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Old June 8, 2012, 11:54 AM   #1
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bullet seating depth vs accuracy

I am from the old school of seating bullets. I always run the bullet to the lands and back off a touch. Will I have a Savage model 12 that seems to have a long throat and can't seem to seat bullets out far enough to touch the lands. So I worked up a couple loads that shoot under 1/2 MOA then tested same load with different seating depth loads. Its interesting how much difference there is( hope the picture works)
Please note my measurements from 1 thru 5 should read
1 2.280
2 2.300
3 2.320
4 2.340 Best depth 1/4"
5 2.360
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Last edited by homesick; June 8, 2012 at 02:10 PM.
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Old June 8, 2012, 03:24 PM   #2
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I don't shoot the .204, but SAAMI lists the COAL at 2.2- 2.26.

You're way the heck out there... you mentioned that you're not even getting close to the lands even when you're loading up to 0.1 longer than max length?

Something doesn't sound right. Are you using a OAL gauge and bullet comparator?

I don't see how you can seat those bullets that far out and still have enough in the neck for good, consistent tension. It's usually recommended that one cal dia. , minimum, of the bullet be seated.

But, hey, if it works...
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Old June 8, 2012, 03:49 PM   #3
Brian Pfleuger
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I have a theory that you should find the seating depth your bullet likes BEFORE you tweak the load.

Someday, I'll get around to testing it.

The idea came from discussions with UncleNick, might well have been his idea, I don't remember now.

Least ways, the theory is that the load you find is based on barrel time. Once you have the load and then start messing with seating depth, you are simultaneously changing barrel time. Two variables, hard to know what's happening.

So, start with seating depth using 90-ish % charges of Trail Boss. Trail Boss burns almost instantly and at pressures much lower than normal rifle loads. Both characteristics reduce barrel vibrations and decrease the importance of barrel time. Tweak the seating depth until you find what works best and THEN do the normal charge sequence. Only one variable, should be easy(er) to isolate.
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Old June 9, 2012, 08:16 AM   #4
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Brian, that sounds like an interesting way to do load development. It would be interesting to see if you could shoot fewer rounds that way than settling on charge weight first and then adjusting seating depth.

Machine guns are awesome until you have to carry one.
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Old June 9, 2012, 09:47 AM   #5
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My old way to develope a load was to find max seating lenght back it off a touch and start working out loads.
Factory ammo bullets are seated at 2.255 / my loads are at 2.340 and I am not touching the lands in fact 2.360 does not touch lands. However at 2.360 the bullets in the case are not stable. ( less then 25% in case neck).
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Old June 9, 2012, 04:23 PM   #6
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Of course it was UncleNicks idea Peetz,, he's a walking talking thesaurus on anything GUN!
Now back to potatoes,,, I think the OP might be making to large of OAL jumps in his testing, most of my concoctions have been tested at like .005 increments until success is achieved, be it tighter groups, or groups closer to Zero.
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Old June 9, 2012, 05:00 PM   #7
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Just my way, I prefer to back off from .025 OFF LANDS AND develop the charge. Sometimes you cant get bullets that long and you just have to develop powder charge. I believe MOST guns you can find a bullet weight/type and then find a powder. may have to change around powders but I believe you can play with charges starting low and working up and most of the times can find that one who can punch nearly a single ragged hole at 100 yards.
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Old June 10, 2012, 08:25 PM   #8
Bart B.
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Folks winning matches and setting records at the longer ranges seat bullets used for single round loading a bit long so they set back a few thousandths when chambered. Then work up a load that shoots the most accurate. It's usually the same load their previous barrel chambered for the same round used. This helps the bullet align a bit more perfectly with the bore when it's fired. And it seems to correct somewhat for bullets that are a bit crooked in the case, too. Bullet runout can be as much as 1% of the caliber (.003" for 30 caliber, .002" for 22 caliber) and excellent accuracy happens.

If the ammo has to feed reliably from a magazine, then whatever the longest OAL lets that happen does just fine. But ammo's got to have pretty straight necks and bullets to do well accuracy wise when seated back off the rifling; the more they have to jump to the lands the straighter they have to be.

Note that if you don't shoot at least 20 shots per test group checking this out, you may get the wrong answer. Most any jump distance can yield the smallest group with 5 or less shots each are fired.
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Old June 12, 2012, 05:12 PM   #9
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I am just gona start over I pulled every thing off the rifle gona start from scratch. I bought some more brass and bullets. The rifle shoot excellent before I started trying to IMPROVE it. I have one 1/4" group / 5 shoot groups at 5/16s and several 7/16s " groups. Then I started to mess with it and the accuracy went to pot. I shouldn't expect more then that from a stock rifle.
The good part is its like having a new rifle in the rack
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