This is a general page about Smart Parts barrels in particular, with information on their design and lineage over the years. I made this page for a reference to be used by other people that have old-school barrels in their possession.

O-rings:
SP barrel fronts use a 19/70 o-ring to seal with the barrel back.
Freak backs use two internal 18/70 o-rings to hold the inserts.
Grease may be applied to the outside of the Freak insert if its fit within the back is especially tight, but you should clean the grease out from the barrel before use.
Barrel threads may be wrapped with an o-ring for a friction fit when installed on the marker, but is not required. Impulse/Ion/NXT/Luxe barrels use an 18/70 o-ring from the factory. Shocker barrels don't use an o-ring from the factory but you can install one if you prefer.

Barrel front threading:
Most "common" Freak and All American barrels share the same threads (7/8"-32-DL), but there are exceptions for compatibility. The old-style AA barrels use different threads and aren't compatible with newer fronts. Additionally, the Freak-XL barrel uses different threads because the front is designed differently, and and Freak-XL barrel fronts aren't compatible with any other barrel back. All Freak-XL backs and fronts are engraved with the letters "FRK XL" aroud their outside surfaces, so they can't be confused with non-XL barrels from the outside.

Barrel lenths:
The advertised total barrel length may vary depending on your combination of Freak or All American backs/fronts. Freak backs are shorter than AA backs, despite the fronts having identical threads, so the advertised lengths apply only to a matching Freak back/front or AA back/front. The chart below shows the combinations:

Late-Model Barrels: 2006 series
Barrels were redesigned into their most-modern cosmetics. Most of them use the same "flared" barrel shape, which is noticeably different from the last series barrels which curved inward instead of outward.
These designs were used up until SP went out of business in 2010. At the time of this writing, the barrels sold by GOG Paintball also follow these designs.

Linear barrel	This is the lowest-end barrel of the current series. The Linear barrel has a flared shape using straight-line porting, instead of the SP trademark spiral porting. This barrel is similar to those found on Ion markers, although the Ion version has less porting.	Linear barrel
06 Progressive barrel	The 06 Progressive barrel is shaped similar to the Linear, but with SP's trademark spiral porting using teardrops for catching paint splatter. This barrel was likely meant to be a cheaper aftermarket solution for low-end markers. Available in multiple colors and lengths.	06 Progressive (12 and 14 inch shown)
Tactical barrel	These are 21 inch length one-piece barrels, with linear porting and a huge muzzle break. This upgraded barrel comes on the SP*8 marker. A slightly shorter Tactical barrel is also used on the SP1 marker, which replaced the SP*8.	Tactical one-piece barrel
06 All American two-piece	New cosmetic milling from the earlier version AA barrel. The porting is similar to past versions with only minor adjustments to the shape of the front. The bore size is 0.693 for Ion/Impulse/ShockerNXT threads, or 0.689 for everything else. Works with all Freak and AA fronts. Available in gloss or dust finish. This was the stock barrel on Shocker NXT markers.	06 All American
06 Freak barrel	The original multi-insert barrel kit, with newer cosmetics. Freak backs are compatible with all Freak and current AA fronts. Available in stainless or aluminum back. Current-style (2006) Freak backs have ovular cutouts to view the inserts, and the current Freak fronts use linear porting with the flared barrel shape. Comes stock on Epiphany markers as well as private label Shockers. Variants of this barrel (with altered cosmetics on the Freak back) also come on DLX Luxe markers and 09 Impulse markers.	06 Freak back and front

Recent Barrels: 2002 series
These barrels are easy to recognize because they all use a similar curved barrel shape (except for the AA). They're also the most common SP barrels floating around, since most of SPs barrels were produced during this time period.

02 Progressive barrel	These one-piece barrels use spiral porting like the All American styling, with a curved barrel shape toward the end. These barrels came stock on classic Impulse markers. This was later upgraded to the 06 Progressive and given teardrop ports.	02 Progressive
Teardrop barrel	Same as the Progressive barrel, except with added teardrop porting. Discontinued in 2006 when the 06 Progressive was released.	Teardrop barrel
02 All American two-piece	This barrel uses a standard 0.689 back and is compatible with all Freak and AA fronts. The back uses new cosmetic porting from the last version (one-piece AA). Compared to the earlier All American, the front is virtually the same (spiral porting with straight barrel shape). This came stock on Shockers at the time, as well as a few private label Impulses.	02 All American
02 Freak barrel	These were used between 2002-2005. The backs had the text FREAK milled out, while the fronts used spiral teardrop porting (using the same barrel shape as the teardrop/progressive). Fronts are compatible with AA backs. Came stock on Evolution Autocockers and many private label Impulses and Shockers.	02 Freak front and back
Stainless Freak	Different milling to compensate for the added weight. These are still used through 2006 and later. Came stock on Nerves.	02 Stainless back
Freak Jr. barrel	These barrels came with a typical 02 Freak back, one insert, and a modified front. The fronts didn't follow cosmetics from neither a Freak nor AA barrel; instead they use a slow-spiral line of ports and had a flared muzzle tip shape. The Freak Jr was intended to be a cheaper version barrel for use on lower-end markers, allowing the possibility for purchasing the Freak insert kit at a later time.	Freak jr front only

Vintage & Rare Barrels: 01 series, and older
These older barrels have been long since discontinued, but can still be seen floating around on older markers. At this point in time, the basic SP barrel shape was mostly a straight tube, although some had muzzle brakes. The threading on these old two-piece barrel fronts is NOT the same as the newer ones. Additionally, the old two-piece barrels are glued together from the factory; they can be separated if necessary but it takes a lot of force.

