Loading different OAL 308 Win

[wvscotsman]

So I did some searching and did not find a specific answer to my question. So here it is.

I have a Savage Model 11 VT in 308 Win. 24" heavy barrel. I am working on hand loading for the first time for this rifle and any rifle. Some research shows me that a lot of bench rest shooters seat the bullet to where the jump to the lands are in the .015 to .020. I have used two methods to find the length that my ogive on this particular bullet hits the lands. I used a partially closed mouth on a bullet that will float and chambered it using the bolt. This gave me a consistent 2.810" OAL where the ogive is right against the lands. The next method I took a fired case and deprimed it. Set it up to where the bullet would move with some coaxing like the previous one. For this method though I drilled out the primer pocket to where a small diameter cleaning rod could be pushed through it. I then pushed the case into battery position. I used the rod to push the bullet out the mouth of the case until it struck the lands. This also gave me a consistent 2.810" in OAL where the ogive hits the lands.

Backing off .015 gives me a COAL of 2.795" as my starting point for these loads to see if this rifle likes the .015 jump

Sorry if I got to detailed on that. HERE is the question. How much should I drop my starting load charge now? According to the manual I am going by, my starting load charge of Varget powder for the 165 Grain bullet I am using should be 38.5 gr. The OAL in the manual is 2.75. Making the OAL longer increases the pressure so to achieve the same muzzle velocity of 2400 FPS I need to drop the charge but I am not sure how much I should be dropping it.

Any help or advice is welcomed. Thanks in advance.

[Jim Watson]

That is so far below the typical working load that OAL and jump to the lands are inconsequential.

The relationship between seating depth and jump versus chamber pressure is not simple or linear.

I doubt if a .015" jump increases pressure very much. I know I am loading at .012" jump with loads only a grain below the maximum.

[arizona98tj]

I was taught that given all things equal, increasing OAL (seating the bullet further out of the brass) will reduce pressure compared to a cartridge that has the same bullet seated deeper. That is why people worry about bullet setback in an autoloader....the bullet gets seated deeper while it is being chambered and so the pressure increases and you run the risk of KABOOM.

[Brian Pfleuger]

You don't have to reduce the STARTING load at all. That's why it's a starting load. It takes into account various difference between firearms and components. Considering all the other differences, a tiny change in seating depth is inconsequential, in this instance.

Plus, 0.015 jump is relatively large. Pressure grow exponentially as the bullet approaches the lands. Very little increase until you get very close and a rapid increase the last small distance.

Rifle rounds are generally the OPPOSITE of handgun rounds. Making the OAL longer creates higher pressure, shorter makes less pressure. Assuming that the reference point in the spot "in-between" where the pressure is a minimum.

[wvscotsman]

I plan on starting at .015 and trying some different jump lengths along with different powder charges.

Thanks for the answer guys. It seems I will be safe starting at the minimum load charge and working up a ladder from there.

Brian, would you recommend my starting jump maybe being .010?

Jim, thanks for the response and info also.

[arizona98tj]

Thank you Brian for the explanation. Much appreciated.

[arizona98tj]

Brian,
What is the reason for rifle and handgun pressures reacting the opposite of each other in regards to bullet seating depth?

[Brian Pfleuger]

Handgun cases typically have very little room in the case. Shortening the OAL typically reduces that space significantly, making the space that the powder begins burning in much smaller. A few thousandths change can increase pressure dramatically.

In typical bottleneck rifle cartridges, shortening the OAL doesn't change the space inside the case significantly.

For example, if I seat a Hornady 139gr bullet to 2.800 inches in a 7mm-08, the space left in the case is approximately 3.163 cubic centimeters. If I shorten it an entire 0.250" it drop to 2.903cc, a change of only 8.3%

If I look at a 125gr Hornady XTP in 9mm Luger seated at 1.169, the space for powder is 0.530cc. If I reduce that OAL just 0.069" to 1.100, the space is down to 0.418cc, a reduction of 18.6%.

Technically, shortening the OAL gives the bullet more "head start" before it contacts the rifling, which does, exclusive of other factors, reduce pressure. However, shortening the OAL also intrudes on powder space which, exclusive of other factors, increases pressure.

The final pressure change depends on which of the factors has a larger effect.

