Effect on cartridge pressure when seating a .308 bullet closer to the lands?

[MCMXI]

I'm breaking in a new barrel that I've had installed on a Remington 700 action. When I ordered the barrel from Kreiger, I specifically asked them to ream the chamber so that 168gr SMK HPBT bullets could be seated 0.020" to 0.040" off the lands and STILL fit in AI magazines for an AICS stock. Last weekend I measured the "optimum" OAL for the SMK 168gr bullet using a Stoney Point (now Hornady) OAL gauge. It turns out that if I seat the bullet 0.020" off the lands, the OAL is 2.865" (I actually use a bullet comparator gauge which measures from the ogive but that measurement would be meaningless here). Lyman lists the OAL of the 169gr HPBT at 2.775" so I'm seating the bullet 0.090" further out. I don't have a chronograph so how will this affect pressure and velocity? I'm using IMR4895 and the maximum load in the Lyman manual is 45.4gr (if I remember correctly) so I made up some loads with 42.5gr, 43.0gr, 43.5gr, 44.0gr, 44.5gr and 45.0gr of powder. I ran out of time and only shot three of the 42.5gr loads since I'm cleaning after every shot (barrel break-in) but they felt very anemic. Could this be due to the increased OAL? Is 0.090" significant here? Is it likely that the bullet seating depth will shift the entire load range up by + 1gr, + 2gr, + 3gr etc?

Thanks.

 

[Shoney]

Yes, there shouild be a pressure decrease by lengthening the OAL. I find it hard to believe it would be detectable without electronic equipment.

The formula that shows the relationship is:

pV=nRT

where

p is the absolute pressure [Pa],
V is the volume [m3] of the vessel containing moles of gas, here it is the volume you allow in the shell case
n is the amount of substance of gas [mol], here it is in solid form as powder until it goes bang
R is the gas constant [8.314 472 m3•Pa•K−1•mol−1],
T is the temperature in kelvin [K].

Think of it as:
Pressure in the case X Volume of case = two Irrelevant numbers RT (for our purposes) X the amount of gas n as being in solid form as powder until the "big bang";
1. the gas constant R remains the same;
2. the temperature T remains constant;
For practical purposes the equation should read PV=n

Your increase in volume is so small that the pressure decrease is also equally small.

 

[USSR]

1858,

As long as your bullet ogive is not contacting the leade, pressure should lower as you seat a bullet farther and farther out. That's alot of IMR4895, what brass are you using?

Don

 

[MCMXI]

Shoney, PV=nRT is the "ideal gas law" so I'm not sure that it can be applied here unless the gas resulting from the combustion of the powder can be modeled as an ideal gas. Also, n would increase as the volume increases.

Don, I'm using new Lapua brass all trimmed at the factory to the correct length (kind of). The load data is from Lyman, Sierra and Speer manuals for IMR4895 and the 168gr HPBT. The max load is 45.4gr (compressed) according to Lyman. The bullet is seated so that the ogive is 0.020" off the lands.

 

[NCsmitty]

.020 off the lands is what I like to use if the mag has the room. The situation that you have is a plus all the way around. I assume your talking 308 Win here, and the less intrusion into the powder space the better with that short neck. I'm quite sure that you'll experience a slightly lower pressure because of it. You may even be able to push the envelope here should you choose, once the break in is complete.
The Hodgdon site also lists 45.4gr Compressed as Max and @2.800 COL with the Sierra BTHP.

NCsmitty

 

[Shoney]

1858
Stop and think a second! What causes the pressure when the cartridge goes bang. The gas produced by the burning powder, which is n.

Since the gas n is constantly changing as the powder continues to burn[ and the volume V is constantly changing as the bullet moves down the barrel, the equation would still be valid, but if you need a mathmatical answer, it would be a series of the formula at points along the path of the bullet, and would be very complex to figure mathmatically.

 

[MCMXI]

I assume your talking 308 Win here, and the less intrusion into the powder space the better with that short neck. I'm quite sure that you'll experience a slightly lower pressure because of it. You may even be able to push the envelope here should you choose, once the break in is complete.

Yep ... .308 Winchester and I was thinking along the same lines as you that the pressure would drop somewhat.

Shoney, at what point after primer ignition is all (most) of the powder "converted" into gas and where is the bullet at that point? I'm probably completely wrong about this but I always thought that the powder serves one main purpose (like gasoline or diesel in the cylinder of a combustion engine) and that's to burn in order to heat the air in the case/cylinder so that it expands and pushes on the bullet/piston.

 

[Jim Watson]

Vihtavuori did some work on that. As I recall, without going to get the book, seating a bullet closer to the lands but not touching increases chamber volume and reduces pressure. Seating a bullet some farther from the lands increases the freebore effect with little resistance to bullet travel reduces pressure.

Seating the bullet into the lands increases the pressure by increasing resistance to initial bullet movement, seating the bullet a lot deeper increases pressure by reducing the initial volume available.

