Group shape

[Demphna2]

I am new to PCP airgun shooting, but have an interesting thought that translates from powder burner reloading and tuning. From what I've noticed just shooting the one PCP I have, you have control over a few things to make the gun shoot like you want. Regulator pressure, hammer spring tension, and pellet selection.

This thread will focus on pellet selection and group shape. These rifles are extremely accurate and consistent and seem to shoot pellets very well. Some pellets shoot great. Stealing from Jack Neary's (HOF benchrest shooter) assessment of group shape indicating what you need to do in regards to powder and seating depth can help determine what's going on with an air rifle if used in the same capacity.

So without messing with the regulator or hammer spring and just testing a lot of different pellets and weights, I think you find a lot that shoot well, but the interesting thing to evaluate would be group shape. Size matters unsure, but shape can give insight as to what is going on harmonically with your barrel. Horizontal, vertical, oval/round circle (optimal), diagonal (worst), double grouping; they all indicate what your barrel is telling you it wants.

Obviously you would have to shoot very well to notice the difference in group size and shape, but does anyone have any anecdotal experience with this? Does your best shooting pellet group well and have a round shape to it? Have you tested pellets that group ok but the dispersion is horizontal, vertical, or diagonal? I'm interested to hear everyone's thoughts.

 

[bigHUN]

Based on your OP I assume you don't have the FX Impact variants that using liners instead of - a simple ordinary barrel, so I will try to translate my experience with my heavy modded Impact MK2.

Uhm, from where to start? The group shape...
Usually I do these barrel tuning from a bench shooting rings @ 100 meters, possibly in a calmest day I can catch. Gun mounted on a one piece rest to minimize the user errors.... lol how easy that sounds...
With the Impact barrel/liner combination we have an option to index the liners. I have couple liners that are more bent and some less (not visible with naked eye). When I rotate the liner CW (clockwise) for example by 15 minutes, shooting 5 shots groups and monitoring how the group "size" POI is rotating around the POA.
But not only the group is rotating but I have noticed the group shape is changing when the group rotates.
I chose the group = position - that is more horizontal shape and possibly closest to scope mechanical center. With a sharpy I mark the liner, so when in future removing for cleaning I can mount it back to that position.
So my choice is a more horizontal group shape. That gives me a higher odd for better score with side winds, but also higher risk with tail/front winds. I gamble this way .

The other thing I need to add to your " >>> Regulator pressure, hammer spring tension, and pellet selection <<< " the Dwell timing... which on Impact we adjust with Valve spring tension how quickly to open the Valve and how long to keep it open. The ultimate goal with that is to have a higher burst of pressure to start accelerating the projectile but stop the air pulse inside the barrel/liner before reaches the muzzle.

I am not going to comment about "projectile" selection, based on my experience I think those not effecting the shape of the group...either works good groups - or not. But the barrel actually chose the pellets, so to find a good combination need a lot of air for disposal.
The barrel harmonics... again a long story.

I assume some other guys can chime in with non FX liners knowledge - how they do it?