Some rifles reach max pressure sooner

[bboswell]

I have always known that not all rifle share the same max load which is why we work up. I have recently come upon an extreme example which made me curious. Both are custom rifles in 280AI shooting 150g LRAB's with same powder and primer.

Rifle 1 - Brass is Nosler factory brass.

Max load is near the minimum charge from Nosler but at the max per Sierra.

Rifle 2- Lapua 30-06 brass fire formed.

Have not reached any pressure signed yet while near Nosler max and a couple grains over Sierra max. But at same charge as rifle 1 it was 150 fps slower at America charge.

I expected that this meant the Lapua brass had more capacity, nope measured and Lapua has 1.4g less water capacity than Nosler brass.

So, what exactly are the deciding factors that makes one chamber pressure up faster than another?

 

[bigthicketboy]

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[Tjay]

You need to use the same brass in both rifles for a valid comparison to eliminate brass as a variable.

I have a 300 RUM that is at max pressure with Nosler brass at starting loads and with R-P brass does not get to max pressure until max loads are reached. Same powder, primer and rifle only change is the brass.

Brass and barrels can change pressure as well as powder lots and primers, also temp and humidity can play a roll.

 

[SJB358]

The difference in brass isn't enough to make too much difference, but the throating, chambering, any freebore, etc will all add to the equation. That is a great example.

Any idea if the reamers were the same that chambered the two rifles. Also, what sorta barrels are on each of them?

Great example. I wouldn't have expected that much difference at all.

 

[bboswell]

The difference in brass isn't enough to make too much difference, but the throating, chambering, any freebore, etc will all add to the equation. That is a great example.

Any idea if the reamers were the same that chambered the two rifles. Also, what sorta barrels are on each of them?

Great example. I wouldn't have expected that much difference at all.

Different reamers. Freebore is similar but I will have to refer to my notes to provide exact distance to lands for each.

Rifle #1 - Nosler brass used in a Schneider Barrel that shoots with 0.040 jump to lands.

Rifle #2 - Lapua brass shoots in a Bartlien that likes 0.120 jump to lands.

 

[SJB358]

The difference in brass isn't enough to make too much difference, but the throating, chambering, any freebore, etc will all add to the equation. That is a great example.

Any idea if the reamers were the same that chambered the two rifles. Also, what sorta barrels are on each of them?

Great example. I wouldn't have expected that much difference at all.

Different reamers. Freebore is similar but I will have to refer to my notes to provide exact distance to lands for each.

Rifle #1 - Nosler brass used in a Schneider Barrel that shoots with 0.040 jump to lands.

Rifle #2 - Lapua brass shoots in a Bartlien that likes 0.120 jump to lands.

Gotcha, that is pretty interesting though. Always learning new things around here.

 

[Dwh7271]

The difference in brass isn't enough to make too much difference, but the throating, chambering, any freebore, etc will all add to the equation. That is a great example.

Seating depth will also change pressure, at least reading the pressure curves it does.
Primer change can influence it along with temperature.
Occasionally a barrel is slightly over specs (some specialty shooters order them that way) which also would change what you see.
Caliber is diameter measured from land to land (bullet sizing also) while bore size is less. For example a 243 might have a bore of .236. If that bore is originally a thousandth larger, the pressure should be lower on comparable rounds than the .236 bore.

Edit: the above was just an example, most 243s are .237 bore

 

[PJGunner]

"Caliber is diameter measured from land to land (bullet sizing also) while bore size is less. For example a 243 might have a bore of .236. If that bore is originally a thousandth larger, the pressure should be lower on comparable rounds than the .236 bore."

Interesting concept. That might even explain the disparity between one of three rifles I have chambered to the 7x57. Loads that are perfectly normal and OK in two of the rifles are way over the top for rifle #3 which is a custom with match grade barrel and chamber.
I did a test for the three where I extrapolated data so as to duplicate the original fairly low pressure of 1993, a 175 gr. bullet at 2300 FPS. Rifles numbers one and two were in the ball park at 2310 and 2330 FPS. Number three however ran 2450 FPS. The number three rifle had a one inch longer barrel FWIW. I don'y believe it would account to the increase in speed.
Maybe, just maybe you might have solved the puzzle of rifle number three.
Paul B.

 

[SJB358]

If you can find pure lead sinkers roughly a little larger than the Bore you wanna slug I usually oil them up good and push them through with a wooden dowel or a good, coated cleaning rod using the Bore guide. I've done it for my 338, 30-06 and 45-70. It helped explain why my 338 took some extra powder in order to even get close to book speeds since I actually measured .339 in the old barrel.