Seating depth and Bearing Surface?

[Shoalwater]

So, I am at a bit of an impass with my next build. I am going to be building a 280 AI for my next rifle, this rifle will be primarily used for deer size game out to 1000, with most shots well inside that. I am putting together some dummy rounds to send to PTG to have a custom reamer made. I plan on shooting the 150gr Nosler LRAB's primarily out of this gun. The LRAB's have a huge boat tail at .250" which means to get .284" of bearing surface in the case it pushes the base of the bullet well beyond the shoulder neck junction, chewing up valuable case capacity. If I seat it to match the bullet base perfectly with the shoulder/neck junction I only get .100" of bearing surface in the neck. Is this going to be enough bullet in the neck to survive recoil in the magazine? I am using just standard 280 AI RCBS dies, not bushing dies. Thanks for the help. The first picture shows a bullet that is seated with .284" of bearing surface in the neck, the other picture shows with just .100" seated.

 

[FearNoWind]

I can't help but wonder why you restrict yourself to only two possibilities; .100 and .284. I'd expect that there would be other possibilities between those two extremes. Or am I missing your point?

 

[Shoalwater]

Most people always seem to recommend seating a bullet at least one times the bullets diameter. So that is where I come up with .284" seating depth, the .100" seating depth is what puts this particular bullet right at the shoulder/neck junction, which is where I would like the bullet to be and still give me maximum case capacity. I know it can be seated less, more, or anywhere in between, Im just wondering if the bullets will stay in place during recoil with just .100" of the bearing surface seated in the neck.

 

[kcebcj]

I would seat it with at least .284 of bearing to start for a hunting rifle. There are many variables that determine just where you want the bullet to end up. If you run all the seating dept scenarios in a program like QuickLoad you will see what works the best for what you are trying to achieve with that bullet. You don't want run out to be a problem or the bullet to move once seated.

 

[FearNoWind]

I would seat it with at least .284 of bearing to start for a hunting rifle. There are many variables that determine just where you want the bullet to end up. If you run all the seating dept scenarios in a program like QuickLoad you will see what works the best for what you are trying to achieve with that bullet. You don't want run out to be a problem or the bullet to move once seated.

There ya go ....
What he said.

 

[Shoalwater]

I understanding seating and how to go about it, I guess what I am really asking is what do y'all feel is the minimum amount of bearing surface necessary in the case neck for a hunting rifle? Keep in mind that I know my seating depth will be dictated by where my particular barrel likes them, I am just trying to decide where to seat the bullets for the dummy round to have my custom reamer made. In other words, I don't want to throat the barrel where the ogive is touching the lands with only .100" of the bearing surface in the neck and everyone here says you really need to have at least .284" of bearing surface in the neck for a hunting rifle. That would leave .184" off the lands which in my experience most bullets shoot better closer to the lands. It all makes since in my head what I am trying to convey, just isn't coming across in my words for some reason.

 

[Bill Johnson]

That would leave .184" off the lands which in my experience most bullets shoot better closer to the lands. It all makes since in my head what I am trying to convey, just isn't coming across in my words for some reason.

That's not exactly true. In a standard 280AI chamber, you have .284 from the chamber mouth to the beginning of the leade. The distance from the bullet ogive to engaging the rifling is probably a lot less than .284, bullet seated so you have 1 caliber in the neck.

Two reasons to keep 1 caliber minimum. One so recoil or rough handling don't change seating and second, you also have to take into account the maximum cartridge length allowed by standard magazines, about 3.350".

That said, the 375 Ruger has with a neck length of only .305. That is 81% of diameter, and that on a hard recoiling rifle. I've seated bullets so I've only had about 75% in bearing but it's just not good to recommend the practice. So you might compromise and set your minimum seating depth so that you have .227 bearing surface in the neck. That's 80% (if my math is correct).

You're having a custom reamer made, so seat one that way and have the reamer made so that it puts you .010 off the lands. Most rounds truly do not like the bullet in to the lands and many perform their best as much as .050 or even way more off the lands. That way you're maximum cartridge length guarantees at least 75% of 1 caliber in the neck, even if you seat to the lands.

 

[7magcreedmoor]

Shoalwater, if it helps, I just finished load development on my 280AI, using berger 168VLDs and Retumbo. PacNor made the barrel with the Nosler/SAAMI spec reamer. I tested 6 COALs, starting 0.015" off the lands (which happens to be magazine length in my Savage 111) and shorter by 0.040" intervals back to SAAMI. Final best accuracy load is max COAL and half a grain over book max charge of Retumbo, which doesn't compress the powder. I wouldn't be worried about losing case capacity to the boat tail. Velocity is 3024 by chrono out of a 26 inch barrel. Doing the COAL test loads, the best group was 0.338", the worst (SAAMI length) was 0.911" @ 100 yards. I will be waiting to hear how you fare with the ABLRs.

 

[Garycrow]

The argument that seating the bullet deeper takes up case capacity is one I've always had a problem with. Case capacity is what it is, the bullet moves so you're not really sacrificing capacity by seating it deeper. As soon as you touch off the round the volume of your case is no longer set by the bullet seating depth so I have a hard time accepting that it's of any consequence when it comes to how much pressure the round will generate. True you can get a bit more powder in the case by seating it out farther but most loads aren't at 100% density anyway so the extra little bit of powder space doesn't come into play.

Given my druthers I'll always take the shorter throat for a couple of reasons. First is the desire to have as much bearing surface as possible in the neck, and second is throat erosion as the rifle is shot. With your second dummy round it won't be long before you can't touch the lands at all while still having the bullet in the case, not a good place to be at. If I were having a custom reamer made I'd pick your first dummy round and have it throated for that. If the throat turned out to be too short it's a simple matter for any competent gunsmith to lengthen it with a throating reamer. You can't put the material back after it's been removed.

 

[Edd]

The LRAB's have a huge boat tail at .250" which means to get .284" of bearing surface in the case it pushes the base of the bullet well beyond the shoulder neck junction, chewing up valuable case capacity.

What is the diameter of the small end of the boat tail?

 

[Shoalwater]

@7magcreedmoor what did your COL end up at with the 168gr Bergers?

@Edd as best I can measure it, the diameter of the boat tail seems to be about .225"

 

[7magcreedmoor]

@7magcreedmoor what did your COL end up at with the 168gr Bergers?

@Edd as best I can measure it, the diameter of the boat tail seems to be about .225"

Subtracting out the length of my comparator body, my base to ogive is 2.791", and that for my throat is a 0.015" jump. This is also magazine length in my savage 111 DBM. The test groups were 0.338" @ 100yds. The next longer test length (+0.040") grouped 0.366" @ 100yds. Even the "worst" group of the COAL test process was not horrible, @ 0.911", and the average of ALL the lengths tested was 0.660".

 

[Shoalwater]

@7magcreedmoor Pac-Nor makes good barrels, I have one on my Savage in 6.5 Creedmoor. With that length how much bearing surface of that 168 is in the case neck? I may just go with a standard reamer if you can seat that far out with a standard reamer.

 

[7magcreedmoor]

First contact of bullet/neck is 3.029", -2.791" final dimension = 0.238" of inside neck contact with bearing surface. Not quite the sacred "one caliber"'s worth, but hey, it seems to shoot anyway.