Anyone have or know of information as how much the COAL affects pressure? Example: If COAL is per the reloading book, say 3.000, how much will it change if we shorten the coal by .010, .020. How much will it change if we lengthen Coal by .010, .020. Basic question is can you affect chamber to any degree by adjusting COAL? NO other changes made.

Yes. When you seat the bullet longer, you increased your internal case capacity. When you increase case capacity on a given powder charge, it will reduce pressure some. The opposite is also true. How much depends on the round, the bullet, and the powder charge.

If you seat the bullet out far enough where the bullet is engaging the rifling, you will begin to increase pressure at that point. The bullet into the lands will increase your initial shot pressure (the pressure needed to begin the movement of the bullet). Once you delay the initial bullet movement some, it's time you can not get back, and the powder burns at a certain burn rate. It should increase your pressure overall and cause higher pressure.

Also, many magnum rounds, like a 7 Mag or 300 Win Mag have data with bullets seated at 3.340" COAL, which is common max mag length on some rifles. But many rifles now have magazines that will accommodate up to 3.700" COAL. So, if you seat these long, heavy bullets out further, you gain 2 big things. You gain powder capacity to run a slower burning powder and more of it to get higher velocities (and often times run powder charges over listed book max). And you get the bullet closer to the rifling to aid it to shoot better. If you can seat the bullet out further, always do it, if the rifle will accommodate the added COAL.

This is also the problem I'm seeing on the 6.5 PRC right now. The 6.5 PRC has a standard length of 2.950" COAL. This is longer than a short action and shorter than a long action. Some rifles are being chambered on a short action (which is common for about 2.800" COAL), and modified to allow the 2.950" length. But the short action rifle is maxed out, and already at max length. The 156 Berger I seat beyond 3.000", and needs a long action to feed it properly. If you seat the 156 Berger to 2.950", the bullet has over .080" jump to the rifling. It will work, but you increase your pressure on a given powder charge when you have to seat it that much deeper. It slows down the round and has a longer bullet jump. It's all about the ogive of the bullet in combination with the COAL of each bullet and each round. When I load ammo, the most important measurement I use is the ogive measurement. This tells me where the ogive of the bullet is in relation to the rifling. I've taught many of reloaders about that to help improve their quality of their ammo.

Loaded my savage short action 7mm08 175's at 2.955 OAL. They are as long as the mag would hold/feed.

I read many places that if I had a blind mag or floor plate instead of a detachable magazine it would be even longer.

What Chad wrote needs to be a sticky.

I put a link to it in our library. (LINKS To Technical Reference Threads:) Thanks guys. Good info to share. Add a link to library anytime you see info like this.

https://texashuntingforum.com/forum/ubbthreads.php/topics/7630089#Post7630089

That's cool! Thanks!

I would have made it a sticky but we already have 5 stickys in Ammo &Reloading and we don't like to see to many stickys in a section. People have to scroll down.

That’s why I built my 6.5 PRC in the Long action.
Nothing particularly magical for me in having a SA over a LA

Chad,

your post can be copied and pasted into the new to reloading sticky. Measuring base to ogive and ogive to lands is a critical concept that most basic manuals do not teach. Good post.

Thanks guys!! The 6.5 PRC is similar to the 6.5x284 with the same issues on the heavy bullets. You have to seat it out long to gain powder capacity and get close to the lands and it best fits on a long action. There’s several rounds in 6.5mm that have had the same problem, like the 6.5 Rem Mag. The PRC is similar to a modern 6.5 Rem Mag without the belt (and a little bigger in size).

The 2.800” short actions are mostly R700 clones. The Montana Rifle Company M70 clone short action is 3.125”, which is very handy.