Home Forums Hunting & Shooting Ask The Gunwriters Speaking of .270 Win loads

After looking thru the "Old school" loads for the .270, I was looking at my loads using the 150 Hornady Interlock FB. Now I've always loaded that to 3.260"-3.265", where the cannelure is about 1/16" from the case mouth[in front]. Never any pressure signs of any type. Accurate as I could ever hope. 57.0gr H4831sc. Now, I look thru my latest Hornady manual and it says the OAL for the 150 Interlock is 3.210"! WTF!? Can you guys explain where this difference comes from. BTW, older Hornady has that bullet at 3.285" OAL. Hodgdon site has it at 3.285". Geez, I need a brew. Thanks guys.

Interesting.....seems that's a question best sent to Hornady.....

That's just the OAL they tested their loads at. Don't sweat it, if you seat bullets out further you'll have slightly less pressure as long as it's not crammed into the lands.........

Actually, seating rifle bullets out further results in a little more pressure. But as you noted, not much as long as the bullet's not into the lands.

???? I thought that increasing capacity by seating bullets out further slightly lowered pressure (as long as you don't jam them into the lands). Didn't you have a rule of thumb for increased case capacity vs velocity vs pressure and the like?

Brian Litz agrees exactly with what I was saying:

"Effects of Seating Depth / COAL on Pressure and Velocity
The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well-known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space."

http://www.bergerbullets.com/wp-content/uploads/2013/03/COAL.pdf

Do you disagree with him? If so why?

It depends on how close the bullet is to the lands, but in general pressure drops as the bullet is seated further from the lands--but only to a certain point. When bullets are seated deeply enough to really compress the powder and reduce powder space, pressures start to rise, but according to the results of the experiments I've seen, this doesn't occur until the bullet's seated very deeply, say 1/4 to 1/2 inch from the lands in a typical centerfire rifle throat.

You can also see this when chronograph. The typical scenario is to start with bullets close to the lands, but if accuracy isn't what's desired, then the bullet gets seated progressively deeper. This almost always results in lower velocities than when the bullet's seated close to the lands, indicating lower pressures.

I think we were just referring to two different variables. Variable 1 is that larger volume means lower pressures for a given powder charge. Variable 2 is the amount of Freebore. Both can have their own effects on pressure. :-)

John,not exactly sure what my distance is to the lands, but, I've marked my bullets with a sharpie, carefully chambered them. No marks, other than normal bolt cycling. Nothing "felt" tight. With this load that I've mentioned above, over the screens, avg MV is 2880+/- a couple fps. Seems my OAL is ok, just can't figure Hornady's difference in OAL in their manuals. Guessing I'm good, perhaps over thinking this. Just seemed a bit odd. Have they[Hornady], changed their design of this bullet somewhat?

Here are a few examples:

.223 Remington 700, 50-grain Ballistic Tips and 28.5 H335:
Seated .001 from lands: 3490 fps
Seated .002 from lands: 3472 fps
Seated .003 from lands: 3472 fps
Seated .005 from lands: 3412 fps

6mm PPC, 65-grain Berger and 30.0 Benchmark:
Jammed lightly into lands: 3514 fps
Seated .002 deeper: 3498 fps
Seated .003 deeper: 3457 fps

.270 Weatherby, with 130-grain Nosler E-Tips (a long bullet) and 80.0 grains of Ramshot Magnum. The results are little different, because of the freebore:
Seated so rounds just fit in magazine: 3386 fps
Seated .05 inch deeper: 3359 fps
Seated .10 inch deeper: 3427 fps

Here you can see the velocity drop with the .05 deeper seating, but then rise again at .10, because the powder room's being compromised.

Have seen a couple different bullet-seating depth experiments done in pressure labs, and they both showed velocities and pressures dropping initially, as bullets were seated farther from the lands, then rising as bullets were seated much deeper.

Thanks, that's a perfect example of the 2 variables.

And I still think the OP doesn't have anything to worry about. :-)

rifletom,

I just went to my library and checked the cartridge OAL for the .270 with 130's in Hornady manuals. The one that shows 3.275" is the 3rd edition publish in 1970.

The 4th edition (2000) lists a 130-grain OAL of 3.180".

The rest of mine Hornady manuals, the 5th through 10th editions, all show the 3.210" length.

One guess is the test barrel was changed several times during this period. The first barrel may have had a longer throat, but the later tests may have all used barrels chambered with the same reamer.

Another possibility is they standardized on a slightly shorter OAL length due to introducing more models of 130-grain bullets, with different ogive lengths. The 3rd edition only lists one, the Interlock Spire Point, but the 10th edition also lists three others--the SST, Interbond and GMX.

Had to reread this three times.. Not sure how one could load a bullet 1/2" from the lands. 1/4" maybe depending on the bullet. One would have to put the bullet way into the case depending the exact bullet. Crimp grooves would not be in the equation.

He was shooting the 150's......

Sorry, my mind (or fingers) somehow switched to 130's--but the 3rd and 10th editions list the same OAL for both weights--which is the difference he's talking about.

Thanks Mule Deer! This is a great example of how seating depth can affect pressure. I didn't know about the increase in pressure when a bullet is seated very deeply so I learned something today.

I was thinking about this some more, and it seems like the example of a rifle cartridge showing more pressure when the bullet is seated very deeply is consistent with the fact that pistol cartridges show higher pressures when the bullet is seated deeper. The difference might be due to the fact that the case of a pistol cartridge is very small, so even a small increase in seating depth would cause higher pressure, whereas in a rifle, the bullet would have to be seated much deeper in the larger rifle case before it shows a pressure increase.

I'm not sure how to account for the initial decrease in pressure as bullets are seated deeper in rifle cartridges except that presumably the effect of the bullet contacting the lands is much greater due to higher velocity. It's interesting to think about at any rate.

John, when you mentioned different "models" of the same weight bullet over the years with possible changes in ogive lengths, this makes the most sense. Not to mention differing testing barrels. I'll stay at my "older" length of 3.260", as it has worked for a number of years. Thanks all.

bowmanh,

The effect of seating depth in handgun cartridges is accentuated by the differences in how handgun and rifle powders burn. Handgun powders are usually "degressive" burning, meaning they start out burning fast, then burn slower as the bullet moves through the bore. This results in a faster rise in the pressure curve when bullets are seated deeper, because of the charge being confined in a smaller area.

Centerfire rifle cartridges are primarily "progressive" burning: They start out burning relatively slowly, then burn faster. This results in a slower pressure rise as the bullet starts down the bore, flattening the pressure curve.

Very interesting. This leads to one more question. In rifle cartridges, what is the primary factor causing the decrease in pressure as the bullet is seated further from the lands (ignoring the situation where the bullet is seated very deeply)? Is it related primarily to the effects of the bullet hitting the lands or to the way rifle powders burn? Or perhaps some combination of these plus other factors?

Deeper seating allows the bullet's velocity to increase more before it hits the lands, and the higher velocity (and hence momentum) allows the bullet to engrave more easily, without as much build-up of pressure.

This is the same basic reason the longer "freebore" throat in most Weatherby rounds tends to flatten the pressure curve: The bullet has more room to increase velocity before hitting the lands.