Dang it, I used to have a nice RSI Pressure Trace II system that could probably have answered the question about relative change in pressure. I've only been tracking velocity, but I guess I'm curious now as to what the pressure is doing. Although the Dasher is so dang short it would be tough to instrument it.
I will say though that my direct experience has been that pressure is increasing along with velocity. On my first barrel, before I was aware of this increase-in-velocity phenomenon, I built up my load in early development to what looked like a safe load. No overt pressure signs and velocities was where I thought it should be, right around 3050fps. I had built up to this fairly slowly, but like I said, no pressure signs. I developed the load over a chrono and then shot it a few times in practice
without using the chrono. I saw a few flattened primers here and there (velocity/pressure increasing), but I wasn't too concerned.
Then I started blowing primers. Like, a ridiculous # of primers. Shot after shot. And yes, I was new enough (and dumb enough) to keep shooting cause I wasn't sure what was going on. Luckily I'm shooting a Barnard which could take that kind of abuse, but I seriously DO NOT RECOMMEND that course of action to others. Me being stupid gives others the chance to avoid it. You start blowing primers, you've got a serious issue to deal with. Granted, at that point, my brass had been pretty stressed from the load I was putting on it. I ended up dumping nearly 80 pieces of brass with shot out primer pockets. After only 4 firings.
I took that load back to the range with a chrono and holy crap! I was well over 3100fps. All other variables were the same- it was the same load I'd been shooting for the past 100 rounds. But it had jumped from 3050fps to 3150fps. From slight (non-troubling) pressure signs, to serious pressure signs. That's when I started wondering about my barrel and started tracking my new barrels from shot 0 and shooting every practice round over a chrono.
So, while what WyleWD is saying makes sense conceptually, I have personally witnessed that pressure *does* increase over those first 150 shots or so of a barrel's life, along with the velocity. It would be cool to have actual measurements to back that up though.
deadlyswift said:
WyleWD said:
queen_stick said:
Jay,
There has to be a pressure increase to jump velocity up that much... is it caused by 'build up' in the bore (carbon, copper, etc..)? I'm thinking that something is causing a restriction on the bullet passing through the bore, causing the pressure increase. If that's what's happening (and I could be very wrong here) then I would think you would run into overcharge issues LATER in the barrel's life, not in the beginning.
Walt
If this is true, it contradicts what I was told by a barrel maker & a trustworthy gunsmith regarding custom made stainless barrels. Was told that as the rifle was shot it would "smooth out" the small imperfections, rough spots, etc in the barrel from the rifling & drilling processes and speed would increase and there might be some DROP or
leveling out in the pressure curve. Thus a reduced load used in break in may be able to be bumped up or a change in powder burn rate after break in.
I know it is debateable without data, but.... there is merit to either side of the debate IMHO. WD
I am leaning toward what WyleWD has posted. From my past experience. plus what my records show and from what 3 top Fclass shooters have told me and one Awesome gunsmith has told me as well. I guess I'm saying 2+2=4 so all adds up to this. Don't hold me too this but I'm very intrested in just finding out for myself and if I can I will share my results.
I think this thread has turn the light bulb on to what my gunsmith was trying to tell me in the beginning. My gunsmith has a Cam adapter for the Hawkeye bore scope. This magnifies things and you can see the microscopic flaws in a barrel and when you see this on a LCD screen it really shows up. I will admit I was a little worried from what I was seeing in a barrel that had not had a single round fired. Believe me this barrel and rifle is simply just a hammering machine. My barrel shot a 1/4" group at 200yds at 38rnds. I stopped and loaded up for a practice match my club was having. Upon reaching 168rnds This barrel went crazy on me! I was using plot sheets and could see just where things happen. As I got stressed in the moment things just got worse so I quit and came back home. Shot some of the same ammo that was left over from the match. Groups at 200 were now at 1"+. Velocities were 75fps faster than from the start. I'm starting to believe now that at this point those microscopic flaws are smoothing out and even tho it's a handlapped custom barrel one still has to test every 300 or so rounds and stay on top of your load as the barrel changes conditions thoughout the life of the barrel. I have made notes and filed those notes with my records for this rifle and barrel. I'm hoping I can stay on top of this and scope the barrel thoughout the life of it with the cam that my smith has. My Hawkeye just will not show what the Cam on the Hawkeye will. I'll ask even if my gunsmith is busy if I can bring my rifle to the shop and scope it myself. I'd just like to see how things do change and if nothing else gain some knowledge for future reference. Like I said don't hold me to it but if I can I will try to keep track of this and I'll post my results. I'm not as good with graphs as JAY is but I will try!
Very intersting Thread!!! Thanks for bringing this up Jay!! Great Topic!!