Book length or at the lands?

[The big greasy]

I'm shooting a tikka 223 with varget and a Sierra match king 77 grain. I can load into the lands or should I load at the book length. Which in general is better accuracy? I see alot of ar rifles shooting these bullets and they are restricted to mag length and are getting good accuracy. So was just wondering what your thoughts are. I know shooting will tell it all, just was thinking.... Book length and lots of jump, or hardly any jump. Thanks.

 

[Harvey_NW]

If you're not limited by mag length I would start with bottom edge of bearing surface at the neck shoulder junction, or at least .030" off the lands to stay out of carbon fouling. In most cases with high BC bullets in decent quality chambers and barrels, seating depth doesn't have near the effect on accuracy people think it does. They just don't shoot large enough samples to confirm.

 

[The big greasy]

If you're not limited by mag length I would start with bottom edge of bearing surface at the neck shoulder junction, or at least .030" off the lands to stay out of carbon fouling. In most cases with high BC bullets in decent quality chambers and barrels, seating depth doesn't have near the effect on accuracy people think it does. They just don't shoot large enough samples to confirm.

Thanks. I've loaded up duplicate rounds from low to high and was planning on doing two ladders at around 500 yards with them. I've seated them to about 25 thou off the lands.

 

[Pharmseller]

I start with the bullet just barely touching the lands, then work backwards in 0.04” increments. I always find accuracy within 4 iterations.




P

 

[Andouille]

If you're not limited by mag length I would start with bottom edge of bearing surface at the neck shoulder junction,

Is the idea here that the bullet would be maximally supported along the bearing surface while providing more powder capacity than if the bullet was seated beneath the neck/shoulder? I'm trying to envision whether the pressure generated by the powder has a different effect on a bullet protruding within the case body versus a bullet with base flush with the neck/shoulder. For instance, a 200 gr bullet seated at COAL for a 300 WSM would have a lot of bullet inside the case body. My gut feeling is that it doesn't matter as the bullet has essentially no give and can only be pushed in one direction - out.

 

[Harvey_NW]

Is the idea here that the bullet would be maximally supported along the bearing surface while providing more powder capacity than if the bullet was seated beneath the neck/shoulder? I'm trying to envision whether the pressure generated by the powder has a different effect on a bullet protruding within the case body versus a bullet with base flush with the neck/shoulder. For instance, a 200 gr bullet seated at COAL for a 300 WSM would have a lot of bullet inside the case body. My gut feeling is that it doesn't matter as the bullet has essentially no give and can only be pushed in one direction - out.

Pretty much. In my mind it optimizes case capacity and neck tension, personal preference really. I haven't noticed any effect at all on the opposite though, I have a 25 Creedmoor that didn't have near as much throat as I expected so the 133 Berger is seated pretty deep, it's my tightest shooting rifle.

 

[sveltri]

My Tikka 223 shot best when I was able to load longer than stock mag length. I think I ended up about .040 off, that produced better groups than jumping from mag length with the 77 TMK.

 

[Andouille]

Pretty much. In my mind it optimizes case capacity and neck tension, personal preference really.

I'm curious what your load density/case fill is with the bullet seated that deep and if that was a contributor to the tight grouping. My understanding is that velocity SD is minimized by having a case near 100% capacity, and a consistent velocity leads to less dispersion and smaller group size. But I realize we might be splitting hairs if we're talking the difference between 95 to 100% case fill; O would expect a perceivable velocity SD difference for a large difference like 80 vs 100% fill due to changes in how pressure builds.

 

[Harvey_NW]

I'm curious what your load density/case fill is with the bullet seated that deep and if that was a contributor to the tight grouping. My understanding is that velocity SD is minimized by having a case near 100% capacity, and a consistent velocity leads to less dispersion and smaller group size.

I doubt it. At 100 yards if there is an affect on dispersion from velocity I'd be hard pressed to believe you could distinguish it. I had a rifle I tested a couple powders in and got results from 20fps ES all the way to 125fps ES for 10 shots, both groups ended up about 1.3-1.5" with the same bullet. And there are a lot of loads out there that with a large sample size produce 50fps ES but hit MOA targets at long range. When you get a good combo that shoots it doesn't always pencil out perfect.

But I realize we might be splitting hairs if we're talking the difference between 95 to 100% case fill; O would expect a perceivable velocity SD difference for a large difference like 80 vs 100% fill due to changes in how pressure builds.

I usually start with a slower powder and try to get good case density, but I have seen sometimes that doesn't yield the best results. My 284 Win for example, shooting 165's I tested H4831, N565, and H4350. The 4350 had the best velocity by 100fps, produced the least dispersion right at 1", and had an ES of 35 over 10 shots with new brass. I think the effect on pressure and case manipulation by seating deeper is negligible in comparison to something like changing the powder and burn rate.

 

[Andouille]

I doubt it. At 100 yards if there is an affect on dispersion from velocity I'd be hard pressed to believe you could distinguish it.

Good point, a quick ballistics app calculation shows 0.15" difference at 100 yds for 300 WSM 165 gr TTSX at 2,900 and 3,000 fps. That means a potentially 0.3" group radius increase which would be lost in the noise for most reloaders such as myself.

 

[Vern400]

If I have a special situation where JAM is needed to shoot best I will go outside mag length.
I don't like to single load, and I PROVE the need with multiple large groups.

BUT. I do avoid that "just touching" place if I can.
Why? Tolerance and variation. It's easy to get in a situation where one cartridge has four or five thousandths of jump, and another is beginning to jam. That is the place where big fluctuations and pressure and velocity can happen. In short, you won't find it at 100 yards. You probably won't find it at 200 yd. But there is a place close to touching where your ES and SD will be notably higher. That will result in more vertical stringing at longer ranges.

So if that's where your gun shoots well it's probably best practice to find your jam cbto and then back off 0.010 or even 0.015. It becomes very important not to push the shoulder back too far when resizing. Because when the firing pin smacks the cartridge forward the shoulder clearance can allow a jam you didn't intend.

Of course the thing that matters is that you get statistically repeatable excellent groups. Just be aware that there is a pothole in the road very close to that "just touching" length.

Not all calibers really care a whole lot. But some of the 30 caliber cartridges like some bullets scary close to the rifling.

 

[49ereric]

I always started at book length and then longer until I saw groups change provided book length wasn’t the best.
some rifles you can watch groups tighten up and then open up when you get to far out.
mauser K98 is a classic example when shooting a 175 Sierra in a chamber made for a 196-198 bullet. Didn’t shoot good until I had only @17K of bullet in the case.

 

[N2TRKYS]

I always load to book COAL. I’ve found zero reason to ever deviate from that.