I don't think it should matter.....unless the end that was in the breach needs to be crowned/recrowned. The rifling twist should be the same and that shouldn't matter either way. In any event, my experience with Daisy is they did not get too fancy with making one end of a barrel different in construction than the other. Pick it up off the assembly line and push it in....fast....without having to think too much. Good quality as fast and cheap as possible was the Daisy way.
Push a few pellets through with a cleaning rod, from both ends. Any taper in the bore should be felt as increasing or decreasing effort required to keep pushing the pellets. Make the loose end the breech, and the tight end the muzzle. Doing the opposite tends to be less accurate, because the pellets would rattle down the barrel. Hence the popularity of chocked airgun barrels.As for which way round would produce more velocity; that probably depends on just how much taper is present. A very small amount of taper towards a loose muzzle might shoot faster, due to lower friction, after the pellet was swaged down early in its travel. A lot of taper, opening up towards the muzzle, and air could leak past the pellet, making it slower.This assumes a decent and appropriate, crown and lead on the particular barrel.Velocity is nothing without accuracy...
Ah, the reason behind the question. That helps.Assuming your pump seal is not leaking, a way to boost pump efficiency is to reduce the free space (increase the compression ratio) at the end of the pump stroke. How exactly is best with your particular pumpgun is for you to figure out. Generally, adding a thin layer of JB Weld to the piston face does the trick. Adding glue inside the cylinder head can be tricky, unless you can remove the cylinder.Adding too much glue and machining it back is one way to go. Whatever you do, don't glue the transfer port shut; or the piston to the cylinder You can experiment with this in a non-committal way by applying modelling clay to the piston face (away from the sealing surfaces that run against the cylinder). It will probably take a layer about 1/16" deep over most of the piston face. That should make a real difference in pumping efficiency.Work up to it. Too much clay and forcing the pump closed will break the linkages...Some early pump guns had a user adjustment for the compression, to take up wear and keep up efficiency. Pity they don't all have it...
First things first. 880 barrels have a definite breech and muzzle. The breach end is freebored about a quarter of an inch.Second. If you want to add anything to the face of your piston, may I suggest using aluminum muffler repair tape? It sticks very well, and is thicker than aluminum foil. Much less messy, as well.