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Jump to tthe lands?


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I have a 16" EA upper that I'm trying to work up some new 175 gr. SMK loads for.  Rather than measuring OAL, I'm not measuring to the ogive using the Hornady Lock n Load comparator tool.  This is the first time using the tool but I have previously checked OAL with Hornady's version of the old Stoney Point tool.

Anyhow, here's my question if you've gone to this extreme yourself.  How much of a jump are you measuring to the lands if you're loading to magazine length?  To stay in the 2.800 - 2.810 ballpark, I'm measuring about 2.225 to the ogive which seems to be alright based on what I've read on some other forums.

This works out to be a jump of about .115 to the lands.  I'm not surprised by this as it's actually a shorter throat than my old 700 had. 

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The Magazine will most likely determine the COL ,some will let you get away with a little more length above the 2.800" & some may jam in them mag. over the 2.800" .

The magpul's are a little generous , of course if your loading one round at a time this means nothing .

The chamber dimensions will also determine the bullet jump ( 308 vs 7.62 x51 ) or who made the barrel .

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Yes, I understand the OAL is limited by the mag, I'm using all factory Armalite.  That's not my concern, I'm just wondering if what I've measured regarding the throat is about average.  If I had to guess I'd say this chamber was probably cut with something along the lines of an M852 reamer by whoever supplied this to Armalite.  Sierras seem to be pretty tolerant to jumping and that's all I plan on reloading for this. 

It seemed like it wanted to be sub-moa with 175s and Varget, but then my $1k scope took a crap.  Yesterday was also the first try with a PWS PRC replacing the A2 hider.  A lot of recoil reduction but LOUD.  Swapped out scopes again and will be back at it for another try next weekend.

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Most do not even bother with that measurement because of mag. COL restrictions . Now with bolt action system you can work the COL .

I would say every bbl. chamber will be some what different , I use the bullet that shoots the best . May not be scientific, but once you find the right bullet & load , ya got it made , but of course you knew that .

If this was a bolt action forum , you would probably get all kinds of answers . With AR's you don't have a lot of COL options.

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  • 10 months later...

I just did a function test of some 168 gr. Nosler Ballistic tips, and had jamming. I noted the 2.8" OAL and ignored it completely.. But that seems to be the problem.

This batch was 2.860. 

  But the neat thing is during the function test, with 10 rounds fired, the fouling shot was wild about an inch, the following 9 cut a ragged hole the 3/4" center to center at 50 yards.

  I should have paid more attention to see if it was a magazine jam, or a chambering jam, but believe it to have been the latter.  It looked like a jam. 


  So I am going to play with seating depth... but as a bolt and break action guy, I want to seat to engraving, or just short of it. Old habit. 

  I don't want to seat away from the lands any more than I have to. 


I am thinking though that a bit of polishing in the frame there might allow a bit more room, and slick up the feeding. Has anybody here played around with that?

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I would really like a bolt action .308. One of those tacticool jobs like Desert Tactical. One good reason is so I can seat close to lands & have fun with all that stuff. But to worry about any of that for an AR doesn't get my nad, I don't get it.

It seems I can go 2.815" OAL with the pmag LR-20. But I keep it at 2.810" for my 168's. I tried seating depths for SMK'S, Amax's & Nosler Silver Tips & they all seem to like it most right there in mine. I do measure from Ogive's.

Edited by NoFail
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  • 3 weeks later...

I used to mess around with seating depth and bullet jump now I just load my pet load to mag length, shut the gas off, shoot single shot with -1/2moa results. Also when you do this, you can neck size only. I grew up shooting bolt rifles and this is how you turn your semi into a sweet a$$ straight pull bolt. I think it's just a matter of finding a good load that is accurate with that big jump rather than loading a round that takes away from the capability of the rifle.

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