Help with large discrepancy in chrono readings

[Horitexan]

While a severely fouled barrel can certainly cause accuracy issues and effect velocity, I can't see (and never have seen) a 300fps difference as a result of a dirty/fouled barrel. On the other hand, I've never (that I can remember) needed 60 patches to clean a barrel. Lol! That's pretty seriously dirty. I would have thought the barrel would have SLOWED after a cleaning since it's typical, for example, for HbN treated barrels to require increased loads to maintain velocity because the bullets slip down the barrel with far less friction. I would have thought that this would be analogous to going from a dirty barrel to a squeaky clean one.

My first guess would be an issue with the chronograph. I'm not familiar with the Pro brand but all the chrono's that use light/shadow to record velocity require the proper lighting conditions (i.e. plastic shields installed, or not, angle of the sun, sufficient light, distance from muzzle, etc.) so, maybe, there was some inconsistency in conditions or setup? I know my Chrony (same style as yours) needs 10' from the muzzle to provide an accurate reading. I use a tape measure every time. If you are off in your setup I can see some of the discrepancy you're experiencing; but certainly not all of it. 300fps is a LOT of difference!

Then there's ammo temp as cattlelackranch mentioned. But, again, I can't see that accounting for such a large discrepancy unless (maybe) the ammo was scorching hot the second round and really cold the first round.

Maybe you had a combination of both of these issues going on. It's weird, though. If it were me, I'd go get another box of that ammo and do some more shooting - with the rest of your current box and then with the new one. You're probably going to get some deviation between the two lots but I'd be looking to see if my readings settle down in terms of consistency; and whether the 300fps discrepancy disappears. Hopefully it was just an anomaly that'll be one of those unsolved mysteries we all have at some time or other. I wish I had the answer for you - I'm really curious what the cause(s) is. I don't know if you're a member but you ought to go over to www.accurateshooter.com and post your issue. There are some great technicians on there and they're always willing to help. Couldn't hurt to get more input.

What are your groups like - aside from the issue of velocity? Also, what was your ES/SD like? You haven't answered that question yet.


Sent from my iPhone using Tapatalk

[bboswell]

Can you expound for me please?

These figures are usually used to evaluate the consistency of handloads, in this case I wanted to use them to evaluate the chrono used.

I have the same chrono, it is great for checking arrow velocity at an ASA event but entirely too light sensitive for rifles.

If you want accurate velocity use Magnetospeed, LabRadar or shoot 100yds & 400 yds and use Ballistics AE to calculate your true velocity

Last option is most accurate.

[Livin'2hunt]

While a severely fouled barrel can certainly cause accuracy issues and effect velocity, I can't see (and never have seen) a 300fps difference as a result of a dirty/fouled barrel. On the other hand, I've never (that I can remember) needed 60 patches to clean a barrel. Lol! That's pretty seriously dirty. I would have thought the barrel would have SLOWED after a cleaning since it's typical, for example, for HbN treated barrels to require increased loads to maintain velocity because the bullets slip down the barrel with far less friction. I would have thought that this would be analogous to going from a dirty barrel to a squeaky clean one.

My first guess would be an issue with the chronograph. I'm not familiar with the Pro brand but all the chrono's that use light/shadow to record velocity require the proper lighting conditions (i.e. plastic shields installed, or not, angle of the sun, sufficient light, distance from muzzle, etc.) so, maybe, there was some inconsistency in conditions or setup? I know my Chrony (same style as yours) needs 10' from the muzzle to provide an accurate reading. I use a tape measure every time. If you are off in your setup I can see some of the discrepancy you're experiencing; but certainly not all of it. 300fps is a LOT of difference!

Then there's ammo temp as cattlelackranch mentioned. But, again, I can't see that accounting for such a large discrepancy unless (maybe) the ammo was scorching hot the second round and really cold the first round.

