actually first touches the rifling. It may be 2/10ths of an inch between the two marks. But at least, it'll let you know the situation.
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  #3  
Old 12-27-2012, 10:11 PM
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Location: Mountians of SW NC, near Asheville
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Re: Calculating OAL

OAL causes lots of confusion new guys and quite a few old hands....that should not be. And there is NO way to "calculate OAL", chambers vary too much for that. There are three ways we can establish an OAL for rifles.

First is the max length that will feed and function through the magazine OR go by factory length - that's the SAAMI standard length.

Second is book OAL length and all that means is the length the guys who developed the listed data used in their rifle. Our's ain't theirs so ....

Third, we can start somewhere off the lands and develop the best shooting lenght for our rigs. There's no need to be highly precise about where the lands are because it's not going to be exactly there anyway. Most factory rifles are going to shoot best with a bullet jump to the lands from about 20 thou to as much as five times that much. ONLY experiments on the range will tell you what works best with YOUR rig and YOUR load!

A lot of us agonize over getting a specific amount of jump from seated bullet to lands contact and that demands a bullet comparitor/gauge that references off the bullet's ogive. Fact is, we can do quite well with our experiments by simply measureing the actual OAL unless the bullet noses are highly variable - most aren't that bad. There's a range of seating that will work well and it seems to run from 10 to 20 thou wide for most rifles so try to find where the 'good' range is and seat in the middle of it so small variations won't matter much, if at all.
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  #4  
Old 12-28-2012, 04:32 AM
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Re: Calculating OAL

Correct Bart the Sierra is #2156 155 Match, Thank you both for such great answers I will try the process with the rods and see what happens

thanks
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  #5  
Old 12-28-2012, 09:46 AM
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Re: Calculating OAL

Note that as the barrel wears out, the lands will start further away from the bolt face. Such is life as steel erodes away from hot gasses. .308's erode their lands about 1/10th inch over 4000 or so rounds; depends somewhat on the peak pressure of a given load and the powder type used.
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  #6  
Old 12-28-2012, 10:15 AM
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Re: Calculating OAL

I agree with posted except the 10-20thou 'range' of seating for optimum performance.
With every gun I've worked with, including factory, best seating depth represented a huge change in the gun's performance. Grouping is sometimes outright halved.
Through further testing from this point(wherever it is), the biggest window I've found to hold this performance has been around +/-3-5thou. Far tighter than 10-20thou.

I have not had to keep chasing normal land erosion. Once that initial best seating depth was determined, I've never needed to adjust it again. Just seat very accurately to that exact same depth and everything holds.
A qualifier to this though; I don't continue shooting barrels past their peak. It ain't worth it, new barrels are cheaper..
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  #7  
Old 12-28-2012, 06:09 PM
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Re: Calculating OAL

Thanks for the comments, I have tried dropping the bullet into the chamber and then measuring etc - it seems that the bullet loosely pushed into the case is 2.955 oal and the same head dropped straight into the lands gives a read of 2.990
- I had been loading these to 2.775 - I have also heard that the Sierra 2156 is a bit picky about the rifles it works in and I was advised to use the 2155, I was thinking of moving up to 168 match.

any tips?
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