.090" is not going to amount to 1, 2, or 3 grains worth.

 

[Shoney]

1858
I'm not trying to be argumentative here, but I think you have been given bogus info on "the heat of the powder burning expands the air in the case, producing the pressure". If this were true, how do cases that carry compressed loads, i.e. no air, produce such high velocities?

The answer is that rapidly burning gun powders produce extremely large quantities of gas. One molecule of gun powder produces approximately 12 molecules of CO2 + 10 H2O molecules + 6 Nitrogen N2 molecules + 1 molecule of Oxygen O2.

Any qustions???

 

[Ridgerunner665]

:banghead::banghead::banghead:

SEATING BULLETS CLOSER TO THE LANDS DOES NOT REDUCE PRESSURE.

It increases it...if you don't believe me just pick up a Hornady 7th edition reloading manual and read pages 25 and 26.

Guys...some of you should be more careful about posting on this type of thing...you're going to get somebody hurt.

 

[Ridgerunner665]

The OP stated .020 to .040 of "jump"...and that is perfectly safe to do...

Here is what happens when a round is fired...normal OAL

The powder ignites and the bullet gets pushed through the "freebore" into the rifling...the freebore acts as a sort of "cushion".

When the bullets are seated longer (to the lands...or real close to it)...it generates higher peak pressure before the bullet gets pushed into the rifling because the bullet does not move as far...their is no "cushion"...pressure and velocity are both increased.

1858,

Just keep working up...you're doing the right thing. Pay attention to how your bolt feels when you lift it...when it starts feeling "sticky" or harder to lift...check the headstamps for anything that resembles these...



Thats ejector marks...if you see those...no matter how slight they may be...back it off 1 full grain since your using IMR 4895 and its winter time...that will be your max load.

FWIW...I'd recommend Reloder 15 and Varget...much better than IMR 4895.

EDITED TO ADD: 44 grains of IMR 4895 is about as high as I would go...that powder is pretty sensitive to temperature changes.
Be very careful...work up slow...and pay attention the the bolt lift (look at the headstamp of EVERY round you fire...not the primer...the headstamp.)

 

[Shoney]

ridgrunner665
I agree with you that putting a bullet into the lands increases pressure. They have a great deal of friction to overcome in order to enter into the lands, and a spike in pressure can be expected.

I respectfully submit to you that the OP did not ask about pressure when the bullet is seated into the lands. As I unserstood his question, he was asking what effect of seating longer than the book COL had on pressure.

The bullet that is seated longer than a book COL, but not closer than 0.02 to the lands, it has inertia as it enters into the lands; and at that point the inertial engergy ovecomes the friction and there is not great spike in pressure. That is the "cushion" that you are talking about.

When you are dealing with pressures from 0.040 to 0.020 off lands, there may be a slight increase in pressure, but I have not seen it in the .308.

To clarify, my general discussion that seating a bullet with less COL than book will produce a higher pressure, and seating a bullet longer than book COL (with the caveats of proximity of less than 0.04 off the lands) does produce lower pressures.

 

[Ridgerunner665]

I understand that the OP was asking about longer OAL...but he also has a Kreiger barrel with a short throat.

That does not leave much room for error....or room to "play with" so to speak.

From my notes:

308 20 inch factory Remington barrel

45.5 grains of Varget
Remington brass
CCI BR2 primer
168 Nosler Ballistic Tip
OAL 2.80
2,585 fps (average for 10 rounds)

Same load at mag length (2.82") gets 2,635 fps...and thats a factory Remington barrel with a throat like a giraffe.

It takes quite a bit of pressure to equal 50 fps...dangerous amounts??? No...but its still an increase.

 

[Shoney]

Ridgrunner665
Out of curiosiity, how far off the lands is the 2.82".

 

[Ridgerunner665]

.123 (nearly 1/8th of an inch)

 

[Ridgerunner665]

And from the measurements the OP gave...the throat in his rifle is .080" shorter than mine.

 

[MCMXI]

Shoney and Ridgerunner665, you're both providing some excellent information that I'm sure many folks here will find useful ... particularly me so thanks. Shoney, I appreciate the explanation on the amount of gas produced by the powder. As for the compressed loads, I figured there was some air in the case between the grains ... now I know I was misinformed on that one since your reasoning makes a lot more sense. Ridgerunner, I'll do exactly as you say re looking at the headstamp and keeping an eye on the bolt lift as I work up to the higher velocity loads. The original Remington barrel did have a very long throat and I couldn't seat the bullet even close to the lands and still have the round fit in the AI magazines. Kreiger used an Obermeyer reamer for the chamber. As for RL15, that's what I've been using this year for .308 and .223 but decided to use up some IMR4895 that I had from a while back. This weekend I'll be breaking in another Kreiger barrel on a Remington 700 chambered in .300 Win Mag. I haven't measured the chamber in that barrel yet but I will before the weekend gets here. I plan on the same 0.020" to 0.040" off the lands with a 190 grain SMK HPBT assuming it fits in the AI magazines so I'll check the headstamp on those cases too as I work up with RL22 powder.

