well known phenomenon
A lot of pistols I've owned have thrown one flyer per mag at first. That's because the lock-up is often a little different with the first manually chambered round.
And especially consistant when using a Ransom rest. Its quite common for autoloaders (even when using the rest) for the first (manually chambered) round to be away from the rest of the group. Usually only a little, but sometimes a serious amount.
Its one of those things, and there is nothing to be done but live with it. Our minds say "they should all be the same", but the machine has other ideas.
Many revolvers will have one chamber that shoots to a slightlly different place than the rest, due to their tolerances. Custom made guns seldom do this, but mass produced ones often do. And again, the difference can be small, or large, depending on the specific factors involved.
Group size is distance between the farthest apart shots. How many shots is proper? Your choice. Most handgun shooters use 6 for revolvers and 5 for autos. And they only use 5 because its a commonly used number for rifles as well. Some rifle shooters use 3 shot groups, others 10. 5 is a good number, if you aren't using a machine rest. Because in reality, you are also part of the group size. And by the time you get to 12, 15, 17 etc., your shooting abilty may play more of a role than the gun does.
