I've been looking and searching through all the threads and I apologize in advance if this has been discussed ad nauseum. I haven't found the info I can relate to.
I have a 20" Kreiger straight (no taper) barreled .223AI I use a 7" TBAC Ultra on.
With the CB brake, I get an average velocity of 2738 fps with 80 gr Berger VLD's.
With the Ultra installed, I get an average velocity of 2817 fps with the same load.
This increase in velocity, along with a change in harmonics (I am guessing, because the 80 fps doesn't account for all), changes the POI by exactly 1 MOA and that is consistent to 600 yards, zero effect on group sizes.
Do any of you see this much increase in velocity with a can? What about the POI vs POA changes?
Nope. I think something else is causing your velocity to vary. It could be anything from temperature differences outside to humidity on to what kind of chrono you are using. If you shoot one day after three weeks of a dry spell vs the day after a good rain you can get slightly different velocity readings.
If you are using the old school optical type of chrono if you are not aligned exactly the same each time it can account for differences in readings. If you shot it exactly straight on for example it will give one reading, but if you were at a slight angle but still over the sensors the angle will cause a slightly different reading.
(side note: those optical chronos work by measuring the time in between when the plane of the first sensor is broken to the time of the second sensor plane being broken--basically it measures the shadows, so any cant or not exact alignment with to the muzzle can cause small variances in actual distances being measured)
Edit: I see that you are using a magneto speed, with that will impact harmonics a lot depending on the set up. Any time you hang something off the barrel you are going to be changing something up)
In my experience you 'might' get a 5 fps difference with can vs sans can all other things being equal.
If you measured one string on a cold day you will get slightly lesser velocity because cold air is much more dense. If you measure the other on a warmer day the air is 'thinner' and it can give velocity differences. The same thing is true at sea level near the coast with crazy humidity vs shooting at 5,000 feet elevation in the mountains.
As far as harmonics go yes a can will alter the harmonics and 1" is about average depending on the rifle, barrel, can and overall layout. The real trick is, is it consistent? If you put the can on, take a shot, then remove the can and replace it, and take another shot, are your baffles lining up exactly the same every time? This is one reason why I prefer taper mount systems to other mounting solutions. They are rock solid and they are very repeatable.
Also usually with a quality can (all other things being equal) it will tend to shrink group size a bit. The harmonics work a bit different with something hanging off the end of your barrel. In your case you are hanging TWO things off the end of your barrel so it's kind of a flawed test.
At distances beyond 100 yards (or even within 100 yards) exterior environmental factors are all in play. If you have a 5mph cross wind now, and 45 seconds later you don't that will impact your groups at longer ranges. In other words I am saying at 100 yards (generally speaking) you can see tighter groups with a good can. But out at 600 yards there are far too many environmental factors to claim it's all on the can or not at all impacted by the can.
