Originally Posted by Tench
Brian I think a lot of the CG data originates from BC's measured by Harry Fuller? It gives a general idea about a pellet but actual values can be quite different.
As Colin pointed out above, many people don't run 8.4's close to the limit as I too have found they are better at around 780 fps, and also like Colin when I was using the heavies I found they were at their best when pushed as fast as you dare set the rifle which does minimise a little of the trajectory difference.
As to a pellets bc performance difference between regged and unregged, I believe there is a difference. it is not the regulator itself that makes the difference but the pressure used to fire the pellet.
In this graph of a shot string from an unregged rifle you can see that when the cylinder pressure is high which would create a short duration to the firing valve open time the bc is higher, as the pressure comes down and the unregged action automatically compensates with a longer valve duration the bc gets lower.
I am sure the velocity also plays a part here, the higher velocity causes greater drag which lowers the bc, this may explain the rise in bc value towards the end of the string as the velocity drops.
Graph originally from Harry.
So if we knew the res pressure around shots 8,9 and 10 we could build a regulated action to run at this pressure to achieve the best performance from that pellet at that velocity in that rifle. I have been working in this area for a couple of years but have so far failed to convince any manufacturer that better than what they are selling us is possible! I believe massive gains in rifle external ballistic performance is possible with proper development. Instead we get useless cosmetic changes between rifle models!
Great info Simon, cheers
Yup I suspected that the BC values we take for granted are arrived at in a rather haphazard fashion, and that's borne out by what Mark found. Although he didn't measure actual BC with chrono's along the trajectory, he did observe that the difference in POI between a 10.3 and an 8.4 was alot more than the 2mm difference at 50m we would expect IF the BC values being used in the calculation were accurate. That's also consistent with him not seeing much of a difference in windage too.
I believe if he had measured the BC with chrono's he would have seen that the BC of the 10.3 in his rifle was quite a lot less than the quoted BC of 0.031.
BC is specific not only to your gun and barrel, but to lots of other factors too. Taking a quoted BC someone else measured on their rifle, applying it to calculations for your rifle and expecting to get the same results is very unlikely.
Your theory about reg pressure is also right I believe. Jon Harris has been saying the same thing for some time and he's taken it further and suggested that it's not only the reg pressure but also the pulse characteristics of the release of air that can alter BC in a given rifle with a given weight and fit of pellet.
That's also consistent with other theories that 'Springers take less wind' - which could be explained by a different and possibly more efficient release of air resulting in a better than average BC, and that also explains why I can get a BC of 0.026 for a certain batch of 8.4's with my springer where quoted figures for PCP's can range from 0.018 - 0.023
To summarise what I'm saying.... don't trust the BC figures from Chairgun, you might find significant differences in your own rifle. Which is why you can't make a sweeping statement like "10.3's are better in the wind than 8.4's so why doesn't everyone use them" because it's very much rifle dependant.