Seating Depth Questions

[JackEllis]

I'm working up a load that uses 100 grain Remington Core-Lokts with IMR 4064 in a .243. The most accurate powder charge seems to be 34.5 grains so I've been adjusting seating depth in in hopes of closing the group size a bit. Assuming this is readable, here's what I found yesterday:

OAL, Average Velocity, SD, ED, Vertical Spread (inches)

2.63 2678 26 71 1 14
2.64 2669 13 29 1 1/16
2.65 2641 43 93 1 1/4
2.66 2710 18 44 1 7/16
2.67 2717 10 23 1

The measured length to the lands for this bullet in my rifle is 2.72" using a Hornady gauge

These were shot in order of increasing OAL, decreasing order of production (longest were made up first), and the last two (2.66 and 2.67) were shot a few hours after the others when I was tired, so I will probably shoot them again because there's a fair bit of horizontal dispersion.

First question: Is it reasonable for MV to increase a bit as OAL increases? Makes sense to me but I'm interested in other opinions.

Second: Is it safe to load these to 2.722 assuming I can still get the bolt closed? I'm thinking so but want to be sure before I start experimenting.

These are hunting loads so I could stop now and be satisfied. They're intended as pronghorn/small deer/coyotes and practice rounds.

[bazineta]

MV depends on a lot of factors, including barrel heat, so it's in general something that is hard to predict.

So far as being safe, probably. However, being that they're hunting rounds and you have a very good SD there with the last one, I'd personally stop before I was wondering if I could get the bolt closed; with hunting rounds you are of necessity going to be unloading quite a lot, and it's super annoying trying to get a stuck bullet out of the lands and getting spilled powder out of the action and magazine.

[bsumoba]

First question: Is it reasonable for MV to increase a bit as OAL increases? Makes sense to me but I'm interested in other opinions.

Assuming you do not change powder, it is generally going to decrease in velocity as you seat it out (assuming you are not in the lands already). Reason being, as you seat out the bullet, you are reducing the pressure in the case, which results in lower MV.

Higher Pressure = Higher velocity....generally speaking.

Second: Is it safe to load these to 2.722 assuming I can still get the bolt closed? I'm thinking so but want to be sure before I start experimenting.

The moment you start going into the lands or touching the lands, you increase pressure. I would back off the powder charge if you decide to go into the lands. My recommendation if these are hunting loads, jump the bullet and don't go into the lands. It would make for a crappy day by sticking a bullet in the bore while ejecting a loaded round while in the wilderness and dumping powder everywhere in your action.

[LynnJr]

If you are developing a load you should always start at your longest length and work shorter.
That means into the lands on a single shot and magazine length on a gun that uses one.

You won't find any top shooters starting way off the lands and working into them.

[Dark Hunt]

With a small jump to the lands the pressure will peak higher because the bullet has less room to accelerate before it hits the lands.

That's why pressures are higher when you seat into the lands. Even though the combustion chamber is bigger due the the bullet being seated out, the pressure climbs faster (and peaks higher) trying to start the bullet engraving from a dead stop.

[LynnJr]

With a small jump to the lands the pressure will peak higher because the bullet has less room to accelerate before it hits the lands.

That's why pressures are higher when you seat into the lands. Even though the combustion chamber is bigger due the the bullet being seated out, the pressure climbs faster (and peaks higher) trying to start the bullet engraving from a dead stop.

Actually if you seat deep into the lands pressure drops off again.
It is the zero jump to around 0.020 into the lands were you see a modest pressure increase on the order of 3000 psi. If your into the lands deeper than 0.020 it starts to fall off again as the engraving has already gotten sufficient enough. This is for jacketed lead core bullets with a 1.5 degree or 3 degree included angle.

[bazineta]

I'm envisioning folks hauling around benchrest rifles for hunting, and it's frankly making me tired just thinking about it.

[LynnJr]

I hunted with a Benchrest rifle this morning without getting tired.

[JackEllis]

My .30-06 sometimes feels like a benchrest rifle when I'm lugging it around in the bush.

Thanks for the helpful suggestions. I haven't fully processed the data yet but it appears that a longer OAL gives me a few tens of FPS speed bump and groups that are acceptable for killing coyotes, pronghorns and small deer. My groups aren't benchrest quality but that may be a function of the bullets and the shooter.

I have two or three lifetimes worth of these bullets and may do some more testing with a different target.