Thanks.

 

[USSR]

Don, I'm using new Lapua brass all trimmed at the factory to the correct length (kind of). The load data is from Lyman, Sierra and Speer manuals for IMR4895 and the 168gr HPBT. The max load is 45.4gr (compressed) according to Lyman.

1858,

Yes, but that max load is more than likely with Winchester brass and not Lapua. Case capacity and powder charge go hand-in-hand in regards to pressure generated. Personally, without a chronograph to confirm velocities, I'd be disinclined to go much above 43.0gr of IMR4895 with a 168gr bullet in Lapua brass.

Don

 

[NCsmitty]

Good point about different case capacities between brands of brass USSR.
As always, differing opinions on an issue that would require specialized equipment to substantiate.
I will continue to load my ammo .020 off the lands to better center my bullets in the bore and go with the old fashioned procedure of watching for pressure signs as I develop my loads.

1858, you obviously have a handle on your wants and needs for your 308 and as previously mentioned watch for the obvious pressure signs.

NCsmitty

 

[MCMXI]

Don,
Both the Sierra (50th Edition) and the Lyman (49th Edition) use Remington brass.

I need to make a correction, the Lyman manual gives a load range of 38.0 to 42.5 grains of IMR-4895. The Sierra manual has 39.0 to 42.1 grains. My source for the 41.0 to 45.4 grains is from the Hogdgon web site as mentioned by NCsmitty. Since I was under the impression that seating the bullet further out, but NOT touching the lands, resulted in a reduction in pressure, my reasoning was that I'm still within published data. That said, I just noticed that Hodgdon lists the COL at 2.800" which would mean that my bullet is seated 0.025" further in! Now I'm really in trouble (confused) but luckily I've only shot three loads, all at 42.5 grains. I've attached a couple of photos of the heads of the three I shot along with three brand new, unfired Lapua cases for comparison. How do they look ... I'm not expert but I don't see any signs of over pressure?

I'm really beginning to see the benefits of having a chronograph. Without one I'm sort of in the dark ...

 

[Ridgerunner665]

No visible signs of pressure there...

And yes...a chrono is the VERY BEST $100 you will ever spend.

 

[USSR]

1858,

Yeah, I agree with Ridgerunner665, no obvious signs there. FWIW, I loaded 175SMK's in Remington brass using 43.6gr of IMR4895 for a buddy's rifle (Remington M700 with a throat WAY out there) without problems. I believe the COAL was 2.86" (had to be single-loaded), and they were still atleast 0.030" off the lands. Remington brass has more case capacity than Lupua, so I would proceed cautiously above 43.0gr of IMR4895 with Lapua brass.

Don

 

[MCMXI]

Remington brass has more case capacity than Lupua, so I would proceed cautiously above 43.0gr of IMR4895 with Lapua brass.

Don, thanks ... I have one of those RCBS kinetic bullet pullers so if I start to see signs of excessive pressure at 43.0gr I'll be banging away late into the night!

Ridgerunner665, thanks for looking at the photos ... I appreciate the extra pair of eyes.

 

[MCMXI]

Don,
I continued breaking in the new Kreiger barrels on the .308 Win and .300 Win Mag today which was slow going with shoot/clean for five rounds, followed by shoot/shoot/shoot/clean and then shoot/shoot/shoot/shoot/shoot/clean. I was using Sweet's 7.62 and noticed a big improvement after the first four rounds with the patch coming out very clean i.e. no copper. I've now managed to put a total of 15 rounds through the .308 and 8 rounds though the .300 Win Mag. Anyway, I wanted to ask if you thought the brass looked ok ... I shot seven loads of 42.5gr of IMR 4895 and five loads of 43.0gr of IMR 4895 (red mark on case). The bolt opened easily with all loads and the 43.0gr loads resulted in very good accuracy. The bullet is seated 0.020" off the lands, but the case is new, unfired Lapua brass so I wasn't expecting such good results this early. I'd appreciate your thoughts on the case head though. I found out that I can use a chronograph at my local range so I'll be ordering one of those soon.

Thanks.

 

[NCsmitty]

The cases look fine. Time to up the charge and watch for signs of pressure as before, and watch if the groups tighten or loosen.
Watch for the case necks smoking about 1/4 of the way down the neck, any more than that means the pressures still too low. If it's a match chambering, it will need to be even less. The higher the pressure the less smoking down the neck will occur. It's an integral part of my own case analysis to show that the cases are obturating properly for optimum gas seal.

The Kreiger barrels are near top of the line and will give you their best after about 50 rounds or so. It's appears to be coming in according to your group. Nicely done.

NCsmitty.