Is back-thrust the same for all .223 based cases? (edited for typo)

[darkroomdan]

Loading manuals mention that pressure for .223 size cases (6mm TCU, 6.5 TCU and 7mm TCU) are all based on the MAP of the .223 Rem. round. And I have read that back-thrust can be thought of as basically peak pressure times cross-sectional area. In other words, the .223 based cases can run higher pressure than say 30-30 or .444 based cases because the smaller diameter reduces the back-thrust. A 7mm TCU runs close to the same velocity in several bullet weights as the 7-30 Waters while using about 1/3 less powder, but running at higher pressure. Both are supposed to be OK in a Contender.
My question is: does bullet weight and diameter have any effect on back-thrust. Will 7mm TCU firing 120 grain projectile have the same back-thrust as a .223 Rem. firing a 55 grain slug - both being loaded to the same pressure? If you start pushing a 140 -154 grain slug to max book velocity does the back-pressure increase as the bullet weight goes up? Apparent felt recoil does.
Been pondering on this awhile...
Thanks,
DRD
(edited for typo .223 instead of .233???) drd

[Steve P]

In my opinion, the aspect of pressure to address is chamber pressure.  As the pressure increases in the chamber, the brass is expanding and gripping the chamber walls.  A lighter load may have less chamber pressure and actually create greater pressure on the frame (your "back-thrust")  because the brass it not gripping the frame walls as much as a little heavier load would.  Your heavier bullets and hotter loads will create greater chamber pressure and become harder to extract.  You may end up having primers backing out and becoming cratered or even penetrated by the firing pin, but I believe you are well passed leveling out the pressure curve against your breach face. 

I would be more afraid of a low powder charge using heavy bullets trying to get low velocity from a .223 case than I would be of attempting to get max loads per published load data.  For example, I have had more 7TCU head separations using military brass with low velocity Unique loads than I have had using near max loads of WW748. As chamber pressures increase, the case expands gripping the chamber wall even tighter.  The "back-thrust" pressure cannot increase any more.  It may not have peaked, but rate of pressure increase is certainly levelled off.

I may be wrong on the "back-thrust" pressure leveling as the chamber pressure goes up.  I look forward to other opinions.

Be safe.  Use book loads.

Steve 

[quickdtoo]

Here's Dan Lilja's opinion on bolt thrust.

http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htm

Bolt Thrust
Bolt thrust is easy to calculate. Only two inputs are required. They are peak chamber pressure in PSI and as mentioned, the inside area of the case head that the gas pressure can work on. The formula then is:

THRUST=AREA*CPSI Where:

AREA=3.1416*(HS/2)^2

HS=the diameter of the inside of the case head.

I sectioned some cases and measured the inside diameters and found that they were as follows:

CARTRIDGE CASE INSIDE DIAMETER (HS)
222 .300"
PPC .370"
308 .385"
MAGNUM .420"
378 WBY MAG .500"
50 BMG .680"

The thrust is measured in pounds per square inch.




And a related article by Varmint Al...

http://www.varmintal.com/a243z.htm

Tim

[onesonek]

As you stated, pressure and casehead size are the factors with frame thrust. If those  remain the same going to more bullet weight will only increase recoil. Recoil and back thrust are 2 different things, at least thats how I unfderstand it.

Dave

[darkroomdan]

Thanks for the replies/info. And thanks for the formula Tim. Pressure and inside diameter - how 'bout that... And since the .222 and .223 share the same case head size, the inside diameter should be the same, right? I had also read that the "blown-out" and more straight 7mm TCU allowed the case to better "grip" the inside of the chamber as it expanded during firing and so reduced the amount of back thrust some.
Ah well, interesting stuff. Since I'm using loads from the manual, all should be good!
Thanks again,
drd