As I said, the change in pressure from moving away from the rifling tends to be relatively small except for the first few thousandths. So, backing the bullet even farther away doesn't decrease THAT component very much more. However, in handgun cartridges, that tiny change in pressure by being farther away from the rifling is MUCH more than off-set by the dramatic change in initial powder burn space.

[Unclenick]

Dr. Lloyd Brownell studied this for DuPont at the U of M in the mid-1960's. Below is a plot showing pressure change with seating depth he measured in a .30-06 with a round nose bullet. Touching the throat is at the left side of the plot. Dr. Brownell's theory was that as you seat deeper, you increase the amount of gas that bypasses the bullet between its release from the neck and the moment it hits the throat and obturates the bore, thus causing a stall in the initial rise in pressure. This stall can actually be seen as a small step in detailed plots. The reason there is a minimum is that when you seat still deeper beyond that point, the fact you are reducing the volume the powder starts burning in as you seat deeper then becomes the dominant peak pressure influence.



But here are a couple of things to keep in mind. A typical round nose bullet has a very gradual taper on the sides of the ogive before the hemispherical (or elliptical, in some designs) section of the nose starts, to allow it to seat out where pointed bullets do. This long taper causes very gradual change in the gap used by gas to bypass the bullet. If you go to a pointed projectile, the sides keep moving in radially, so the transition from a large to a small gap is more abrupt.

Below is an example of a short pointed bullet at work, reproduced with kind permission from Jim Ristow at RSI. It is two sets of strain gauge pressure traces made using his Pressure Trace instrument of the same 6mm PPC cartridge, bullet and powder charge in the same rifle. The three upper traces are with the bullet touching the lands, the four lower traces are with the bullet backed out 0.030" off the lands. The peak pressure difference, you can see, is about 18%. A 10% powder charge reduction will produce about 20% to 30% reduction in pressure, depending on the chambering and powder used. It is also the standard recommended starting load reduction, so it's a good number to use when starting in the lands.



In the Precision Shooting Reloading Guide (Precision Shooting Publications, 1995) the author of the chapter on precision loading, Dan Hackett, relates how, in changing bullets one day, he turned his competition seating die's micrometer adjustment the wrong way and wound up with 20 rounds loaded 0.050" off the lands instead of 0.020" off the lands, as he'd intended, before he noticed. The dogma he had subscribed to at the time said nothing over 0.025" off the lands would shoot well. Faced with pulling down the loads or just shooting them in practice, he opted for the latter. To his amazement, this gun (a 40X KS in 220 Swift), which had never before printed a 5-shot group smaller than ⅜” at 100 yds, gave him two ¼” groups and two bugholes in the low ones with these “incorrectly” seated bullets.

So, rule one, don't buy the dogma automatically. Berger bullets found out this is true especially for VLD designs. They used to recommend everybody jam them into the lands. But some of their customers could never get them to shoot like that. Based on customer experiments, they concluded that some guns want them set back as far as about 0.120" off the lands. They recommend trying jamming the lands, then trying them backed up in 0.030" steps to see where the gun wants them. You don't usually see the need for that much jump, but if you play with bullets enough, you find out that many of them actually have two seating depth sweet spots, with one a lot further back than you might suppose.

I recommend, if want to minimize the time involved in all this, that you don't make seating depth step adjustments smaller than 0.020" until you see signs of group tightening, then you can fiddle with finer adjustments as your time and inclination steers you to do, though I think you'll find the difference hard to discern below that resolution.

You use the standard starting load to begin with a bullet in the lands, but it helps to adjust it to best its best precision (smallest group) before you start moving bullets back. Dan Newberry's system seems to work well in identifying this. Keep an eye open for pressure signs while working up.

Use a chronograph to adjust loads at different seating depths to have the same average velocity. This comes close to giving them all the same barrel time (though those needing larger charges to reach the same velocity will actually have slightly longer barrel times), so it tends to prevent you falling into the trap of thinking you are finding the best seating depth when you are actually tuning the barrel time simultaneously. You need to get these variables isolated to know for sure which is causing what.

[arizona98tj]

Thank you both for your explanations. Much appreciated.

[wvscotsman]

Fantastic information!!! Thanks to everyone.