Have a seat and get comfy. I have a few things to say on the subject of barrels.
You will see a minor improvement with an aftermarket barrel, but the difference will not be staggering. By far the largest impact on accuracy is the paint itself, but accuracy is still going to be pretty bad compared to any other ballistic device you may be used to. After all, you're shooting a large caliber liquid filled gelatin shell projectile. There is a reason modern firearm bullets are no longer spheres. A paintball is not aerodynamically sound, and even if it starts out perfectly round with no defects (which is a best-case scenario), the gelatin will deform in flight simply because of the hydraulic/pneumatic forces we use to propel it. We generally consider 50-75 feet (15-23 meters; sorry, but from here on out I'll be using SAE measurements) to be just about maximum range for typical accuracy. Given three rounds, I can usually hit a man-sized target at 75 feet. In most cases, if I shoot at someone farther than that, I'm just trying to keep their head down so that I or my teammates can move closer.
So yes, a barrel will help, but it won't give you miracles, and this is simply because of the inherent inaccuracy of paintballs. As I said earlier, the quality of the paint itself with regard to dimples, fill consistency, and seam deformation has the biggest impact on accuracy. The fewer the dimples and the smoother the seam, the better the accuracy. This kinda sucks when you've got to deal with a "field paint only" facility, but at least everyone has the same disadvantage.
In this game, paint size to barrel bore size match as well as the velocity consistency of the marker itself are the two big factors that you can control as far as accuracy is concerned. If you stay around in the forums long, you'll see people beat the "paint to bore match" thing to death. This is one of the things to consider when picking a barrel.
There are two schools of thought on paint to bore match: overboring and underboring. Tests have shown that neither has any accuracy advantage (see the Punkworks results here
), but there are other differences. Interestingly, these same tests showed that perfect bore matching was actually a detriment to accuracy.
Overboring simply means that the barrel has a slightly larger inside diameter than the paintball. A paintball sitting in an overbored barrel will roll around freely, providing little to no resistance. You don't want a huge gap, just a couple thousandths of an inch; just enough so it rolls without getting stuck. If you have a closed bolt marker (ie: Autococker, pump marker, etc.) where the paintball sits at rest in barrel, overboring is usually a bad idea, as the ball can roll down the barrel before it's fired, causing inaccuracy. Additionally, overboring allows a small amount of air to pass by the paintball, which adds up to a noticeable but not severe difference over the course of an air fill.
On the other hand, underboring simply means that the barrel bore is slightly too small to accommodate the paintball. The reason this works is because paintballs are flexible. On a correct underbore, you can place a paintball into the breech end of the barrel, hold it vertically, and the ball will not move. However, the bore should not be so tight that you are unable to blow the paintball out of the barrel with lung power. Additionally, looking down the bore of an underbored barrel that has a paintball in it, you will see that the paintball is making contact with the barrel typically in three (occasionally four) locations. Unfortunately, because of how precise you need to be to underbore correctly, this usually requires the use of a barrel kit that comes with different sized bores. The advantage of underboring is that you don't get the blow-by that you do with overboring, and closed bolt markers will not experience roll outs.
Your Tippmann is an open bolt marker, and as such does not require underboring. Additionally, the valve system on your Sierra is not what we would call hyper-efficient anyway, so you won't miss any waste caused by overboring blow-by.
Typically you'll find that most tournament players overbore. The reasons behind this are the fact that most of them use open bolt markers, and because tournament paint is typically incredibly brittle, and there is less chance of breaking paint with an overbore. Mind you, a correct underbore usually won't break paint any more often than an overbore, but tournament thinking is that it's better to be safe than sorry.
As a recreation player with a myriad of both open bolt and closed bolt markers, I prefer to underbore where possible. Occasionally I will run into paint that is either smaller than my smallest bore, or it has tournament grade brittleness combined with poor shell quality (dimples, etc) that will force me to overbore.
If you choose to overbore (which is easier and requires less equipment), I recommend getting an accurate measurement of the type of paint you typically use, and order your barrel with an appropriate diameter. In an ideal overbore, you want 0.002" to 0.005" clearance. Try not to go too much more than that if you can help it. IIRC, the stock Tippmann barrel I have around here somewhere measured out at something like 0.695", whereas most paint lately is measuring in at 0.684" and below.
On another front, you'll want to consider barrel length. Punkworks has shown that under typical conditions barrels need to be at least 6" long in order to correctly guide a paintball to provide a stable flight trajectory, but usually an 8" barrel is preferred for minimum length. Additionally, any barrel longer than 14 inches is simply wasting air by having to push the paintball farther down the bore. The theory that adding more length will improve accuracy has been proven to be unfounded with paintballs. As a result, I always recommend a barrel length between 8 and 14 inches, with an acceptable allowance up to 16".
Finally, you may want to think about barrel porting. This has little effect on accuracy, but it has always been a big point of contention and confusion. In the old days when custom shops were everywhere, you were able to to choose whether or not you want porting (sometimes called venting) in the barrel, and occasionally they let you choose how much of it you wanted. Palmer's Pursuit Shop is really the only manufacturer left that still gives you the option to pay for different amounts of venting when you order their Brass barrel (which is one of my all-time favorite one-piece barrels), but you can still choose a barrel with the amount of porting it has in mind. Porting has two impacts: it decreases the sound volume of the shot at the cost of decreased gas efficiency. By dissipating some of the air used to fire the paintball through tiny holes in the side of the barrel before it exits the muzzle, the sound caused by the plane of the shockwave breaking the muzzle is lessened, but it costs you a little bit of extra gas to push the paintball past those ports. Tournament players typically love porting. I could take it or leave it; in fact, my own Palmer's Brass barrel has no porting at all.
A final thing I just thought of to consider is the number of pieces your barrel comes in. Don't be fooled: a two-piece barrel is not inherently more accurate than a one-piece barrel. In fact, it's been shown that the transition between pieces can actually detriment accuracy. The reason this myth came about is because your typical barrel kit comes as a two-piece barrel where the back section can either be replaced for different bore sizes or has different sized bore inserts. As such, the term "two-piece barrel" became associated with accuracy, which is not necessarily correct. In fact, if anyone made a barrel kit consisting entirely of a kit of one-piece barrels, I'd probably own it. The thing you have to be careful with in this one-piece/two-piece debate is that occasionally a manufacturer will make different grade barrels, and each different grade may have a different number of pieces. Custom Products (CP) is famous for this.
If you need anything explained further or have any more questions (or even to tell me that this wall of text is daunting and unnecessarily long), don't hesitate to post up.