I am currently on the high end of pressures on my 7mm Mag. I know that if I go much higher on my IMR4350 the bolt gets sticky. I load 60.5 grains for a 160ab and shoot 2,950 on average with little variance. If I go to 60.7 my velocities jump to 3,000 and the bolt gets a fairly sticky (but no blown primers or anything like that).
I was wondering since I am on the higher end of pressures if it effects my barrel life and how many shots should I expect before the barrel needs to be replaced? After my first 400 or so rounds my chamber length went from 3.360 to 3.386. Is that throat erosion normal or excessive possibly from pressure?
As long as I stay at 60.5 I don't have any pressure issues so far. I want my MV 2950 at least and it originally grouped 1/2inch at 100. I did back down to 60.4 and velocies went down to 2930 with no differences. So I just stuck with it since that was my goal. I did try to push to 3,000 but that's when it gets sticky.
I full length resize. Not sure how much I am bumping the shoulders back. I have the die touching the shell holder then another half turn so I think I am good on that. I haven't measured how much it's been bumped.
Just a little worried I my be toasting my barrel a bit....
I could be wrong since I'm not there and you are. So, I'm only going by the information you post.
You say there are no signs of pressure, but then you talk about the bolt sticking. I was taught that a sticking bolt is a pretty important sign of too much pressure.
And even if it wasn't, who wants to talk about the one that got away because you couldn't open the bolt for a follow up shot? "...Yeah, but that first one I wounded him with was a smokin' 4200fps."
If you drop it to 60.0 grains and it shoots accurately and reliably at 2850fps, you won't see much difference in the trajectory curve and your brass will last longer.
Otherwise, try a different powder such as H4831sc. I loaded to 2900+ using 168g Bergers with this one and definately no sticking. But, the primer was just beginning to flatten around the edges. So, I stopped.
Yes. Extreme pressure (heat mostly) will wear on your throat faster than lesser heat. If you've already fired 400 rounds like this, it's a little late to worry about toasting your barrel. Either shoot the barrel out completely and replace it, or take it to a smith and get him to do a setback. If it's a factory barrel, I would wait to rebarrel when you feel the accuracy is no longer acceptable.
You need to look at a powder burn rate chart. but rel-22 or maybe 7828 will drive your 160 faster with less pressure. If i'm doing chrono load work up's i get to where the powder is finally so slow for a given rifle that the speed drops off and revert back to the slightly faster powder.
Barrel life is reduced with fast burn powders which induces a quick pressure spike.
heat and powder induced erosion are the real killers. On our small squad we always had the ammo carrier with an extra barrel for the belt fed. Not because .308 is powerful. heat would just take the rifleing out. I've looked down barrels that you could not hardle see any rifling left.
.270 Ackley improved 29 inch 1.250 dia. target barrel 7mm STW 28 inch 1.250 target barrel. I also love my .458 mag for varmints and the biggest game in the world.
Barrel life or throat erosion can be dependent on a number of things. The primary culprits are heat & pressure.
I don't see where an additional half grain of powder will make much difference in throat erosion. But if you are running into possible pressure signs, then that might not be the right question to be asking. Safety is always a top priority when reloading. Adding a half grain or so will probably not make much difference ballistically, but it could represent a substantial decrease in safety. For me, the minimal additional return is not worth the increased risk at those velocities.
As others suggested, changing powders may get you faster speeds at lower pressures, etc.
Your rate of fire can also have a huge impact on barrel life. Most of us will stop sending rounds down range long before our barrels get toasty. Firing rounds in a super-heated barrel will create more throat erosion faster than anything else I can think of. I always get a kick out of those young guys at the range who put a 30 round clip in their AR-15/M4 and then proceed to empty it as fast as they can pull the trigger. I suspect the silly grin that follows would be wiped away if they understood what they had just done to that barrel's accuracy.