[for clarity purposes, my earlier posts on here pointed out the issue at the time with first strike that they had been questioned with regard to 'frangibility'. Since then they have been formally tested and are accepted as frangible, the advice given by ukpsf has been updated and they can therefore be permitted at more sites/events. The decision goes to the organizers as to whether they are allowed at a specific game. More details are in the 'paintball and the law' thread]
Using first strike in cqb is inappropriate.
I have been shot by first strikes, but not at close range. So can't comment on how they feel up close
As a principle the first strike holds its velocity for a longer distance, as opposed to a paintball which quickly loses velocity
A paintball at 280fps and a first strike at 280fps will both be at a similar velocity when at close range, but at a distance the paintball will have lost a lot of velocity whereas the first strike will still be near to its original muzzle velocity
A first strike is bigger, and has the fin, but is only half a sphere
It's a long time since I held a first strike and a paintball together to compare, but don't recall one being particularly heavier than the other - thus i don't recall the first strike will have a significant mass. But a bit of physics tells me that as the first strike has to have the gun adjusted to stop it firing hot then the first strike probably has less mass then the paintball (based on less air required = less energy required to push it down the barrel)
A difference between the first strike and the average paintball that I do recall is the tougher first strike shell. Like some paintballs we had in the old days, for the woods, that went through bushes. You knew you were hit!
There is quite a lot of American feedback and discussions going on about first strike, and in quite a few places they still operate on an 'off world' basis with games played outside of organized sites, certainly with a lot of the scare stories about first strike
In the U.K. they are likely to have been used in urban environments, not intended for cqb, but when in and around buildings it could be close up when you meet a player, and sometimes it's automatic to shoot
If using a combined magazine/hopper such as with the dye dam then the player may switch between them - but if you lined up with first strike, then flick to hopper then the one in the breach is first strike - and easy to forget about