Maybe you had a combination of both of these issues going on. It's weird, though. If it were me, I'd go get another box of that ammo and do some more shooting - with the rest of your current box and then with the new one. You're probably going to get some deviation between the two lots but I'd be looking to see if my readings settle down in terms of consistency; and whether the 300fps discrepancy disappears. Hopefully it was just an anomaly that'll be one of those unsolved mysteries we all have at some time or other. I wish I had the answer for you - I'm really curious what the cause(s) is. I don't know if you're a member but you ought to go over to www.accurateshooter.com and post your issue. There are some great technicians on there and they're always willing to help. Couldn't hurt to get more input.

What are your groups like - aside from the issue of velocity? Also, what was your ES/SD like? You haven't answered that question yet.


Sent from my iPhone using Tapatalk

I agree with this. I suppose since the OP hasn't responded about the SD and ES, he doesn't have those figures at hand. It sounds to me like it may sometimes be a chrono set up issue with lighting but sounds more like a terribly fouled barrel is now clean as a whistle and the rounds were also cooking in the heat or chamber before they were fired. Those two factors alone could account for the difference in velocity.

I don't use factory ammo so, I have no basis for comparison. In my hand loads, if after three shots I see the ES and SD going poorly, I stop that group and those bullets get marked to be pulled. I don't bother to finish testing that load and I move along to the next one. In short, the SD and ES are critical in this conversation to make an accurate assessment.

[Horitexan]

The main reason I think the ammo is less likely the culprit here (aside from the possibility of it having gotten cooked one time and cooled another) is that his two sessions seemed to produce fairly consistent readings DURING each session. Despite the lack of reply on ES /SD, the OP provided some velocities from the 2nd session and his wording indicates that the deviation was between two averages (velocities) he calculated following each individual shooting session. Had it been a true ammo issue, I expect, he would have seen a 300fps spread during a single shooting session. In order for this to have been an issue with the ammo, he would have had to (magically) grabbed only the slower rounds during one session and, then, grabbed only faster rounds on the next one. That's statistically impossible. Lol!

It just has to be something other than the ammo. More I think about it, it's got to be the chrono. I didn't know there was a company other than Chrony that made that style. Lol! Never heard of "Pro" but I'm always suspect of companies that need to tout their (Pro) ability in their name! IME, those products were rarely even close to "professional quality." My Chrony works great but it's such a PITA and inconvenient to setup that it gathers dust since I got my MagnetoSpeed. I even use my MS for load development, without issue.


Sent from my iPhone using Tapatalk

[Livin'2hunt]

I may have misread but I thought he wrote that he was shooting with a terribly fouled bore at first and a shiny, clean bore for the second. If that's not the case, I have no clue. I don't seem to have these sort of issues but then again, I meticulously load my own and if I get crazy readings, I KNOW that load just isn't working and I move along. It's hard to make these assessments from afar and without knowing all the pertinent information.

I would recommend taking a rifle with known velocities and shooting it thru the chrono to see what it reads. The thing may need to be calibrated or have a 264 bullet run through its brain.

[Horitexan]

I may have misread but I thought he wrote that he was shooting with a terribly fouled bore at first and a shiny, clean bore for the second. If that's not the case, I have no clue. I don't seem to have these sort of issues but then again, I meticulously load my own and if I get crazy readings, I KNOW that load just isn't working and I move along. It's hard to make these assessments from afar and without knowing all the pertinent information.



I would recommend taking a rifle with known velocities and shooting it thru the chrono to see what it reads. The thing may need to be calibrated or have a 264 bullet run through its brain.

Sounded like he didn't have any "known" velocities to compare to; but he did shoot some other rifles and said the velocities were in the ballpark of what the factory claimed and were relatively consistent. You're correct about it being really dirty the first round and squeaky clean the next. But, even so, 300fps seems like too much to be solely a bore cleanliness issue. Besides, you'd think cleaning the bore would slow things down, not increase speeds.


Sent from my iPhone using Tapatalk