All American old-style two-piece 1989-2001	The original Smart parts product; this barrel was the first to feature their spiral porting. This was originally available with different sized barrel backs (0.686 light gray, 0.691 dark gray, and 0.689 black) but all except the standard 0.689 were dropped in 1998. The threading on these was changed in 2000 when SP created the Freak barrel and switched to the barrel front threads that they use today. These old-style AA barrels came stock on pneuVentures Shockers as well as SP Shocker Sports at the time.	Old-style glued All American barrel
All American Stainless 1999-2000	Same as the old-style AA two-piece backs, except made from stainless steel. These also functioned with old-style aluminum AA fronts if the user special ordered it. Stainless front version available in 14" only.
Smart Stainless 1999-2001	Similar in function to the Progressive barrels, except with a different shape, and of course made from stainless steel. These were one-piece barrels made entirely from stainless and using spiral porting. They were available in 12" only.	Smart Stainless barrel (picture from J. Sickal) Smart Stainless barrel
Big Daddy barrel 1999-2000	These are old-style AA barrels using the spiral teardrop-ported fronts when they were first developed. At the time, the only way to get the teardrop porting was to buy this barrel, which was considered an upgrade to the typical All American barrel. This style of porting ultimately evolved into the Freak front when SP's barrel design shifted to new-style barrels, at which time the old-style AA barrels were discontinued.	Big Daddy barrel Big Daddy barrel
All American Teardrop	Another name for the Big Daddy barrel.
Progressive original 2000	This barrel used a straight shape with spiral porting. This was the stock barrel for the Impulse of the time, and it also came stock on some 2001 Shocker Sports instead of the AA two-piece.	00 Progressive barrel 00 Progressive barrel
Teardrop original 2000	Upgraded version of the Progressive; this one-piece used spiral teardrop ports.	00 Teardrop barrel
Freak original 2000-2001	Freak backs originally said SMARTPARTS as pictured. Also, the original Freak fronts used spiral teardrop porting that was first used on their Big Daddy barrels, but weren't curved like the 02 versions (the front was straight like all the other 2000 barrels, see the Big Daddy barrel for pictures).	Freak original

Freak Insert Variations: 2002-present
Freak inserts have been in production since the year 2000, although there have been some changes made over the years. Here's a list of the details.

Standard aluminum inserts	These are the typical Freak inserts that are most often used by SP barrels. Inserts are anodized aluminum with silver laser-engraved sizes on the side. They come in the following sizes: .679 (silver) .682 (light green) .684 (purple) .687 (red) .689 (blue) .691 (dark green) .693 (yellow) .695 (pink)	Freak aluminum insert kit
Stainless inserts	Stainless steel versions of the common aluminum inserts. These are more or less identical except for the silver finish. They're silkscreened with sizes on the outside. These were popular since they were theoretically more durable than the aluminum ones, however cost at least $50 more than the aluminum kit. Stainless insert kits came with one size larger (.697) and weren't made in the smallest .679 alumium insert size. Therefore, the smallest stainless insert was .682	Freak stainless insert kit (picture from MasterSconi@pbnation)
GOG Boremaster kit	Rebranded version of the aluminum freak insert kit. The insert colors changed to these: .679 (silver) .682 (tan) .684 (orange) .687 (red) .689 (blue) .691 (silver) .693 (black) .695 (dark green)	GOG Boremaster inserts
Empire Superfreak kit	Rebranded version of the aluminum freak insert kit, manufactured by KEE/Empire. Insert sizes and colors are as follows: .675 (red) .680 (pruple) .685 (blue) .690 (black) .695 (green)	Empire Superfreak insert kit
Limited black insert kit	This full Freak insert kit is all anodized black and engraved with each individual size. These inserts are fairly rare because it could only be purchased along with an entire marker (private label Ion or Shocker NXT). While it's true that GOG and Empire both made black inserts in the future, neither of them made a full kit consisting of ALL black inserts.	Black insert kit
SP stainless reball insert	A special stainless steel insert with a bore size of 0.670, made in 2006. SP only made 1 or 2 batches of these, so they aren't easy to find in current times.
GOG aluminum reball insert	GOG Paintball started producing these in 2011, however the bore size was increased to 0.675 and the inserts are made from aluminum.	GOG aluminum reball insert