This is a very good gun. It is very easily modified. It combines parts of the A-5 and the 98. It has a trigger mechanism all together encased in plastic. It also has the A-5 barrel adapter and comes with an M-4 Barrel and handgaurd. You can also buy adapters for 98 barrels. It has two Piccatinny rails, one on top with an M-4 sight and one on bottom for a grip. It is a great gun for military training, scenario games, or just woodsball games. The only thing i had to worry about is the barrel adapter. When i got it, it was a little wobbly. But I added a small sliver of paper and it was fine. I also recommend getting a sight rail that goes on top of the hand guard and a red dot sight. If you do that, then you need to get the BT Rip Clip.
The BT Paintball Design BT-4 Assault is newer, so it should be commonly available, both new and used. Check out the Places to Buy links below for places to purchase it new.
Tippmann 98 Pro
RAP 4's T68
Smart Parts SP-8
BT-4 Assault at the base
Red Dot sight
adjustable sight rail
2 point strap
grav fed hopper
Easy to use. Inexpensive for new players
Can be a little touchy during longer games if not cleaned out
the BT-4 Assault is a great gun for both the newer player and the seasoned player of woodsball its reasonably easy to find and usualy inexpensive the list of attachments that can be used on the gun is pretty fair mainly due to the fact that you can take acual parts from the M4A1 carbine (usualy found at military surplus) and slap them on to your marker with a bit of tinkering.
My recommendation is this is a great gun for both new players and older. simply because its good. I've never had a problem with my marker during a game and I assume to never have a problem I would highly recoment this marker to anyone looking for a new gun or a person looking to get started hard into the game