that is actually the worst for consistency. and its because of inconsistently sized paintballs. we underbore exactly because paint is not consistent.
Originally Posted by athomas
you see, at paint to barrel match, the barrel is a fixed size. meanwhile, every paintball is a different size (even good paint). so each ball is at a different place on the over to undebrore scale ... which means the drag to seal ratio is different for each ball, which means each one will come out a different speed. meanwhile, at a large overbore, every ball will not touch, so the drag to seal ratio will be very close on the balls. conversely, at a reasonable underbore, every ball will seal tight, so the drag/seal will be consistent, providing consistent velocity.
a large over or underbore will give you good consistency, with underboring .003-.005 being a bit more consistent. underboring is also much more efficient (up to 30 fps). paint to barrel match will be the worst consistency.
the answer to your question though OP is another one "what size is your paint" without knowing what size paint you are shooting, its impossible to tell you what bore size to use. my general rule of thumb is to aim for .003" to .005" underbore. i select this by blow testing the paint, and then when i find a match, use teh barrel 3 to 5 under it. we have not found underboring, even with extremely brittle paint, to break more paint until .007" - 009" underbore. so smaller is not always better, the reason underboring caught that bug was because before it was popular, getting barrels in the sizes to make a .003 to .005 underbore to work was nearly impossible due to shrinking paint. so we all cheered when the return of .679 and smaller barrels came out, as paint is consistently below .683 here.
using high speed videos from jack wood, and simon, and our own work, we have determined most "barrel breaks" are really minor damage to the ball during the loading cycle which later fail completely when subjected to the power pulse. a true "barrel break" where a perfectly normal, undamaged ball, breaks int he barrel is extremely rare, and has a distinctly different symptom to most "barrel breaks"
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