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How do bullet seating depth variations relate to accuracy? Does distance to the lands or changes in pressure/velocity have more to do with it? How much variation does it take to be noticable?
I can't say, with certainty, whether distance to lands or pressure has more effect on accuracy, but I know seating depth does, in general... and can also effect feeding reliability. I just suggest making various loads, within min/max cartridge overall length for your bullet profile, and see how it changes your groups. Are you talking pistol or rifle?
Every rifle is different. My 6.8 SPC AR likes loads as close to the lands as I can get them and still fit in the magazine. I'm loading up a bunch of 300 WinMag for my Remington 700, and I'll experiment with it. Some rifles don't like the long loads, they want short ones. You just have to experiment.
deeper in the case reduces volume and is supposed to increase pressure.
seating a bullet out far enough to contact the lands when the round is chambered is supposed to increase pressures
Seating a bullet deeper increases the distance between the bullet and lands and is supposed to result in decreased accuracy because the bullet may not be aligned as well when the cartridge is fired
The round has to both fit in the magazine and feed reliably. Too long and it may drag in the magazine or even hang up. Too short and it may lead to feed issues when chambering the round.
When I want to load for a single rifle (all chambers are slightly different) I first seat the bullet deep enough to fit into the magazine and feed out of it. Then I chamber the round and eject it. Do I see marks on the bullet from contacting the lands? If yes, I seat it a little deeper, put it in the box of freshly reloaded shells and adjust the die to seat the next bullet slightly deeper and then chamber it and eject it to look for land marks. I do this till I find a seating depth that leaves the bulelt just off the lands.
Yes, seating depth does affect accuracy and pressure. Pressure is mostly obvious, less volume = more pressure. However, odd things can occur with a specific combo which may result in higher peak pressures when jamming the bullet into the lands. I'd advise you do so reading on how to identify excessive pressure signs. There are tons of good Internet sources, but most are just indications...the best IMHO is case head stretching.
Accuracy is much less certain, but some rules of thumb can be found. As someone stated, every rifle is different and there is some truth to that. Two identical rifles may have very different pet loads. It's far more common with low quality firearms as there often are significant tolerance variances. The type of bullet you are shooting usually determines the best starting depths. For long range bullets there are basically secant and tangent ogives. If you care, Google it.
Bullets like the Serria Match Kings are tangent ogives and are much more forgiving of seating depth. It's one of the reasons they are so popular. IIRC, 0.050" is a good starting depth off the lands for these types of bullets, but they typically work at a lot of different depths. Secant ogive bullets like Lapua's Scenars and Berger's VLDs are much less forgiving of seating depth and typically like being jammed into the lands or just off of them. They offer better BCs, but can be finicky in some rifles. Berger has seen a few rifles that like their bullets "jumped" like the SMKs, but that seems to be the exception. Essentially the ogive of these different designs dictates how "straight" they end up going down the bore and that impacts accuracy. Tangents ogives tend to be "self-centering," but with lower BCs, while secants aren't and have higher BCs.
Lot's of "magic" and myth out there, but the only sure way to know for your rifle is to shoot them.
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6 posts • Page 1 of 1
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