Question about Pressure and Case Capacity

[fullup3]

I've been reloading for a couple of years now, and load for several different calibers including .270, 35whelen, 7mm-08, .223, 6.5 Creedmoor, .243, and now I have just got started with the .260. I am making my .260 brass by upsizing new .243 brass in RCBS dies, and this leads me to my question....
How is it that in my .243 I can have a 105grain bullet and the max load for H4350 with this particular bullet will be lets say 39.7 grains of H4350 (this is mostly hypthetical, but is actually a load I shoot). I can then take this same .243 case and run it through my .260 dies to stretch the neck out to .260 and load a 120 grain bullet with 45.5 grains of H4350 with zero pressure signs. I don't really understand how I can add 15 grains of bullet, and at the same time add almost 6 grains of powder to the equation without any pressure problems in the same case. The only thing I've changed is the neck diameter and the bore diameter of the rifle? I guess that's enough to reduce the pressure enough? Can you guys help me understand this. I would imagine that if I went up 15 grains to a 120 grain .243 bullet, I would have to significantly drop the charge weight of powder?

 

[spclark]

You have it right when you factor in the bore diameter increase; the case isn't where the propellant gas volume is generated, the propellant is burning as the bullet travels the bore's length, so the gas volume's expanding at the same time the 'system' volume is too. Also the increasing surface area acts to cool down the propellant gasses, further reducing pressure build.

If you look up info on pressure curves compared in different rifles using similar cases (243, 260, 308 for example) you'll get a visual explanation of what's going on.

In your example another factor might be a difference in 'bearing surface area' (that part of the bullets that's in contact with your bores). The 120 might have less than the 105 which gives the 120 relatively less friction-generated resistance (and a reduction in force needed to swage the bullet into the rifling at the start) during it's travel.

 

[fullup3]

thanks for the reply. that makes sense....less surface area on the bullet and a larger hole for the gas to escape = less pressure. Got it!

 

[243winxb]

The bigger bore may like a faster burn rate of powder. The 243 likes IMR 4350/85gr, the 308/150gr likes IMR 4895. At least for me. ;D

 

[Steve Blair]

Your example shows a 14.3% bullet weight increase but an 18% bullet cross sectional area increase. The force on the bullets is pressure * area. The larger bullet has 18% more force propelling it and proportionately less weight.

 

[gunsandgunsmithing]

Look at it like this...Instead of looking only at the actual case capacity, consider the chamber area PLUS the area of the bore in front of the chamber to where peak pressures occur as the combustion chamber. Now the difference is more apparent. The larger bore has more area and is where most of the work is happening, leveling out the pressure curve by having more expansion area for the burning propellant. It's also why a larger bore tends to like faster powders, to get to the same peak pressures in a larger area. Hope this helps.--Mike Ezell

 

[savagedasher]

Case capacity is chamber pressure . Best example is lake city brass has less capacity then commercial brass . When loading you need to ALWAYS check it. Different brands can have big capacity difference some times even different lots. Always start low and work up. Myself i use Ouickload and still check the books. NEVER TAKE WHAT OTHERS SAY AS A FACT GoodShootingLarry